"Monopole Hall effect in a metallic chiral magnet"Dr. Kotaro Shimizu , RIKEN, Japan [Host: GiaWei Chern]
ABSTRACT:
Topological spin textures like magnetic skyrmions exhibit a stable particle nature due to their topological protection, leading to their utilization as an information carrier in magnetic memory devices. Although exploring the fundamental currentinduced dynamics is a pivotal topic for such applications, the studies have been mostly limited to twodimensional skyrmions [1] and the systematic investigation for threedimensional (3D) magnetic hedgehogs remains largely unexplored. The topological defect at the core of the hedgehog where spin length vanishes, corresponding to monopoles of the emergent magnetic field for electrons, could bring about currentinduced dynamics distinct from skyrmions [2]. 
Condensed Matter Seminar Thursday, June 6, 2024 3:00 PM Physics, Room 323 https://kotaroshimizu.netlify.app/ 
"Latest Trends and Opportunities in Photoemission Spectroscopy"Luca Moreschini , University of California, Berkeley [Host: Despina Louca]
ABSTRACT:
The transition of many materials of interest for the condensed matter community, particularly in energyrelated applications, from crystalline materials to heterostructures and complex architectures, where different compounds are combined, stacked, interfaced and twisted in different ways, has forced the experimental techniques to evolve accordingly. Angleresolved photoemission (ARPES), which has been for decades a spectroscopy of reference for accessing the low energy excitations in solids, had to implement a number of updates to keep pace with the progress made in materials fabrication. This talk will discuss some of the latest trends in ARPES, how it has advanced and to where it is moving in order to remain an invaluable tool for studying the electronic properties of materials. 
Condensed Matter Seminar Thursday, April 25, 2024 2:00 PM Physics, Room 323 
"Voxelated Bioprinting: Digital Assembly of Viscoelastic Bioink Particles"Liheng Cai , University of Virginia [Host: Bellave Shivaram]
ABSTRACT:
Analogues of pixels to twodimensional (2D) pictures, voxels –– in the form of small cubes or spheres –– are the basic units of threedimensional (3D) objects. Digital assembly of bioink voxels may provide an approach to engineering heterogeneous yet tightly organized 3D tissue mimics. However, this approach requires precisely manipulating highly viscoelastic bioink voxels in 3D space, which represents a grand challenge in both soft matter science and biomanufacturing. In this talk, I will introduce a voxelated bioprinting technology that enables the Digital Assembly of Spherical bioink Particles (DASP). First, I will discuss the criteria for the ondemand generation, disposition, and assembly of viscoelastic bioink droplets in an aqueous environment without the help of large interfacial tension. Second, I will describe how to use DASP to create 3D structures consisting of interconnected yet distinguishable bioink particles. Finally, I will share our recent progress in applying DASP to encapsulate islets into multiscale porous scaffolds to treat type 1 diabetes. I will also discuss immediate applications and emerging challenges associated with voxelated bioprinting. 
Condensed Matter Seminar Thursday, April 11, 2024 3:30 PM Gibson Hall, Room 211 
"Computational Approach to Compositionally Complex Materials "Diego Ibarra , University of Virginia [Host: Joe Poon]
ABSTRACT:
High Entropy Alloys (HEAs), also known as Compositionally Complex Alloys (CCAs), embody a transformative class of materials consisting of at least four elemental components. These new types of material open new horizons in alloy design for exploring new structural and functional material properties unknown in traditional alloys. However, the combinatorial nature of HEAs can result in compositional possibilities reaching into the billions or even trillions, making traditional studies challenging. This talk presents an indepth exploration of HEAs, starting with a foundational understanding of their unique characteristics and the importance of their complex phase behavior. It highlights the inherent challenges posed by the expansive compositional space and limitations of conventional materials discovery and design methodologies. The talk emphasizes the pivotal role of computational techniques that provides a strategic blueprint for highthroughput alloy design that accelerates the exploration and optimization of HEAs but also provides a deeper insight into their fundamental behaviors. 
Condensed Matter Seminar Thursday, April 4, 2024 11:00 AM Physics Building, Room 323 
"Linearintemperature conductance in electron hydrodynamics"Leonid Levitov , MIT [Host: Dima Pesin]
ABSTRACT:
Linear temperature dependence of transport coefficients in metals is habitually ascribed to nonFermiliquid physics. In this talk we establish this behavior for 2D electron fluids, systems in which carrier collisions assist conduction, leading to resistance decreasing with temperature. As we will see, electron fluids with simple Fermi surfaces obey nonclassical hydrodynamics described by a loop representing the Fermi surface shape evolving in space and time. Replacing the fluid velocity dynamics with an amoebalike loop dynamics leads to a large family of longlived excitations manifest as multiple viscous modes. A cascade of these modes results in a linear T dependence that extends down to lowest temperatures, as well as a Kolmogorovlike fractional power 5/3 scaling of conductivity vs. wavenumber. These dependences provide a smoking gun for nonclassical hydrodynamics and are expected to be generic for stronglycorrelated 2D systems with nearcircular Fermi surfaces. 
Condensed Matter Seminar Thursday, March 28, 2024 2:00 PM Monroe Hall, Room 118 
"Exploring the photoluminesce photoswitching properties of photochromic molecule crosslinked PbS quantum dots"Ephraiem Sarabamoun , University of Virginia [Host: Joshua Choi]
ABSTRACT:
Pairing PbS quantum dots (QDs) with photochromic molecules (PCMs) allows for the synthesis of efficient and reversible near infrared photoluminescence (PL) photoswitches. In our work, we explore the utility space of this hybrid system by systematically comparing and contrasting different types of PCMs and different sizes of QDs. We demonstrate that the amount of photoswitching observed can be affected by (1) varying the size of the QDs, (2) varying the length of the PCMs, (3) fluorinating the PCMs, (4) varying the end group of the PCMs. We further investigate this system to parse out the mechanisms which may be responsible for this behavior. We present strong evidence to suggest that the mechanism driving this switching effect is an interQD tunneling process. We demonstrate a possible link between the energy levels of the PCMs and the magnitude of the switching effect and outline a rough empirical model which can guide the future design of QD/PCM photo switches to produce customized switching properties. 
Condensed Matter Seminar Thursday, March 28, 2024 3:30 PM Gibson Hall, Room 211 
"Intertwining Topology and Geometry: Topological hydrodynamics on curved magnetic membranes"Chau Dao , UCLA [Host: GiaWei Chern]
ABSTRACT:
In this talk, I will discuss our recent work on transport phenomena stemming from the topological properties of magnetic textures. As a specific illustrative case, we study the transport of vorticity on curved dynamical twodimensional magnetic membranes. We find that topological transport can be controlled by geometrically reducing symmetries, which enables processes that are not present in flat magnetic systems. To this end, we construct a vorticity 3current obeying a continuity equation, which is immune to arbitrary local disturbances of the magnetic texture as well as spatiotemporal fluctuations of the membrane. We show how electric current can manipulate vortex transport in geometrically nontrivial magnetic systems. As an example, we propose a minimal setup that realizes an experimentally feasible energy storage device and discuss its thermodynamic efficiency in terms of a vorticitytransport counterpart of the thermoelectric “ZT” figure of merit. 
Condensed Matter Seminar Thursday, March 21, 2024 3:30 PM Gibson Hall, Room 211 
"Study of magnetic and electronic properties of honeycomb layered MTiO3 (M= Mn, Co, Ni) ilmenites"Srimal Rathnaka , University of Virgina [Host: Despina Louca]
ABSTRACT:
Despite identical R3 crystal structures, honeycomb layered MTiO_{3} ilmenites exhibit diverse magnetic orders and transition temperatures (T_{N}): Gtype antiferromagnetic for MnTiO_{3} (T_{N}=68 K) and Atype antiferromagnetic for CoTiO_{3}(T_{N}=38 K) and NiTiO_{3} (T_{N}=22 K). This work focuses on this intriguing interplay between local structure, electronic properties, and magnetic configurations. CoTiO_{3} has two magnon peaks around 514 meV with a distinct gapless Dirac node nestled between them are observed and the magnon modes are renormalized to lower energies. For CoTiO_{3}, magnetic excitations attributed to spinorbit exciton multiplet transitions show the same temperature dependance as magnon with the intensity dissipating quickly above T_{N}. The energy levels arising from crystal field and spinorbit coupling are gradually thermally populated through T_{ }and reaching maximum at 100 K. However, the NiTiO_{3} system shows a single low energy magnon peak around 24 meV which is renormalized into lower energies, but it does not show Dirac magnon properties. The calculated exchange interactions using SpinW confirm the weaker interplane interaction in CoTiO3 than NiTiO3. Across three system, both transition metal M^{+2} ion and Ti^{+4} ions are in distorted octahedra environment, and the first four nearest neighbors are TiO < MO < TiO < MO with the given bond length order. Across three systems TiO and short MO bond length variation is minimum. However, MO bond length (MnTiO_{3}=2.28 Å, CoTiO_{3}=2.17 Å and NiTiO_{3}=2.12 Å) variation is significant which follows the same variation as reported dielectric constants (MnTiO_{3}=20.4, CoTiO_{3}=19.5 and NiTiO_{3}=17.8 ) and T_{N} and confirms the interplay between these parameters. Within the measured 100 K to 500 K, temperature dependance of local structure is insignificant and for the reported relative dielectric values, the variation is almost constant. This suggests that the interplay between local geometry and magnetic interactions governs the diverse behaviors observed in these honeycomb materials. 
Condensed Matter Seminar Thursday, February 22, 2024 11:00 AM Physics, Room 323 
"Exploring emergent quantum phases in twodimensional flat band systems"JiangXiazi Lin , Brown University [Host: Seunghun Lee]
ABSTRACT:
Quantum phases such as superconductivity and ferromagnetism are among the most important topics in condensed matter physics research. Recently, a family of twodimensional flat band systems, including magicangle twisted graphene, uncovered an abundance of symmetry breaking and novel quantum phases. In this talk, I will introduce the recent advances in these materials and give two examples of how we engineered and revealed new quantum phases of matter in twisted graphene. These include an orbital ferromagnetic state induced by spinorbit coupling and a zerofield superconducting diode effect. Towards the end of the talk, I will mention our ongoing effort of studying a new type of Coulombdriven rotational symmetry breaking state in the moiréless bilayer graphene. These examples establish the twodimensional flat band systems as a versatile platform with multiple tuning knobs, where new physics emerges from the interplay between various quantum phases. 
Condensed Matter Seminar Monday, February 12, 2024 2:00 PM Physics, Room 323 A recording of this talk is available at this link (use passcode #D8FWkr?). 
"Ultranodal state in multiband spin1/2 superconductors"Peter Hirschfeld , University of Florida [Host: Bellave Shivaram]
ABSTRACT:
Recent measurements on the tetragonal phase of the ironbased superconductor FeSe,S support the existence of a remarkable phase where the superconducting state supports a finite residual density of states arising from patchlike nodal surfaces[1,2]. This ``ultranodal"> state can arise in situations where conventional intraband spin singlet pairing is highly anisotropic and coexists with timereversal symmetry breaking interband spin triplet interactions [3]. Here I present a microscopic scenario including ferromagnetic interactions that can account for nonunitary pairing and C4 symmetry breaking in the superconducting state that is also observed in recent experiments.
1) Sato, Y. et al. Abrupt change of the superconducting gap structure at the nematic critical point in FeSe1xSx. Proc. Natl Acad. Sci. 115, 1227??1231 (2018). 2) Hanaguri, T. et al. Two distinct superconducting pairing states divided by the nematic end point in FeSe1xSx. Sci. Adv. 4, eaar6419 (2018). 3) ``Topologically protected ultranodal state in ironbased superonductors", S. Setty, S. Bhattacharyya, Y. Cao, A. Kreisel and P.J. Hirschfeld, Nat. Comm. 11, 523 (2020).

Condensed Matter Seminar Thursday, February 8, 2024 3:30 PM Physics, Room 323 
"Nanoscale quantum sensing of programmable quantum matter"Shaowen Chen , Harvard University [Host: Seunghun Lee]
ABSTRACT:
Characterization and quantum control of complex quantum matter is one of the shared goals for condensed matter and quantum information science research. Toward this end, my research uses van der Waals materials to synthesize topological and correlated states, and quantum sensors based on spin defects to uncover their microscopic picture. Focusing on superconductivity as the theme of this talk, I will first present pathways to program the electron correlation by exploiting the lattice degree of freedom, both in the planar and vertical directions of moiré materials. The challenges to fully characterize the moiré superconductivity will be discussed. In the second part, I will show new experimental observables unlocked by the nanoscale quantum sensing platform can uncover hidden physics. As an example, quantitative visualization of the super current flow in a Josephson junction is used to reveal electrically configurable ground states in the zeroresistance regime. A surprising role of the kinetic inductance and the implications for the Josephson diode effect will be discussed. Finally, I will share my vision to explore intertwined topology and correlation by integrating the programmable quantum materials with nanoscale quantum sensors. 
Condensed Matter Seminar Thursday, February 1, 2024 3:30 PM Physics, Room 323 A recording of this talk is available at this link (enter passcode *0m4DSym). 
"Novel Fabrication of Quantum Wires: Towards Fractionalized Excitations"Tomoya Asaba , Kyoto University [Host: Seunghun Lee]
ABSTRACT:
The quest for novel quantum states in condensed matter physics often hinges on the reduction of system dimensionality. In particular, onedimensional systems are theoretically predicted to host a range of fractionalized excitations. These include the TomonagaLuttinger liquid, which exhibits spin and charge separation, and the Majorana particle, a cornerstone for faulttolerant quantum computing. However, fabricating nearperfect onedimensional quantum wires has been a significant challenge, especially those involving strongly correlated electrons. 
Condensed Matter Seminar Monday, January 29, 2024 2:00 PM Physics, Room 323 A recording of this talk is available at this link (enter passcode ^Sa3J2OZ). 
"Nextgeneration artificial van der Waals quantum materials "Dr. Bumho Kim , University of Pennsylvania [Host: Seunghun Lee]
ABSTRACT:
If one can control the atomic symmetries of a material at will, the intrinsic properties of the material will be significantly modified. However, the atomic structures of conventional materials are often constrained by the equilibrium phase of matter. Here, we overcome this fundamental limitation using recent advances in twistronics, enabling precise control over all the individual point group symmetry elements – inversion, mirror, and rotational symmetries – in twisted van der Waals (vdW) material in a new 3D configuration [1]. The resulting 3D twisted materials exhibit emerging optical responses that are fundamentally different from those of natural vdW materials. This novel approach to control symmetries can enable nearly infinite vdW quasicrystalline phases, promising a practical platform to study lessexplored structureproperty relationships of quasicrystals. In addition, we will discuss an ultraclean vdW crystal synthesis method [2]. A selfflux synthesis method we developed has yielded vdW materials with ~ 2 orders of magnitude lower point defect density compared to commercial vdW materials grown by a chemical vapor transport method. These ultraclean vdW materials reveal intrinsic excitonic properties that were previously obscured by lowquality materials. The combination of these ultraclean materials with the symmetry design approach holds great promise for the development of highperformance artificial material systems for nextgeneration technologies.
References:

Condensed Matter Seminar Thursday, January 25, 2024 3:30 PM Physics Building, Room 323 A recording of this talk is available at this link (enter the passcode @mJua4k1). 
"Roomtemperature and manybody quantum states in topological materials "Md. Shafayat Hossain , Princeton University [Host: Seunghun Lee]
ABSTRACT:
Topological states of matter combine quantum physics with topology—a branch of mathematics that explores geometric properties preserved under deformation. Quantum topology can lead to incredible properties. For instance, in a topological insulator, conducting edge states exist within an insulating bulk. Despite continuing progress, the search for such new quantum phases remains a central theme of condensed matter physics. In this talk, I will introduce two of the most soughtafter quantum states—roomtemperature topology and topological exciton insulator. I will first discuss our spectroscopic observation of topological edge states in Bi_{4}Br_{4}. I will show that the topological states, which typically can only be observed at temperatures around absolute zero, survive here at room temperature. I will also show how we probe the quantum transport response of this edge state using quantum interference. These observations mark the first steps in demonstrating the potential of topological materials for energysaving applications. In the second part of my talk, I will discuss our discovery of a unique topological state in Ta_{2}Pd_{3}Te_{5}. Here, the Coulomb interactions pair fermions (electrons and holes) into bosons (excitons), leading to a superfluid condensate state in the bulk while hosting topological edge states on the boundary. Finally, I will touch upon how these discoveries suggest exciting possibilities. This includes new devices and experimental techniques to discover the fundamental physics of topological quantum matter, opening doors for more efficient roomtemperature devices and quantum information technology. 
Condensed Matter Seminar Monday, January 22, 2024 2:00 PM Physics, Room 323 
"Nonlinear Optical Spectroscopies for Resolution of Electronic Structure and Dynamics"Veronica Policht , U.S. Naval Research Laboratory [Host: Despina Louca]
ABSTRACT:
Rapid and efficient charge transfer following absorption of light is a process of intense interest from the 
Condensed Matter Seminar Thursday, January 18, 2024 3:30 PM Gibson Hall, Room 211 A recording of this talk is available at this link (enter the passcode UEd*U6Wr). 
"Ising Superconductivity and Nematicity in Bernal Bilayer Graphene with Strong SpinOrbit Coupling"Ludwig Holleis , University of California Santa Barbara [Host: Bellave Shivaram]
ABSTRACT:
Superconductivity is an almost ubiquitous feature in the low temperature phase diagram of multilayer graphene allotropes – moire or crystalline. While the microscopic electronic structures of these systems differ, supporting devices with monolayer WSe2 has been shown to increase superconductivity along many axes of the phase space like density, magnetic field and temperature. Here, we study two superconducting domes (SC1 and SC2) in Bernal Bilayer graphene on WSe2 in light of their resilience to inplane magnetic fields. While SC1 appears in a symmetry unbroken phase, quantum oscillation measurements show that the normal state of SC2 is nematic, breaking C3 symmetry. Despite this difference, both superconductors violate the Pauli limit consistent with spin singlet pairing between opposite valleys protected from depairing by Ising SOC. Our results suggest that the induced SOC is central to the observed enhancement of superconductivity in many graphene multilayer systems  favoring pairing between time reversal symmetric partners. 
Condensed Matter Seminar Thursday, November 30, 2023 3:30 PM Clark Hall, Room G004 Join Zoom Meeting https://virginia.zoom.us/j/94608914338?pwd=a2xtYTNSMFpubThmRTA4Q1dkT29KZz09 Meeting ID: 946 0891 4338 Passcode: 170968 
"TOPOLOGY IS EVERYWHERE"Maia Vergniory , DIPC and MPI for Chemical Physics of Solids [Host: Dmytro Pesin]
ABSTRACT:
Quantum materials are a collection of atoms with interacting electrons and nuclei displaying emergent behaviour and topological properties—properties robust to local defects. The past two decades has witnessed an explosion in the field of topological materials: from weak interacting electrons to strongly correlated ones, topological materials represent one of the most exciting discoveries bot hat fundamental and application leve. High performance electronics, quantum information or ultrafast spintronics are just a few of the possible technologies that can be developed based on these materials. In this talk I will discuss the route to go from pure mathematical prediction of topological properties, through high throughput materials search to experimental realization. I will discuss both topological insulators, in non magnetic and magnetic phases as well as topological (chiral) semimetals using the the modern theory of topological band structure—Topological Quantum Chemistry — built upon symmetrybased considerations and complemented with chemical theories of bonding, ionization, and covalence. Consequently, it describes the universal global properties of all possible band structures and materials. Going beyond the singleparticle perspective, I will introduce our formalism grounded in Green’s functions. This approach is designed to uncover topologically correlated phases in materials exhibiting electronic entanglement, such as Mott phases. Additionally, I will discuss recent results centered on Green’s function zeros, which are aimed at diagnosing topology in correlated materials. 
Condensed Matter Seminar Thursday, November 16, 2023 3:30 PM Clark Hall, Room G004 Join Zoom Meeting https://virginia.zoom.us/j/92144577000?pwd=alY5TFVIdXo4Y052NmplMExWUVZnZz09
Meeting ID: 921 4457 7000 Passcode: 574393 
"Recent insights into the metalinsulator transition of vanadium dioxide (VO2) "Mumtaz Qazilbash , William and Mary [Host: Bellave Shivaram]
ABSTRACT:
Metalinsulator transitions are among the most fascinating and least understood phenomena in condensed matter physics. Metalinsulator transitions lead to significant changes in the electronic conductivity and optical properties, and are generally accompanied by structural and magnetic transformations due to a complex interplay between charge, spin, orbital, and lattice degrees of freedom. The thermallydriven metalinsulator transition (MIT) in bulk vanadium dioxide (VO_{2}) is accompanied by a structural distortion that leads to pairing of all the vanadium atoms in the insulating phase. This VV pairing has long been thought critical to the emergence of insulating behavior. We shall present our latest experiments on ultrathin VO_{2} films grown on TiO_{2} substrates. We demonstrate that the MIT in ultrathin VO_{2} films occurs without the VV structural distortion. Our results establish a route to a purely electronic MIT that is driven by electronelectron interactions. We shall also present our recent experiments and results on infrared nanoimaging and nanospectroscopy of VO_{2} films. The development of tabletop, broadband infrared light sources in my lab has enabled nanospectroscopy experiments on VO_{2} and other materials. 
Condensed Matter Seminar Thursday, November 2, 2023 3:30 PM Clark Hall, Room G004 
"Giant microwave absorption in superconductors"Boris Spivak , University of Wisconsin [Host: Dima Pesin]
ABSTRACT:
I will discuss a mechanism of microwave absorption in conventional and unconventional superconductors which is similar to the Debye absorption mechanism in molecular gases. The contribution of this mechanism to AC conductivity is proportional to the inelastic quasiparticle relaxation time rather than the elastic one and therefore it can be much larger than the conventional one. The Debye contribution to the linear conductivity arises only in the presence of a DC supercurrent in the system and its magnitude depends strongly on the orientation of the microwave field relative to the supercurrent. The Debye contribution to the nonlinear conductivity exists even in the absence of the supercurrent. It provides an anomalously low nonlinear threshold. I will also discuss a closely related problems of resistance of superconductornormal metalsuperconductor junctions, and the resistance of superconductors in the magnetic flux flow regime. 
Condensed Matter Seminar Thursday, October 26, 2023 3:30 PM Clark Hall, Room G004 
"Probe and Control of Coherent Phonons in Multifunctional Materials"Giti Khodaparast , Virginia Tech [Host: Olivia Pfister]
ABSTRACT:
The desire for multifunctional devices has driven significant research toward exploring multiferroics, where the coupling between electric, magnetic, optical, and structural order parameters can provide new functionality. While BiFeO_{3} is a wellstudied multiferroic, recent research has shown that the addition of BaTiO_{3} can improve material properties.^{1} In this talk we focus on coherent phonon (CP) generation in BaTiO_{3}BiFeO_{3} (BTOBFO) layered structure as well as nanorod arrays. ^{ }Usually, CPs are used to provide detailed spectroscopy information and to characterize surfaces and buried interfaces. However, the ability to generate strain via ultrafast optics offers the intriguing possibility of dynamically manipulating the strain with ultrashort optical pulses and opens the possibility of creating a new class of devices, where the strain is manipulated in time to control the properties and operation of a device. In our nanorod arrays, we demonstrated several coherent modes, with possible signatures of the coexistence of CPs and magnons.^{2}^{ }While magnons, in general, are hard to manipulate and control, a strong magnetoelastic interaction between phonons and magnons can be important for a variety of reasons: (i) Coherent Acoustic Phonons generated with ultrafast optical pulses can propagate long distances from the surface, into the sample. With strong magnetoelastic coupling, they can carry the spin information along with them into the sample, perhaps between different regions of a chip. (ii) Strong interactions with phonons can enhance the excitation, manipulation, and detection of the magnons for possible applications in memory devices. In this talk, I will present our observations in several BTOBFO films and nanorod arrays with different interfaces to demonstrate the tunability of CPs and discuss the possibility of the coexistence of CPs and magnons. I will also discuss the possibility of controlling these coherent states using external magnetic fields which have been demonstrated to increase the sensitivity of the CPs’ detection in other systems.^{3}
References: [1] S.C. Yang, A. Kumar, V. Petkov, and S. Priya, J. of Appl. Phys., 113, 144101 (2013). [2] R. R. H. H. Mudiyanselage, B. A. Magill, J. Burton, M. Miller, J. Spencer, K. McMillan, G. A. Khodaparast, H.B. Kang, M.G. Kang, D. Maurya, S. Priya, J. Holleman, S. McGill, and C. J. Stanton, J. Mater. Chem. C, 7, 14212 (2019). [3] B. A. Magill, S. Thapa, J. Holleman, S. McGill, H. Munekata, C. J. Stanton, and G. A. Khodaparast, Phys. Rev. B 102, 045306 (2020).
Acknowledgment: This material is based upon work supported by the Air Force Office of Scientific Research under award number FA95501710341 and DURIP funding (FA95501610358). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR1644779 and the State of Florida. 
Condensed Matter Seminar Thursday, October 19, 2023 3:30 PM Clark Hall, Room G004 Join Zoom Meeting https://virginia.zoom.us/j/99696365426?pwd=SXRuZ29NYzdqOHlQM2MxeUtHOEpVUT09
Meeting ID: 996 9636 5426 Passcode: 303138 
"Ultraslow dynamics, fragile fragmentation, and geometric group theory"Shankar Balasubramanian , MIT  Massachusetts Institute of Technology [Host: Israel Klich ]
ABSTRACT:
A recurring theme in classical and quantum dynamics is finding examples of systems which fail to thermalize. We introduce a class of 1D translationally invariant classical and quantum dynamics that have an unusual approach to equilibrium. For certain examples, expectation values of local operators relax in a time which is exponentially large in system size, implying an unusual kind of hydrodynamics. In other examples, thermalization only occurs when the system is connected to a bath which is at least exponentially large in system size, a phenomenon that we call fragile fragmentation. A field of mathematics called geometric group theory plays an important role in constructing these examples, and we discuss extensions of these results to 2D. 
Condensed Matter Seminar Thursday, October 12, 2023 3:30 PM Clark Hall, Room G004 
"Spin Vortices and Phase Transition in 2D Janus Colloidal Crystal"Myeonggon Park , Brandeis University [Host: Marija Vucelja]
ABSTRACT:
Colloids have provided versatile model systems to investigate rudimentary characteristics of material phases and have been employed as elementary units for spontaneous and directed selfassembly, mimicking atoms. Their size observable in the microscopy is feasible to track individual positions and orientations, so many phenomena including melting and nucleation have been studied at the singleparticle level for better understanding atomic scale dynamics. In the same manner, we used Janus colloids to imitate spins in the observable scale under the microscope. We proved the phase transition of Janus spheres’ spin (orientation) order in Janus colloidal crystal, where the spin interaction is controlled by the external electric field. The spin configuration evolves from a random pattern to vortex and zigzag patterns, as increased the electric field. During this process, we measured the spin arrangement is changed from the shortrange order to the quasilongrange order through the power law decay of spatial correlation functions. Furthermore, we found the density of topological defects, which are vortex and antivortex, is correlated with the spin phase transition that is corroborated with the susceptibility of the spin order parameter. To describe the spin phase transition, we suggested the 2D Heisenberg model. Therefore, we expect that the designed system provides a platform to help understand spin behaviors in the singleparticle level as well as plenty of phenomena induced by orientational interaction in atomic and molecular materials. 
Condensed Matter Seminar Thursday, October 5, 2023 3:30 PM Clark Hall, Room G004 
"Criticality at the quantum Hallsuperconductor interface "Vlad Kurilovich , Yale [Host: Dima Pesin]
ABSTRACT:
Topological superconductors provide a promising route to faulttolerant quantum computing; however, it proved hard to find or engineer them. Recently, topological superconductivity was predicted to arise at the interface between quantum Hall and conventional superconducting states. Since both ingredients are readily available in the lab, topological superconductivity seemed to be within the reach. The predictions, however, focus on the idealized “clean” case, whereas only strongly disordered superconductors are compatible with high magnetic fields needed for the quantum Hall effect. Can topological superconductivity survive the presence of disorder?
We develop a theory of two counterpropagating quantum Hall edge states coupled via a narrow disordered superconductor. We show that, in contrast to the cleancase predictions, the edge states do not turn into a topological superconductor. Instead, the disorder tunes them to the critical point between the trivial insulating phase and the topological phase. We determine the manifestations of this criticality in the charge transport, finding that the critical conductance is a random, samplespecific quantity with a zero average and unusual bias dependence. The developed theory of disordered superconductorquantum Hall interfaces offers an interpretation of recent experiments. 
Condensed Matter Seminar Thursday, September 28, 2023 3:30 PM Clark Hall, Room G004 
"Resonance from Antiferromagnetic Spin Fluctuations in SpinTriplet Superconductor UTe2"Chunruo Duan [Host: Despina Louca]
ABSTRACT:
UTe2 has drawn a lot of attention among the superconductivity (SC) community recently for the promising secondary evidence of spintriplet Cooper pairing shown in this system through temperatureindependent NMR Knight shift, the highly anisotropic and unusually large upper critical field, powerlaw behavior of the specific heat, and possible ferromagnetic spin fluctuation hinted by the magnetic susceptibility. Under an extremely large magnetic field (~30 T above Hc2 in the bcplane) the system shows reentrance of the superconducting phase. Our previous experiments at CNCS on a coaligned UTe2 sample in its 0KL and HK0 plane showed two types of rodlike excitations at Brillouin zone boundary, and one of them develops a spin gap and a spin resonance upon entering the SC state. SEM study shows development of charge density wave (CDW) in the (011) cleaved edge at the Brillouin zone corner, which is similar to the other excitation we observed with neutron scattering. Because the resonance has only been found in spinsinglet unconventional superconductors near an AF instability, its observation in UTe2 suggests that AF spin fluctuations may also induce spintriplet pairing24 or that electron pairing in UTe2 has a spinsinglet component. 
Condensed Matter Seminar Friday, July 7, 2023 4:00 PM Ridley, Room 127 
"Hydrogenatom Spin Liquid Mechanism of Hot Superconductivity in Metal Hydrides"Prof. G. Baskaran , MatScience Institute, IIT Madras and Perimeter Institute, Toronto [Host: Shivaram]
ABSTRACT:
Conventional BCS mechanism of HTSC in hydrides, a la' Ashcroft works remarkably well in H_3S, LaH_{10} etc. We pointed out (GB 2015) [1] that Hatom networks in hydrides is a Mott insulating subsystem, which supports RVB superconductivity via internal doping. We will present a case for occurrence of both BCS and RVB pairing to varying degrees, in different hydrides. It resolves several existing puzzles. In our theory, HCS and LuNH systems have dominant RVB pairing, which favors competing orders create, encouraged by structural variations and disorder, wide variation in Tc (like in cuprates). On the other hand, H_3S and LaH_{10} is a dominant phonon mediated BCS system; consequently Tc is protected by Anderson theorem and varies less. We provide support for our electronic mechanism from the works of Ng et al. (1996) and Eder et al., (1997) on emergence of Kondo insulating states in LaH_3 and YH_3, via Kondo coupling of band electrons to localized H atom spins. 
Condensed Matter Seminar Thursday, May 25, 2023 11:00 AM Ridley Hall, Room 123 
ABSTRACT:
Superconducting qubits have emerged as the prominent building block of a scalable quantum computer. While inventing novel qubit designs has enabled the progress towards multiqubit systems that have demonstrated quantum advantage, there much more to be gained by properly engineering the qubit materials. Coherence time of the qubit, the relevant figure of merit, is limited by materials losses. In particular, the main limiting factor is the presence of “parasitic” twolevel systems (TLS) that manifest in increased losses of the superconducting circuits at millikelvin temperatures and singlephoton powers. Amorphous oxides, residues and transition layers at interfaces and grain boundaries are identified as sites harboring such TLS states. In this talk, I will present techniques to carefully address each interface, superconducting films, and substrate, and ultimately reduce the losses in the superconducting circuit in the quantum regime (millikelvin temperatures, singlephoton occupancy). This will ultimately allow to push the envelope regarding coherence times and allow for quantum processors with larger number of qubits and larger quantum volume.

Condensed Matter Seminar Friday, May 12, 2023 2:00 PM Thorton, Room C311 
ABSTRACT:
What is the connection between Aristotle and an ice cream? The answer is encoded in the Mpemba effect, the counterintuitive and controversial phenomenon that hot water cools faster than cold one. Here I will provide an analog quantum definition of this phenomenon, which has been studied in a quantum quench of a spin chain whose initially broken global U(1) symmetry is restored dynamically. Studying this evolution, we find that the more the symmetry is initially broken, the faster it is restored. We dub the measure detecting this effect entanglement asymmetry, which is a measure of symmetry breaking inspired by the theory of entanglement in manybody states. Although symmetry breaking is a pillar of modern quantum physics, quantifying how much a symmetry is broken is an issue that has received little attention so far: The merit of the entanglement asymmetry is to provide a quantitative tool for this. 
Condensed Matter Seminar Thursday, April 27, 2023 4:00 PM Ridley Hall, Room 177 
ABSTRACT:
Realization of artificial spinorbit (SO) coupling in the cold atom systems has opened a broad spectrum of phenomena to be observed experimentally in bosonic systems. The physics of spontaneous symmetry breaking leads to the several exotic quantum phases such as superfluid and more recently the supersolid. In the latter case, both diagonal and offdiagonal longrange order, although being mutually exclusive, are present and it breaks two U(1) symmetries. One of the ways to realize this supersolid phase is using the condensates of Alkali atoms in a double well optical lattice. Such SO coupled systems have interesting phase diagrams depending on the interaction and SO coupling strength. In presence of weak coupling and negligible interspecies interaction the system remains in the phase known as striped phase or miscible phase. In this case, the Raman beam couples two condensates and provides a momentum kick along a perpendicular direction, which indeed leads to a onedimensional density modulation due to interference. Two continuous symmetries are broken by the superfluid phase and the breaking of translation symmetry along one direction, satisfying the definition of a supersolid. Spinor BECs and their relative phase coherence has been a topic of interest for quite some time. The dephasing of the relative phase has important implications in the density modulation and its experimental observation. I will consider a specific realization of a pseudospin1/2 BEC, in which the two spin components are represented by spatially separated lowlying states in a doublewell potential. I will demonstrate that even if the system is initialized with a given relative phase between the two components, there is an apparent dephasing of the relative phase. The dephasing is a consequence of a very weak interaction between the spin species. Furthermore, I will demonstrate that the dephasing rate is a nonmonotonic function of the Raman coupling strength. The minimum of the dephasing rate corresponds to the critical value of the Raman coupling, above which miscible to immiscible phase transition occurs. 
Condensed Matter Seminar Tuesday, April 25, 2023 12:00 PM Rotunda, Room 102 
"Taming energy dissipation in driven topological systems"Iliya Esin , Caltech [Host: Israel Klich ]
ABSTRACT:
In this talk, I will discuss energy dissipation and heating in slowly driven quantum systems, focusing on topological driving schemes. In the first part of my talk, I will present a system in which manybody dynamics leads to the emergence of a quasisteady state with a high entropy density and yet robust topological transport. I will explain the mechanisms behind this phenomenon and demonstrate the emergence of the quasisteady state on an exactly solvable strongly coupled fermionic model. In the second part of my talk, I will show that the dissipation of energy in nearly adiabatic quantum systems is linked to the quantum geometry of the problem. Interestingly, this result implies a topological bound on the energy dissipation rate in a class of topological systems. Our findings uncover new connections between topology and dissipation in slowly driven quantum systems, shedding light on their fundamental properties and potential for practical applications, such as the development of optimized driving protocols for topological drives. 
Condensed Matter Seminar Thursday, April 20, 2023 4:00 PM Ridley Hall, Room 177 
"Hydrodynamics and rheology of fluctuating, semiflexible, inextensible, and slender filaments in Stokes flow"Aleksander Donev , New York University [Host: Marija Vucelja]
ABSTRACT:
Every animal cell is filled with a cytoskeleton, a dynamic gel made of inextensible filaments / biopolymers, such as microtubules, actin filaments, and intermediate filaments, all suspended in a viscous fluid. Similar suspensions of elastic filaments or polymers are widely used in materials processing. Numerical simulation of such gels is challenging because the filament aspect ratios are very large. 
Condensed Matter Seminar Thursday, April 13, 2023 4:00 PM Ridley Hall, Room 177 
"Structural Dynamics and Optoelectronic Properties of twoDimensional Hybrid OrganicInorganic Perovskite"Haritha Sindhu Rajeev , UVA [Host: Seunghun Lee]
ABSTRACT:
Twodimensional hybrid organicinorganic perovskites (HOIPs) have shown promising progress in lightemitting diodes applications. The threedimensional (3D) HOIPs, commonly used as novel solar cells, exhibit extended charge carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection from defects^{ [1]}. It has been experimentally shown that the reorientation of the polarized organic molecules can facilitate the polaron formation, where polaron is referred as a quasiparticle formed by the Coulomb interaction between an excess charge (an electron or a hole) and the ionic lattice, enhance the screening effects on charge carriers, and thus prolong the charge carrier lifetime^{ [23]}. Whereas the pure inorganic 3D perovskites without organic molecules, such as CsPbI_{3}, also show a moderate photovoltaic performance, which brings researcher’s efforts into investigations on dynamics of the inorganic perovskite framework^{ [4]}. An understanding of the microscopical mechanisms behind extended charge carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection in HOIPs is lacking. We performed timeofflight neutron spectroscopy for two perovskites, butylammonium lead iodide (BA)_{2}PbI_{4} (BA) and phenethylammonium lead iodide (PEA)_{2}PbI_{4} (PEA). From the obtained spectra we identified and quantitatively separated the rotational and phonon contribution for BA. Similar analysis would be performed for the PEA spectra in future. We try to understand how both inorganic vibrational dynamics and organic molecule rotational dynamics contribute to charge carrier lifetime and hence power conversion efficiency of solar cells. The study is important to get an idea on how to engineer new HOIPs by exploiting these dynamics for higher device performance. By examining the corresponding temperature dependence, we revealed that the rotational dynamics of organic molecules in these materials tends to suppress their photoluminescence quantum yield^{ [5]} while the vibrational dynamics did not show predominant correlations with their optoelectronic properties.
[1] Mei, Anyi, et al. science 345.6194 (2014): 295298. [2] Miyata, Kiyoshi, Timothy L. Atallah, and XY. Zhu. Science Advances 3.10 (2017): e1701469. [3] Chen, Tianran, et al. Proceedings of the National Academy of Sciences 114.29 (2017): 7519 7524. [4] Wang, Kang, et al. Nature communications 9.1 (2018): 4544. [5] Gong, Xiwen, et al. Nature materials 17.6 (2018): 550556.

Condensed Matter Seminar Wednesday, April 12, 2023 3:00 PM Physics, Room 120 
"Anomalous thermal relaxation in Langevin dynamics and reaction networks "Matthew Walker , UVA [Host: Marija Vucelja]
ABSTRACT:
In recent years anomalous cooling and heating effects in the far from equilibrium limit have gained attention. One anomaly is the so called Mpemba Effect, in which the time to relax towards thermal equilibrium does not grow monotonically as a function of distance to the target. Instead, it has been proposed that there exist shortcuts in the relaxation process that allow both faster, and even exponentially faster heating and cooling. In this talk I will discuss recent works [1,2] that have progressed our understanding of such shortcuts by studying the Mpemba effect using Overdamped Langevin dynamics. I will show when and where you can get the effect, and that our models are in good agreement with experimental findings. Lastly, I will touch upon current works where we study the effect using Markovian jump processes on linear reaction networks.
Matthew R Walker and Marija Vucelja J. Stat. Mech. (2021) 113105
Matthew R Walker and Marija Vucelja arXiv preprint arXiv:2212.07496 (2022)

Condensed Matter Seminar Thursday, April 6, 2023 4:00 PM Ridley, Room 177 
"Scanning tunneling spectroscopy of unconventional superconductors"Pavlo Sukhachov , Yale [Host: Dmytro Pesin]
ABSTRACT:
Motivated by recent experimental observations of unconventional superconductivity in twisted bilayer and trilayer graphenes, we develop a theory describing the differential conductance between a normal STM tip and a 2D superconductor with an arbitrary gap structure. Our analytical scattering theory accounts for Andreev reflections, which become prominent at larger transmission between the tip and the superconductor. Exploiting the dependence of Andreev reflection on the relative position of the STM tip with respect to the lattice symmetry points, we show that the structure of the superconducting gap can be extracted by combining weak and strongtunneling limits of differential conductance. Furthermore, the theory incorporates a tip/impurityinduced scattering potential within the 2D material, which allows us to describe subgap resonances. 
Condensed Matter Seminar Thursday, March 30, 2023 4:00 PM Ridley Hall, Room 177 
"Altermagnetism: a third type of ordered collinear magnetism"Igor Mazin , George Mason University [Host: Dima Pesin]
ABSTRACT:
Since many years, the canonincal classification of ordered magnets included noncollinear (with many further subdivisions) and two collinear types: antiferromagnets (AF), which have net magnetization zero by symmetry, and ferro/ferrimagnets (FM), which do not have this property. The two have distinctly different micro and macroscopic properties. It was supposed, for instance, that only FM can exhibit spinsplitting of the electronic bands in absence of spinorbit coupling AND lack of inversion symmetry, have anomalous Hall effect (i.e., Hall effect driven by variation of the Berry phase), magnetooptical effects, suppressed Andreev scattering in contact with a singlet superconductor etc. A surprisingly recent development (~2019) is that this classification is incomplete: there are collinear magnets that would belong to AF by this classification, but show all characteristics of FM, *except the net spin polarization*! They were recently dubbed by Mainz group "altermagnets", AM. Incidentally, what has also not been fully appreciated was that there are also materials that have strictly zero net magnetization, but enforced not by symmetry, but by the Luttinger's theorem, and therefore truly belonging to the FM class ("Luttingercompensated ferrimagnets"). In this talk I will present the new classification and explain, in specific examples, what are the symmetry conditions for AM, why these are a truly new class deserving a new name, and how their unusual properties appear. 
Condensed Matter Seminar Monday, March 27, 2023 3:30 PM Physics Building, Room 313 
"The Mpemba effect for phase transitions by Landau theory"Roi Holtzmann , Weizmann Institute of Science [Host: Marija Vucelja]
ABSTRACT:
The Mpemba effect describes the situation in which a hot system cools faster than an identical copy that is initiated at a colder temperature. In many of the experimental observations of the effect, e.g. in water and clathrate hydrates, it is defined by the phase transition timing. However, none of the theoretical investigations so far considered the timing of the phase transition, and most of the abstract models used to explore the Mpemba effect do not have a phase transition. In this talk, I will suggest a definition for the phase transition time in a nonequilibrium state using the Landau theory for phase transitions. Using this definition, I will show that a Mpemba effect with respect to phase transitions can exist in such models, namely that the hotter system undergoes the transition before the colder one when quenched to a cold temperature. 
Condensed Matter Seminar Thursday, March 23, 2023 4:00 PM Ridley Hall, Room 177 
ABSTRACT:
Simulation of quantum manybody physics, such as looking for ground state properties and real time dynamics, plays an important role in the study of condensed matter physics, high energy physics and quantum information science. The recent advancement of machine learning provides new opportunities for tackling challenges in simulating quantum manybody physics. In this talk, I will first discuss a class of wave functions via neural network transformation, neural network backflow, which can fulfill the antisymmetry property and capture the correlation and the sign structure for stronglyinteracting fermionic physics. Next, I will talk about recent progress of simulating continuum quantum field theories with neural quantum field state [2], and lattice gauge theories such as 2+1D quantum electrodynamics with finite density dynamical fermions using gauge symmetric neural networks [3,4]. Finally, I will present a neural network representation based on positivevalueoperator measurements for quantum circuit and open quantum system dynamics simulation [5]. 
Condensed Matter Seminar Monday, March 20, 2023 4:00 PM Physics Building, Room 313 
"NonLoudonFleury Raman scattering in spinorbit coupled Mott insulators "Yang Yang , University of Minnesota [Host: GiaWei Chern]
ABSTRACT:
The LoudonFleury form of the Raman operator has been used to compute magnetic Raman responses inside magnetic insulators for decades. The formalism provided by Loudon and Fleury [1] only considers the lightinduced direct hopping between two magnetic ions. However, this is an oversimplified scenario for spinorbit coupled Mott insulators. For example, the microscopic origin of the superexchange interaction inside Kitaev materials shows multiple indirect superexchange paths involving ligand ions can also produce a significant anisotropic interaction [2]. Therefore, to correctly construct the Raman operator for spinorbit coupled Mott insulators requires considering all the nonnegligible direct and indirect superexchange paths.
In this talk, I will present our work on constructing the Raman operator for the spinorbit coupled Mott insulators which involve multiple superexchange paths [3], and I will also show how our revised theory can be applied to the threedimensional hyperhoneycomb Kitaev material β−Li2IrO3 [4], where we show a qualitative modification of the polarization dependence, including, e.g., the emergence of a sharp onemagnon peak at low energies, which is not expected in the traditional LoudonFleury theory.
[1] P. A. Fleury and R. Loudon, Phys. Rev. 166, 514 (1968). [2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009). [3] Yang Yang, Mengqun Li, Ioannis Rousochatzakis, and Natalia B. Perkins Phys. Rev. B 104, 144412 (2021). [4] Yang Yang, Yiping Wang, Ioannis Rousochatzakis, Alejandro Ruiz, James G. Analytis, Kenneth S. Burch, and Natalia B. Perkins Phys. Rev. B 105, L241101(2022). 
Condensed Matter Seminar Thursday, March 2, 2023 4:00 PM Chemistry, Room 402 
"Stripe discommensuration and spin dynamics in holedoped 214nickelates Pr2xSrxNiO4+delta"Aveshek Maity , Research Neutron Source Heinz MaierLeibnitz (FRM II), Technical University of Munich, Garching, Germany [Host: Despina Louca]
ABSTRACT:
Magnetic excitations in the spinstripe phases of Labased 214nickelates have been vigorously explored using inelastic neutron scattering (INS) study for almost last three decades and still have remained an exciting research field, especially to understand their differences yet of their structural similarities with highTc 214cuprates. In view of so far reported twodimensional antiferromagnetic nature, outofplane magnetic excitations are generally not expected in 214nickelates. In this talk, I will present our recent results from INS measurements on the stripe discommensurated phases of Srdoped Pr_{3/2}Sr_{1/2}NiO_{4 }samples, showing a compelling evidence for the presence of a sizable outofplane interaction suggesting a threedimensional nature of magnetic excitations near the halfdoped region [1,2]. The measured magnetic excitations are in good agreement with our linear spin wave (LSW) theory based calculations in the discommensurated spin stripe models. Additionally, I will discuss the effect of shortrange vs. longrange spin stripe correlations on the spin wave dispersion by comparing the results with our INS study on an Odoped sample Pr_{2}NiO_{4+}_{δ} [3]. 
Condensed Matter Seminar Thursday, February 23, 2023 4:00 PM via Zoom, Room TBA Join Zoom Meeting https://virginia.zoom.us/j/91523887660?pwd=TE5NSEhzZy9RTVN4Nmg0V0VvVFM1Zz09
Meeting ID: 915 2388 7660 Passcode: 845220 
"Xray and Neutron Scattering on Square Lattice Antiferromagnets: 214nickelates and melilite Ba2CoGe2O7"Rajesh Dutta , Institut fur Kristallographie, RWTH Aachen University [Host: Despina Louca]
ABSTRACT:
Neutron scattering and its complementary Xray scattering techniques help to probe many novel and [1] A. Maity, R. Dutta, and W. Paulus, Stripe discommensuration and spin dynamics of halfdoped Pr3/2Sr1/2NiO4, Phys. Rev. 
Condensed Matter Seminar Monday, February 20, 2023 3:30 PM , Room Zoom Join Zoom Meeting https://virginia.zoom.us/j/97901615040?pwd=YTh4WEs0RmRCVHRKUjQwTmpDUzQ4QT09
Meeting ID: 979 0161 5040 Passcode: 920090 
"Theoretical study on the spin dynamics induced by a magnetic field in a onedimensional chiral magnet"Kotaro Shimizu , University of Tokyo [Host: GiaWei Chern]
ABSTRACT:
Swirling spin textures, such as helices and vortices, appear in chiral magnets, and these noncolinear and noncoplaner spin textures bring about characteristic resonance structures and quantum transports. Recently, a onedimensional chiral magnet hosting a conical spin texture and a chiral soliton lattice (CSL) have attracted a lot of interest since resonance frequencies as well as a magnetic modulation period can be flexibly controlled by an external magnetic field [1,2]. However, the dynamical properties of these spin textures associated with the oscillating magnetic field and the resultant emergent electromagnetic phenomena have not been systematically clarified.
In this study, we theoretically study the resonance modes and the emergent electromagnetic phenomena in a onedimensional chiral magnet by numerically solving the LandauLifshitzGilbert equation (LLG eq.) and by using the linear spin wave theory. We systematically clarified the magnon band structure by varying the external magnetic field and find that the band gaps increase with the magnetic field perpendicular to the chiral axis. We also find the edge modes appear within the band gap and clarify that the swirling spin textures penetrate into the system from the edges by activating the edge modes. In the talk, we will also discuss the enhancement of the emergent electric phenomena and AC magnetic field drive of swirling spin textures. 
Condensed Matter Seminar Thursday, February 16, 2023 4:00 PM Claude Moore Nursing, Room G120 [1] J. Kishine and A. S. Ovchinnikov, Phys. Rev. B 79, 220405(R) (2009). 
"UltraLow Energy Manipulation of Spin in Nanostructures"Weigang Wang , University of Arizona [Host: D. Louca]
ABSTRACT:
Information technology backed by sophisticated semiconductor devices have deeply changed our society in the past two decades, with AIdriven tools such as ChatGPT posited to bring even deeper impact. However, in the physics governing devices behind most of these applications, we have only utilized the charge carried by electrons, while ignoring the other inherent quantum property, the spin. My research focuses on the understanding of the spin degree of freedom of electrons at nanoscales. First I will give an introduction on a few key phenomena in the field of spintronics, such as the coherent tunneling of spins by controlling the symmetry of the wavefunctions, and the exchange scattering effect in materials with compensated magnetization where the spins can be manipulated in the picosecond time scale. Then I will present in detail one of our research directions in which we attempt to control the order parameter of magnetic systems by using electric fields, instead of magnetic fields or spinpolarized currents. Through the voltage controlled magnetic anisotropy effect where the energy of the system can be modified by the redistribution of wavefunctions induced by external electric potentials, a 100fold reduction in switching current density has been realized. We have demonstrated that both the magnetic anisotropy and saturation magnetization of a metallic ferromagnet can be controlled by voltage, leading to a new method to modify the interlayer exchange coupling of the system, directly verified by insitu Xray magnetic circular dichroism experiment. In addition to the ferromagnetic order, the antiferromagnetic order can also be effectively manipulated by electric fields. Finally, I will describe our recent effort to reduce the switching energy of magnetic tunnel junctions. By controlling the spinorbit interaction of the system using a remote doping technique, we have achieved a recordlow switching energy of ~3 fJ using subns voltage pulses. 
Condensed Matter Seminar Monday, February 13, 2023 3:30 PM Monroe Hall, Room 110 
"Order by disorder in classical kagome antiferromagnets with chiral interactions"Kiril Shtengel , University of California, Riverside [Host: Israel Klich ]
ABSTRACT:
The Heisenberg antiferromagnet on the kagome lattice is an archetypal example of how large ground state degeneracies arise, and how they may get resolved by thermal and quantum fluctuations. Augmenting the Heisenberg model by chiral spin interactions has proved to be of particular interest in the discovery of chiral quantum spin liquids. I will focus on the classical variant of this chiral kagome model, which exhibits, similar to the classical Heisenberg antiferromagnet, a remarkably large and structured groundstate manifold combining continuous and discrete degrees of freedom. This allows for a rich set of orderbydisorder phenomena. Degeneracy lifting by thermal and quantum fluctuations occurs in a highly selective way that, among other interesting effects, provides a semiclassical route to an emergent Z_2 spin liquid. 
Condensed Matter Seminar Thursday, December 1, 2022 3:30 PM Monroe Hall, Room 124 
ABSTRACT:
Height models and random tiling are wellstudied objects in classical statistical mechanics and combinatorics that lead to many interesting phenomena, such as arctic curve, limit shape and KadarParisiZhang scaling. We introduce quantum dynamics to the classical hexagonal dimer, and sixvertex model to construct frustrationfree Hamiltonians with unique ground state being a superposition of tiling configurations subject to a particular boundary configuration. An internal degree of freedom of color is further introduced to generate long range entanglement that makes area law violation of entanglement entropy possible. The scaling of entanglement entropy between half systems is analysed with the surface tension theory of random surfaces and under a qdeformation that weighs random surfaces in the ground state superposition by the volume below, it undergoes a phase transition from area law to volume scaling. At the critical point, the scaling is L logL due to the socalled "entropic repulsion” of Gaussian free fields conditioned to be positive. An exact holographic tensor network description of the ground state is give with one extra dimension perpendicular to the lattice. We also discuss an alternative realisation with sixvertex model, inhomogeneous deformation to obtain subvolume intermediate scaling, and possible generalisations to higher dimension. 
Condensed Matter Seminar Thursday, November 10, 2022 3:30 PM Monroe Hall, Room 124 
"Local structure optimization in disordered layered BiCh2based superconductors"Yoshi Mizuguchi , Tokyo Metropolitan University [Host: Despina Louca]
ABSTRACT:
Since the discovery of cuprate superconductors, layered compounds have been one of important target systems for developing new exotic superconductors and studying unconventional mechanisms. In this seminar, I will talk about material development of layered BiCh_{2}based (Ch: S, Se) superconductors, which were first discovered in 2012 [1,2]. In the BiCh_{2}based compounds, intrinsic local structural disorder (local distortion) has been observed by various experimental probes, and the suppression of local disorder is essential for the emergence of bulk superconductivity in the system [3]. After obtaining bulk superconductors with less local disorder, we could see some interesting superconducting properties in BiCh_{2}based superconductors: absence of isotope effect [4], nematic superconductivity [5], and huge upper critical fields [6] in La(O,F)Bi(S,Se)_{2} will be briefly explained.
[1] Y. Mizuguchi et al., PRB 86, 220510 (2012) [2] Y. Mizuguchi et al., JPSJ 81, 114725 (2012) [3] (review article) Y. Mizuguchi, JPSJ 88, 041001 (2019) [4] K. Hoshi, Y. Goto, Y. Mizuguchi, PRB 97, 094509 (2018) [5] K. Hoshi et al., JPSJ 88, 033704 (2019) [6] K. Hoshi et al., Sci. Rep. 12, 288 (2022) 
Condensed Matter Seminar Monday, October 17, 2022 4:00 PM Chemistry Building, Room 206 
"Transport on an interacting helical edge with resonant impurities"Youjian Chen , University of Virginia  Department of Physics [Host: Prof. Dima Pesin]
ABSTRACT:
The quantum spin Hall insulator, also known as a twodimensional (2D) topological insulator, is a topological state of matter supporting the helical edge states, which are counterpropagating, spinmomentum locked 1D modes protected by time reversal symmetry. It exhibits special magnetotransport properties under external magnetic field. In my talk, I will construct modified Anderson impurity models to study the magnetoconductance of the quantum spin hall insulator. Firstly, I will solve the transmission through single impurity on helical Luttinger liquid in the presence of magnetic field using LippmannSchwinger equation. I will show the analytical expression for trans mission and reflection coefficient in terms of the difference between energy of particle and the impurity level, the hybridization coefficient and the magnetic field. Then, I will show the effect of Coulomb interaction on helical Luttinger liquid at HartreeFock level. Using renormalization group, I will derive the temperature dependence of conductance. Lastly, I will show the coherent transport of transmission through many impurities with different energy and hybridization coefficient in the presence of magnetic field. I will compare my theoretical result with experiment. 
Condensed Matter Seminar Thursday, April 21, 2022 3:30 PM Clark Hall, Room G004 
"Probing correlations in fermionic triangular Hubbard systems"Jirayu Mongkolkiattichai , University of Virginia  Department of Physics [Host: Prof. Peter Schauss]
ABSTRACT:
Quantum gas microscopes have expanded the understanding of manyparticle physics with their unique ability of single atom resolved imaging. Quantum gas microscopes provide microscopic information of quantum manybody states through spatial correlation functions. Relying on the unique tunability of ultracold atoms in atomic interactions via Feshbach resonances, density, and spinimbalance, we study a wide parameter range in the phase diagram. Interestingly, a triangular lattice is the simplest example of geometric frustration because three spins with antiferromagnetic interactions cannot be antiparallel, leading to large degeneracies in the manybody ground state [1]. In this talk, I present a Mott insulator of lithium6 on a symmetric triangular lattice with a lattice spacing of 1003 nm. The lattice is imaged via a Raman sideband cooling technique with imaging fidelity of 98% [2]. We calibrated tunneling by extracting lattice depth from band excitation and the interaction is determined using doublon formation. We can access singlespecies singles components with the use of doublon hiding [3] and spin removal techniques [4] to detect spinspin correlations. We compare the results to Determinantal Quantum Monte Carlo calculations, plan to investigate 120° Neel ordering in Heisenberg antiferromagnets, and search for quantum spin liquids in the triangular lattice Hubbard system. [1] L. Balents, Nature 464, 7286 (2010). [2] J. Yang, et al., PRX Quantum 2, 020344 (2021). [3] P. T. Brown, et al., Science 357, 6358 (2017). [4] M. F. Parsons, et al., Science 353, 1253 (2016). 
Condensed Matter Seminar Thursday, April 14, 2022 4:00 PM Clark Hall, Room G004 
"Driven Majorana Zero Modes: A Route to Synthetic px+ipy Superconductivity"Lingyu Yang , University of Virginia  Department of Physics [Host: Prof. GiaWei Chern]
ABSTRACT:
In the Kitaev's toy model with a constant chemical potential, the Majorana zero modes (MZMs) can exist but stay localized at the edges of a 1D spinless chain. In this talk, I will introduce the Kitaev's toy model with a sitedependent chemical potential. In this case, one is able to create segments of topological and normal superconducting phases. The MZMs exist at the domain walls between the two phases, but not necessarily at the edges of the whole chain. By tuning the chemical potential such that the domain walls can change in space, the MZMs are able to move in space as well. We call this motion of MZMs the Majorana pump and argue that it leads to px+ipy superconductivity. I will present how the py pairing emerges, and how to realize this model in experiments. 
Condensed Matter Seminar Thursday, April 7, 2022 2:30 PM Clark Hall, Room G004 Join Zoom Meeting:https://virginia.zoom.us/j/94358162934?pwd=aVZ3UWp2UzltUnlWREV1azdjcUFqUT09Meeting ID: 943 5816 2934 Passcode: 414834 
"Seas of Spin liquids: a glimpse from Kitaev Model"Professor G. Baskaran , The Institute of Mathematical Sciences and IIT Madras, India; Perimeter Institute for Theoretical Physics, Waterloo, Canada [Host: Prof. Bellave Shivaram]
ABSTRACT:
P.W. Anderson envisaged a novel situation of quantum paramagnetic (quantum spin liquid) phase of low spin Mott insulators, back in 1973 and described it using Pauling's resonating valence bonds states. RVB idea got a resurgence, with the discovery of high Tc superconductivity by Bednorz and Muller in 1986. Low dimensionality and frustrations enhance quantum fluctuations in low spin systems, resulting in a variety of spin liquids and many ideas  emergent gauge fields, Majorana Fermi sea, to topological phases etc. I will discuss a delightful and exactly solvable model by Kitaev, which realized dreams of RVB theorists and more, in an exact fashion. This fertile model is experimentally realized now, thanks to Khaliullin and Jackeli's prediction. There are continuing surprises, including very recent discovery of anomalous nonlinear susceptibility by Shivaram and collaborators. 
Condensed Matter Seminar Thursday, February 24, 2022 3:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/91531198174Meeting ID: 915 3119 8174 Passcode: 486564 
" New Direct Electron Imaging Techniques for Quantum Materials"Dr. Kayla Nguyen , University of Illinois UrbanaChampaign [Host: Utpal Chatterjee]
ABSTRACT:
Electron microscopy is transforming the physical sciences. Aided by a new generation of direct imaging detectors, cryoelectron microscopy won the 2017 Nobel Prize in Chemistry for advancements in visualization of biomolecules. To go beyond traditional electron microscopy, new detectors must also be developed for the diffraction imaging; here, the scattered electron beam encodes a wealth of information about the structure, chemistry, electrical, optical, and magnetic properties of matter. During my PhD, I coinvented the electron microscopy pixel array detector (EMPAD), a fast, highly efficient detector designed to capture the full scattered electron information. The EMPAD has been licensed to Thermo Fisher Scientific and sold around the world. In my talk, I will highlight how the EMPAD enables new characterization techniques for imaging topological magnetic and ferroelectric structures. These approaches can be used to uncover polarization fields, orbital angular momentum and chirality of polar and magnetic textures. By developing new characterization methods in combination with theoretical predictions, new physics in emerging quantum materials can be revealed with electron microscopy at atomic resolution.

Condensed Matter Seminar Thursday, December 16, 2021 3:30 PM Online, Room via Zoom Join via Zoom: https://virginia.zoom.us/j/97711165035?pwd=aHc3VHYxNkZUL29oaEZHMExjMUJ1UT09 
"Tlinear resistivity down to Tâ0 at the pseudogap critical point of holeddoped cuprates and Planckian dissipation"Dr. Anaelle Legros , John Hopkins University [Host: Utpal Chatterjee]
ABSTRACT:
: The perfectly linear temperature dependence of the resistivity observed as T→0 in a variety of metals close to a quantum critical point is a major puzzle of condensed matter physics. In cuprates, this phenomenon is observed in the vicinity of the pseudogap critical point p*. Using high magnetic fields to suppress superconductivity, one can access the normal state properties down to T→0 close to this critical point. I will present highfield magnetotransport measurements of two holedoped cuprates, near their respective p*, supporting that Tlinear resistivity as T→0 is a generic property of cuprates, associated with a universal scattering rate. We measured the lowT resistivity of Bi2Sr2CaCu2O8+δ just above p* [1] and found that it exhibits a Tlinear dependence, quantitatively similar to other very different cuprates. We also observed, using the Drude formula, that in various cuprates showing this lowT phenomenon the slope of this Tlinear resistivity is given by a universal relation implying a specific scattering rate for charge carriers: 1/�� = αh/2πk_{B}T (corresponding to what is called the Planckian limit [2]), where h is Planck’s constant, k_{B} is the Boltzmann constant and α a constant of order unity. Finally, we directly measured the scattering rate in La_{1.6}_{−}_{x}Nd_{0.4}Sr_{x}CuO_{4}, just above p* and in the lowT limit, using angledependent magnetoresistance measurements [4]: these experiments reveal an inelastic scattering rate which is isotropic and linear in temperature, and whose magnitude is consistent with Planckian dissipation. 
Condensed Matter Seminar Thursday, December 9, 2021 3:30 PM Online, Room via Zoom Join Zoom Meetinghttps://virginia.zoom.us/j/96936622285Meeting ID: 969 3662 2285 Passcode: 792554 
"Crossover functions for entanglement entropy in manybody energy eigenstates and universality"Thomas Barthel , Duke University [Host: Israel Klich ]
ABSTRACT:
We consider the entanglement entropies of energy eigenstates in quantum manybody systems. For the typical models that allow for a fieldtheoretical description of the longrange physics, we find that the entanglement entropy of (almost) all eigenstates is described by a single crossover function. The eigenstate thermalization hypothesis (ETH) implies that such crossover functions can be deduced from subsystem entropies of thermal ensembles and that they assume universal scaling forms in quantumcritical regimes. They describe the full crossover from the groundstate entanglement scaling for low energies and small subsystem size (area or logarea law) to the extensive volumelaw regime for high energies or large subsystem size. For critical 1d systems, the scaling function follows from conformal field theory (CFT). We use it to also deduce the scaling function for Fermi liquids in d>1 dimensions. These analytical results are complemented by numerics for large noninteracting systems of fermions in d=1,2,3 and the harmonic lattice model (free scalar field theory) in d=1,2. Lastly, we demonstrate ETH for entanglement entropies and the validity of the scaling arguments in integrable and nonintegrable interacting spin chains. References: PRL 127, 040603 (2021); PRA 104, 022414 (2021); arXiv:2010.07265. 
Condensed Matter Seminar Thursday, December 2, 2021 3:30 PM Physics Building, Room 204 
"Quantum information processing based on spins in semiconductor quantum dots"Dr. Yinyu Liu , Harvard University [Host: Utpal Chatterjee]
ABSTRACT:
The field of Quantum Information is of great excitement in both fundamental physics and industry. One promising platform for quantum computing is gate defined quantum dot in semiconductors. The greatest limiting factor currently is that delicate quantum states can lose their quantum nature due to interactions with their environment. Other open challenges are to develop methods to entangle quantum bits that are separated by significant distances and can be measured quickly with high fidelity. Siliconbased materials are promising due to the long lifetimes of electrons’ quantum states, but also challenging due to the difficulty in fabrication and valley degeneracy. I will report a singlettriplet qubit with a qubit gate that is assisted by the valley states. This work would potentially relax the design and fabrication requirement for scaling. Moreover, this research field has achieved strong coupling between electron spins and photons in hybrid circuitQED architecture. Quantum optics, long distance quantum entanglement and communication via photons are promised. To address that, I will present my project on indium arsenate (InAs) double quantum dots (DQD) that are embedded in circuitQED architecture. We demonstrated the direct evidence of photon emission from a DQD in the microwave regime and further achieved stimulated emission in a similar system. By achieving stimulated emission from one DQD in these works, we invented a semiconductor single atom maser that can be tuned in situ. I will demonstrate that a semiconductor based quantum dot is a promising platform for quantum information as well as for fundamental physics. 
Condensed Matter Seminar Thursday, November 18, 2021 3:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/98992639357Meeting ID: 989 9263 9357 Passcode: 575442 
"Quantum spin Hall effect in monolayer WTe2"Wenjin Zhao , Cornell University [Host: Prof. Dima Pesin]
ABSTRACT:
WTe_{2} is an example of a twodimensional semimetal. It shows incredibly diverse and intriguing behavior such as the quantum spin Hall effect (QSH), superconductivity, ferroelectricity, and excitonic insulator, providing a new platform for studying the interplay between topology and correlations. In this talk I will discuss the helical nature of the QSH edge state in monolayer WTe_{2} and the proximity effect of a magnet upon it. In the first part, I will describe how we explore the spinmomentum locking in the QSH edge state and determine the spin axis by studying the magnetic anisotropy. In the second part, I will discuss the magnetic coupling between a twodimensional antiferromagnet, CrI_{3}, and the QSH edge state.

Condensed Matter Seminar Thursday, November 11, 2021 3:30 PM Physics Building, Room 204 
ABSTRACT:
Optical photons are excellent flying qubits for longdistance quantum networks due to negligible thermal noise and decoherence at room temperature. In this talk, I will discuss how frequency encoding can be combined with nonlinear optics and fiber and integrated photonic technologies to address challenges in scaling future photonic quantum networks. Frequency multiplexing has had a profound impact on classical telecommunication networks, creating low loss and inexpensive hardware that can be exploited for quantum applications. I will describe quantum photonic applications where frequency encoding provides a distinct advantage in terms of scaling losses and resource overhead compared to polarization, spatial or temporal mode encoding. Coherent manipulation of light in the frequency domain at the singlephoton level requires a strong, noisefree nonlinear process. I will discuss our implementation of fourwave mixing (FWM) in a commercial dispersionshifted fiber to achieve quantum frequency conversion with nearunity efficiency and low noise. I will discuss how we used this process as an active "frequency switch" to realize a lowloss multiplexed singlephoton source that can be scaled to the deterministic regime. Next, I will discuss how we used this process as a frequency beamsplitter to demonstrate twophoton HongOuMandel type interference between entangled photons of different colors a hallmark of quantum indistinguishability. Finally, I will discuss our realization of a FWMbased "time lens" for the generation and detection of singlephoton waveforms with picosecond resolution. Based on Joshi et al., Nat. Comm. 9, 847 (2018), Joshi et al. Phys. Rev. Lett. 124, 143601(2020) 
Condensed Matter Seminar Wednesday, November 3, 2021 2:00 PM Online, Room via Zoom Join Zoom Meetinghttps://virginia.zoom.us/j/97076553627Meeting ID: 970 7655 3627Passcode: 968730 
"Triangular GrossPitaevskii breathers and DamskiChandrasekhar shock waves"Professor Maxim Olchanyi (Olshanii) , University of Massachusetts Boston [Host: Prof. Israel Klich]
ABSTRACT:
The recently proposed map [arXiv:2011.01415] between the hydrodynamic equations governing the twodimensional triangular coldbosonic breathers [Phys. Rev. X 9, 021035 (2019)] and the highdensity zerotemperature triangular freefermionic clouds, both trapped harmonically, perfectly explains the former phenomenon but leaves uninterpreted the nature of the initial (t=0) singularity. This singularity is a density discontinuity that leads, in the bosonic case, to an infinite force at the cloud edge. The map itself becomes invalid at time t=T/4. Here, we first map  using the scale invariance of the problem  the trapped motion to an untrapped one. Then we show that in the new representation, the solution [arXiv:2011.01415] becomes, along a ray in the direction normal to one of the three edges of the initial cloud, a freely propagating onedimensional shock wave of a class proposed by Damski in [Phys. Rev. A 69, 043610 (2004)]. There, for a broad class of initial conditions, the onedimensional hydrodynamic equations can be mapped to the inviscid Burgers' equation, a nonlinear transport equation. More specifically, under the Damski map, the t=0 singularity of the original problem becomes, verbatim, the initial condition for the wave catastrophe solution found by Chandrasekhar in 1943 [Ballistic Research Laboratory Report No. 423 (1943)]. At t=T/8, our interpretation ceases to exist: at this instance, all three effectively onedimensional shock waves emanating from each of the three sides of the initial triangle collide at the origin, and the 2D1D correspondence between the solution of [arXiv:2011.01415] and the DamskiChandrasekhar shock wave becomes invalid. 
Condensed Matter Seminar Thursday, October 28, 2021 3:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/my/israel.klich 
"Predicted Nearly Room Temperature Superconductivity in Binary Metal Hydride Systems"Tianran Chen , NIST Center for Neutron Research [Host: Prof. SeungHun Lee]
ABSTRACT:
Due to the low atomic mass and high electronphonon coupling strength in hydrogenrich materials, hydride compounds under extremely high pressures are most promising in the search of highTc superconductors. Firstprinciplesbased computational search has become extremely important not only in predicting new materials but also in guiding highpressure experimental measurements. In this work, we have developed a superefficient and fast method for searching highT hydride superconductors. We introduce new "metrics" that are strongly correlated to strong electronphonon coupling and T but it is much faster to calculate them. Using our new method, we have searched more than 100,000 binary hydride systems and discovered several new highT superconductors. Among them, we report our prediction of hightemperature superconductivity at relatively low pressure in a novel binary metal hydride which may break the current record. A detailed mechanism of the superconductivity, phonons, and electronphonon coupling, anharmonicity, as well as the abnormal T pressure dependence, will be also discussed. 
Condensed Matter Seminar Thursday, October 14, 2021 3:30 PM Physics Building, Room 204 
"Machine Learning for Material Properties and Design"Aravind Krishnamoorthy , University of Southern California [Host: Utpal Chatterjee]
ABSTRACT:
The accelerated discovery and design of new quantum materials requires atomiclevel information about chemical reactions, phase transformations, mechanical deformations and other collective and emergent quantum phenomena. Several techniques have been developed recently that can learn the potential energy surface (PES) of complex materials. Machine Learning (ML) models, particularly deep neural networks, have proven capable of learning highly complex nonlinear relationships between atomic structure and properties and theory and experiments. In this talk, I will describe two examples of MLdriven MD called neuralnetwork quantum molecular dynamics (NNQMD) to tackle problems related to large systems and long trajectories that cannot be investigated by Quantum Molecular Dynamics (QMD). First, we use NNQMD for quantitatively characterizing the intermediate range order, manifested as first sharp diffraction peak in GeSe_{2}. In the second example, we compute the dielectric constant, ε_{0}, and its temperature dependence for liquid water using fluctuations in macroscopic polarization using two coupled neural network models. The first network, NNQMD, learns the PES of liquid water from QMD training data. The second network, neuralnetwork maximally localized Wannier functions, NNMLWF, is trained to predict dipole moments. I will also briefly discuss applications of ML to discovery of new dielectric polymer materials with high breakdown strengths and to optimization of chemical vapor deposition synthesis of quantum materials. VIDEO:

Condensed Matter Seminar Thursday, September 30, 2021 3:30 PM Physics Building, Room 204 
"Coulomb Universe in a Jellium Droplet"Professor Genya Kolomeisky , University of Virginia  Department of Physics [Host: Prof. Israel Klich]
ABSTRACT:
Analogy between the Coulomb law of interaction between charges and the Newton law of gravitational attraction between masses is familiar to every physics student. In this talk I demonstrate that this analogy implies that a system of identical charges can evolve with time in a manner that parallels cosmological evolution of the physical Universe with hallmarks such as Hubble's law and Friedmanntype dynamics present. The Coulomb and Newton laws are also dissimilar because the electrostatic force is many orders of magnitude larger than the gravitational force whose manifestations are only noticeable on astronomical scale. On the other hand, analog cosmological evolutions driven by Coulomb interactions are predicted to be observable in laboratory experiments involving Coulomb explosions and electron density oscillations in conductors. 
Condensed Matter Seminar Thursday, September 2, 2021 3:30 PM Physics Building, Room 204 Joint Condensed Matter and Gravity Seminar 
"Towards Room Temperature Superconductivity in HydrideBased Materials Under Pressure"Dr. Eva Zurek , SUNY Buffalo [Host: Bellave Shivaram]
ABSTRACT:
The pressure variable opens the door towards the synthesis of materials with unique properties, e.g. superconductivity, hydrogen storage media, highenergy density and superhard materials. Under pressure elements that would not normally combine may form stable compounds or they may adopt novel stoichiometries. As a result, we cannot use our chemical intuition developed at 1 atm to predict phases that become stable when compressed. To facilitate the prediction of the crystal structures of novel materials, without any experimental information, we have deve loped XtalOpt, an evolutionary algorithm for crystal structure prediction. XtalOpt has been applied to predict the structures of hydrides with unique compositions that become stable at pressures attainable in diamond anvil cells. In the ternary hydride system two different classes of superconductors composed of S and H atoms have been discovered  methane intercalated H3S perovskites with the CSH7 stoichiometry, and phases containing SH honeyco mb sheets. We also predict a superconducting RbB3Si3 phase in the bipartite sodalite structure that could be synthesized at mild pressures and quenched to 1 atm. 
Condensed Matter Seminar Thursday, April 29, 2021 3:15 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/97001220790Meeting ID: 970 0122 0790

"The anomalous thermal relaxations in linear chemical reactions"Saikat Bera , University of Virginia  Department of Physics [Host: Marija Vucelja]
ABSTRACT:
Thermal quenching is the process of rapidly cooling or heating a material. It has been practiced since ancient times to obtain desirable mechanical properties in materials, especially metals. The dynamics in play during quenching fall in the regime of nonequilibrium dynamics and is a subject of interest as most processes in nature happen out of equilibrium. A curious phenomenon during out of equilibrium processes is the so called Mpemba effect. The Mpemba effect is a phenomenon where a system prepared at a hot temperature (Thot) “overtakes” an identical system prepared at a warm temperature (Twarm) and cools down faster to be in equilibrium with a cold environment (Thot > Twarm > Tenvironment). My project involves studying the dynamics and behavior of linear chemical reaction networks during this kind of out of equilibrium process. Chemical reaction networks are a good model to study various biochemical processes, which are integral to the study of biochemical pathways and thus the functioning of cells. I am especially searching for the existence of a Mpemba like behavior in these kinds of systems and trying to characterize their behavior and dependence on the different parameters of the linear chemical reaction network. In this seminar I will be detailing on the methods used to study the out of equilibrium dynamics of linear chemical reaction networks and will be presenting the preliminary results which indicates the existence of Mpemba like behavior. This understanding will eventually lead to the optimization of chemical production for industrial application and characterization of biochemical pathways. 
Condensed Matter Seminar Thursday, April 22, 2021 3:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/95253668001Meeting ID: 952 5366 8001

"Gutzwiller Quantum Molecular Dynamics Simulation in Liquid"Chen Cheng , University of Virginia  Department of Physics [Host: GiaWei Chern]
ABSTRACT:
The Gutzwiller approximation is a method for stronglycorrelated systems, it is the simplest theory that successfully captures the correlated induced metalinsulator transition, i.e. mott transition. Density function theory (DFT) is a very efficient method to deal with manyelectron systems, thus currently quantum molecular dynamics (QMD) simulations are dominantly based on DFT, however DFT fails to describe many strong electron correlation phenomenon, for example the mott transition. We proposed a new scheme of quantum molecular dynamics based on the Gutzwiller method, the Gutzwiller quantum molecular dynamics (GQMD). A liquid Hubbard model is studied by GQMD, two schemes of mott metalinsulator transition is found at different densities, based on which a phase diagram can be given to describe different states of the Hubbard liquid system. An effort to apply GQMD to real materials is also made on hydrogen system at high temperature and pressure conditions.

Condensed Matter Seminar Tuesday, April 20, 2021 3:30 PM Online, Room via Zoom Special Condensed Matter Seminar Join Zoom Meeting:
Meeting ID: 260 917 9512

"Machine Learning Enable the Large Scale Kinetic Monte Carlo for FalicovKimball Model"Sheng Zhang , University of Virginia  Department of Physics [Host: GiaWei Chern]
ABSTRACT:
The FalicovKimball (FK) model was initially introduced as a statistical model for metalinsulator transition in correlated electron systems. It can be exactly solved by combining the classical Monte Carlo method for the lattice gas and exact diagonalization (ED) for the itinerant electrons. However, direct ED calculation, which is required in each timestep of dynamical simulations of the FK model, is very timeconsuming. Here we apply the modern machine learning (ML) technique to enable the firstever largescale kinetic Monte Carlo (kMC) simulations of FK model. Using our neuralnetwork model on a system of unprecedented 10^{5} lattice sites, we uncover an intriguing hidden sublattice symmetry breaking in the phase separation dynamics of FK model. 
Condensed Matter Seminar Thursday, April 15, 2021 3:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/8377396898Meeting ID: N/A

"A timedependent approach to inelastic scattering spectroscopies in and away from equilibrium: beyond perturbation theory"Adrian Feiguin , Northeastern University [Host: Israel Klich ]
ABSTRACT:
I present a new computational paradigm to simulate time and momentum resolved inelastic scattering spectroscopies in correlated systems. The conventional calculation of scattering cross sections relies on a treatment based on timedependent perturbation theory, that provides formulation in terms of Green’s functions. In equilibrium, it boils down to evaluating a simple spectral function equivalent to Fermi’s golden rule, which can be solved efficiently by a number of numerical methods. However, away from equilibrium, the resulting expressions require a full knowledge of the excitation spectrum and eigenvectors to account for all the possible allowed transitions, a seemingly unsurmountable complication. Similar problems arise when the quantity of interest originates from higher order processes, such as in Auger, Raman, or resonant inelastic Xray scattering (RIXS). To circumvent these hurdles, we introduce a timedependent approach that does not require a full diagonalization of the Hamiltonian: we simulate the full scattering process, including the incident and outgoing particles (neutron, electron, photon) and the interaction terms with the sample, and we solve the timedependent Schrödinger equation. The spectrum is recovered by measuring the momentum and energy lost by the scattered particles, akin an actual energyloss experiment. The method can be used to study transient dynamics and spectral signatures of correlationdriven nonequilibrium processes, as I illustrate with several examples and experimental proposals using the timedependent density matrix renormalization group method as a solver. Even in equilibrium, we find higher order contributions to the spectra that can potentially be detected by modern instruments. 
Condensed Matter Seminar Thursday, April 8, 2021 2:30 PM via Zoom, Room Online Join Zoom Meeting:https://virginia.zoom.us/my/israel.klichMeeting ID: N/A

"Quantum Wakes and Measurement Induced Chirality"Matthew Wampler , University of Virginia  Department of Physics [Host: Israel Klich]
ABSTRACT:
We study the long term behavior of lattice fermions undergoing repeated particle detection, extraction, or injection interspersed with unitary evolution in two specific regimes. First, we investigate the wake pattern formed behind a moving probe performing these operations. These disturbances show dramatically different behavior where, notably, at halffilling the “measurement wake” vanishes and the “extraction wake” becomes temperature independent. Second, in analogy with the edge modes found in topologically trivial systems when undergoing floquet driving, we provide a protocol of repeated local density measurements that induces edge modes in a topologically trivial system while the hamiltonian remains time independent. In the limit of rapid measurements, the socalled Zeno limit, we connect this system to a novel stochastic dynamical system and discover an interesting double step structure in the charge transport in this regime. 
Condensed Matter Seminar Thursday, March 25, 2021 3:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/92170693950Meeting ID: 921 7069 3950

"Ferrimagnetic materials for room temperature small skyrmions"Wei Zhou , University of Virginia  Department of Physics [Host: Joe Poon]
ABSTRACT:
The magnetic skyrmions are topologically protected spin configuration, which stabilized by DzyaloshinskiiMoriya interaction (DMI). Due to skyrmions’ ability to be small, stable, and controllable by electric current [1], they have considerable potential for highdensity data storage applications. It is theoretically predicted that ferrimagnetic materials prefer holding small skyrmions at room temperature (RT) [2,3]. 1015nm ferrimagnetic CoGd heterostructures and 1015nm ferrimagnetic Mn_{4}N heterostructures were fabricated by magnetron sputtering for holding small skyrmions at RT. Magnetic force microscope images show skyrmions. A designed compound layer is capping on the top of the magnetic layer to adjust the interfacial DMI, thus tune the size of skyrmions. The micromagnetic simulation was performed to study the effect of DMI on the size of skyrmions Mn_{4}N.
Reference:
[1] Fert, A., et al. Magnetic skyrmions: advances in physics and potential applications. Nat Rev Mater 2, 17031 (2017).
[2] Büttner, F., et al. Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications. Sci Rep 8, 4464 (2018).
[3] C.T. Ma., et al. Robust Formation of Ultrasmall RoomTemperature Neél Skyrmions in Amorphous Ferrimagnets from Atomistic Simulations. Sci Rep 9, 9964 (2019).

Condensed Matter Seminar Thursday, March 11, 2021 3:30 PM Online, Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/95308132506Meeting ID: 953 0813 2506

"Probing the Universality of Topological Defect Formation in a Quantum Annealer"Adolfo del Campo , Ikerbasque & DIPC [Host: Israel Klich]
ABSTRACT:
The number of topological defects created in a system driven through a quantum phase transition exhibits a powerlaw scaling with the driving time. This universal scaling law is the key prediction of the KibbleZurek mechanism (KZM), and testing it using a hardwarebased quantum simulator is a coveted goal of quantum information science. Here we provide such a test using quantum annealing. Specifically, we report on extensive experimental tests of topological defect 
Condensed Matter Seminar Thursday, November 5, 2020 9:30 AM Online, Room via Zoom Join Zoom Meeting: https://virginia.zoom.us/my/israel.klich 
"Crystal structure, rotational dynamics and vibrational dynamics of a twodimensional perovskite"Xiao Hu , University of Virginia  Department of Physics [Host: SeungHun Lee]
ABSTRACT:
Metal halide perovskites (MHPs) have come to the forefront in the photovoltaic and lightemitting devices due to their attractive optoelectronic properties, such as tunable bandgaps, long charge carrier lifetime, and bright luminescence. The interactions between charge carriers and crystal lattice, such as electronphonon coupling, polaron formation and low thermal conductivity, are proposed to be the major reasons for their extraordinary device performance. Our previous studies have demonstrated the significant role of the organic cation mobility in screening the excited charge carriers and further extending the charge carrier lifetime in threedimensional (3D) perovskites. Compared with 3D perovskites, however, 2D perovskites exhibit a oneorderofmagnitude longer degradation time and some of them could have extremely high photoluminescence quantum yields, which make them popular in the LED field. This talk will summarize the experimental investigations on a 2D MHP and examine the relationships between the lattice dynamics and optoelectronic properties in this type of 2D perovskites. 
Condensed Matter Seminar Wednesday, November 4, 2020 10:00 AM Online, Room via Zoom Special SeminarJoin Zoom Meeting:https://virginia.zoom.us/j/92287909487Meeting ID: 922 8790 9487

"Magnetic skyrmions and their applications"Hamed Vakili , University of Virginia  Department of Physics [Host: Avik Ghosh]
ABSTRACT:
Skyrmions are topologically protected magnetic quasiparticles. An isolated skyrmion is a metastable state of ferromagnet. The metastable state of skyrmions has a finite lifetime at non zero temperature which depends on energy barrier and attempt frequency. Materials with different symmetry groups can support different kinds of skyrmions (Bloch, Neel, Antiskyrmion). We will see how these different types of symmetries can be used to control movements of a skyrmion. Skyrmion and domain wall racetracks can be used for temporal memories in race logic. Locally synchronized racetracks can spatially store relative timings of wavefronts and provide nondestructive readout. 
Condensed Matter Seminar Thursday, April 16, 2020 3:30 PM , Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/96102363843Meeting ID:96102363843Password: 32572 
"Atomic and Electronic Correlations in the Change Density Wave phase of Dichalcogenides"Sharon Philip , University of Virginia  Department of Physics [Host: Despina Louca]
ABSTRACT:
Studies on transition metal dichalcogenides (TMD) is of great significance due to their interesting topological properties and remarkable electronic behavior. Among these materials,1T TaX_{2} class of TMDs, where X = S, Se, has spurred considerable interest due to their multiple first order phase transitions between different charge density wave (CDW) states. The effects of CDW formation in these compounds are attributed primarily to inplane reorientation of Ta atoms to StarofDavid formation. But this alone doesn’t explain the notable electronic behavior of 1TTaS_{2} and 1TTaSe_{2} and why they differ from one another despite having the same trigonal symmetry. At very low temperatures, 1TTaS_{2} undergoes a metal  insulator transition and is proposed to harbor a quantum spin liquid behavior whereas 1TTaSe_{2} remains metallic. Investigating the local structure of pristine 1TTaS_{2} and 1TTaSe_{2 }in the CDW regime could tell us the differences in local atomic correlations in these compounds. 
Condensed Matter Seminar Tuesday, April 14, 2020 2:00 PM , Room via Zoom Join Zoom Meeting:https://virginia.zoom.us/j/775042517Meeting ID: 775042517 
"Investigation of the structural phase transitions in Weyl semimetal (Mo, W)Te2"Yu Tao , University of Virginia  Department of Physics [Host: Despina Louca]
ABSTRACT:
Mo_{1x}W_{x}Te_{2} belongs to the family of layered transition metal dichalcogenides (TMD) that are of intense interest recently because of their fascinating topological properties. The end members of this series, MoTe_{2} and WTe_{2} are Weyl semimetals upon cooling to the orthorhombic Td phase. Mo_{1x}W_{x}Te_{2} undergoes a structural phase transition from a hightemperature monoclinic 1T' phase, to a noncentrosymmetric orthorhombic T_{d} phase at low temperatures through a firstorder structural phase transition. Both 1T' and T_{d }phases are comprised of weaklybound layers, and differ mainly by shifts of the layers along the caxis. Despite much research, the structural properties of Mo_{1x}W_{x}Te_{2 }have not been thoroughly investigated. Neutron scattering experiments at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory were carried out on single crystals of Mo_{1x}W_{x}Te_{2}. Structural changes including changes in interlayer disorder were observed by focusing on the elastic scattering along (2, 0, L) on cooling and warming through the hysteresis loop at the transition. A T_{d}* phase was discovered for the first time across the T_{d}1T’ phase boundary in Mo_{1x}W_{x}Te_{2 }with x up to ~0.2. In WTe_{2}, a sharp transition from T_{d} to 1T′ was observed at ambient pressure in the single crystal near ∼565 K, the transition proceeds without hysteresis. These results should clarify in microscopic detail the nature of these phase transitions. 
Condensed Matter Seminar Thursday, March 26, 2020 11:00 AM Online, Room via Zoom (Zoom link and meeting ID provided above) WebinarJoin Zoom Meeting

"Design principles of biological and chemical intelligence"Zhiyue Lu , University of North Carolina at Chapel Hill [Host: Marija Vucelja]
ABSTRACT:
Living systems respond to external stimuli by utilizing chemical reaction networks that function as cellular information processors. What can we learn from living systems as the design principle of intelligent active materials? One salient example of a biological intelligence is the singlecell circadian clock (i.e. the KaiABC oscillator in cyanobacteria). Such circadian clocks process external signal (sunlight intensity) and computes the time during the day/night. These microscopic computers are naturally challenged by two main sources of uncertainty the internal thermal fluctuations and the external noisy signal. To optimize its performance, we find that a clock must make a tradeoff between resisting internal thermal fluctuations and external signal noise. This noise tradeoff relation can be explained through the geometry of its energy landscape. I will also discuss the use of the energy landscape in designing intelligent responses into mechanochemical materials.

Condensed Matter Seminar Thursday, February 27, 2020 3:30 PM Physics Building, Room 204 
ABSTRACT:
't Hooft anomalies provide a unique handle to study the nonperturbative dynamics of stronglycoupled theories. Although this type of anomalies was known since the 80's, recently it has been realized that one can generalize them by turning on 't Hooft twists in the color, flavor, and baryon number directions. Such generalized anomalies put severe constraints on the possible realizations of the global symmetries of a given theory in the infrared. In this talk, I will explain how one can construct such 't Hooft twists and give examples of the constrains the generalized anomalies can impose on strongly coupled gauge theories.

Condensed Matter Seminar Thursday, February 20, 2020 12:45 PM Physics Building, Room 313 Special Seminar 
"Exact results for 5dimensional superconformal field theories"Christoph Uhlemann , University of Michigan [Host: Peter Arnold]
ABSTRACT:
5dimensional superconformal field theories play an intriguing role in the general understanding of quantum field theory. They provide stronglycoupled UV fixed points for many perturbatively nonrenormalizable 5dimensional gauge theories, and upon compactification they provide new insights into many lowerdimensional theories. This makes them both useful and interesting in their own right. In this talk I will discuss recent progress in the understanding of 5dimensional superconformal field theories through AdS/CFT dualities and nonperturbative field theory methods, leading to interesting exact results and many cross checks. 
Condensed Matter Seminar Thursday, February 13, 2020 12:45 PM Physics Building, Room 313 Special Seminar 
ABSTRACT:
Despite its many successes, the Standard Model of particle physics cannot be the final description of nature at the most fundamental level. Additional elementary particles and interactions are an absolute necessity but have so far evaded our experimental efforts. I will highlight the importance of searches for processes that are forbidden within the Standard Model, as these make for clean signatures of new physics. Important examples are searches for lepton flavor violation and baryon number violation, which will be tested to unprecedented levels in upcoming experiments. 
Condensed Matter Seminar Thursday, February 6, 2020 12:45 PM Physics Building, Room 313 Special Seminar 
ABSTRACT:
Soft Collinear Effective Theory (SCET) is a framework for modeling the infrared structure of theories whose long distance behavior is dominated by soft and collinear divergences. SCET is utilized to compute processes in Jet physics, WIMP annihilations, and more. My collaborators and I showed that SCET can be made compatible with supersymmetry, and that such a theory can be conveniently formulated in "Collinear Superspace". In this talk I will introduce a new set of effective field theory rules for constructing Lagrangians in collinear superspace. This new formalism represents a general way to derive onshell superspace Lagrangians directly from the symmetries of the theory. However, I will also demonstrate how the nonpropagating off shell degrees of freedom i.e. F and D terms, can be reintroduced into the theory. This framework paves the way to constructing theories with N > 1 supersymmetry directly from lowenergy considerations, and has potential implications for supergravity, the Scattering Amplitudes program, and more. 
Condensed Matter Seminar Thursday, January 30, 2020 12:45 PM Physics Building, Room 313 Special Seminar 
"Duality between Spacelike and Timelike Processes in Gauge Theory"Duff Neill , Los Alamos National Lab [Host: Peter Arnold]
ABSTRACT:
Since the original papers of DrellLevyYan, there has been a desire to unite the process of production of hadrons in the final state to the process of probing the structure of a hadronic initial state. Specifically, one wants to relate the process of single inclusive annihilation (SIA) to deep inelastic scattering (DIS) via crossing and analyticity. It has long been known that any straightforward relation between the two processes fails, however, pursuing this relation has lead to a deeper and richer connection between SIA and DIS, now known as the spacetime reciprocity relation (arXiv hepth/0612247). I shall give an introduction to this spacetime reciprocity relation, and argue that the relationship is a consequence of the deeper connection between final and initial state dynamics governed by an underlying conformal symmetry which maps between the two, upto anomalous terms which should cancel as regulators are removed, but from which one is never free due to the initial conditions of the scaling evolution. As a consequence, I will give a timelike BFKL equation that resums the soft region of the fragmentation function of QCD, which maps to the spacelike BFKL equation that governs the soft region of the parton distribution function. Time permitting, I will discuss both possible formal implications and phenomenological applications. This will be a blackboard talk. 
Condensed Matter Seminar Thursday, January 23, 2020 12:45 PM Physics Building, Room 313 Special Seminar 
ABSTRACT:
Hydrodynamics is a theory of the collective properties of fluids and gases that can also be successfully applied to the description of the dynamics of quarkgluon plasma. It is an effective field theory formulated in terms of an infiniteorder gradient expansion. For any collective physical mode, hydrodynamics will predict a dispersion relation that expresses this mode’s frequency in terms of an infinite series in powers of momentum. By using the theory of complex spectral curves from the mathematical field of algebraic geometry, I will describe how these dispersion relations can be understood as Puiseux series in (fractional powers of) complex momentum. The series have finite radii of convergence determined by the critical points of the associated spectral curves. For theories that admit a dual gravitational description through holography, the critical points correspond to levelcrossings in the quasinormal spectrum of a dual black hole. Interestingly, holography implies that the convergence radii can be orders of magnitude larger than what may be naively expected. This fact could help explain the “unreasonable effectiveness of hydrodynamics” in describing the evolution of quarkgluon plasma. In the second part of my talk, I will discuss a recently discovered phenomenon called “poleskipping” that relates hydrodynamics to the underlying microscopic quantum manybody chaos. This new and special property of quantum correlation functions allows for a precise analytic connection between resummed, allorder hydrodynamics and the properties of quantum chaos (the Lyapunov exponent and the butterfly velocity). 
Condensed Matter Seminar Thursday, January 16, 2020 12:45 PM Physics Building, Room 313 Special Seminar 
"Monopole Superconductivity and DensityWave Order in Weyl Semimetals"Professor Yi Li , Johns Hopkins University [Host: Dima Pesin]
ABSTRACT:
Although the existence of magnetic monopoles is admitted by the fundamental laws, the real monopoles in nature remain elusive. Nevertheless, variations of monopoles appear in realistic condensed matter systems, from quantum Hall effects to topological superconductivity, which spur a race to discover new exotic topological phases of matter. In this talk, we will present a dramatic effect arising from topological Fermi surfaces  a novel topological class of superconductivity and densitywave orders: When the ordered pairs acquire nontrivial twoparticle Berry phases, their pairing phases cannot be globally welldefined in the momentum space. Therefore, the conventional description of superconducting pairing symmetries in terms of spherical harmonics (e.g. s, p, dwaves) ceases to apply. Instead, they are characterized by topologically protected nodal gap functions represented by monopole harmonic functions. This socalled “monopole harmonic order” is expected to be realized and detected in Weyl semimetal materials. 
Condensed Matter Seminar Thursday, December 5, 2019 11:00 AM Physics Building, Room 313 
ABSTRACT:
We obtain a new relation between the distributions μ_t at different times t ≥ 0 of the continuoustime TASEP (Totally Asymmetric Simple Exclusion Process) started from the step initial configuration. Namely, we present a continuoustime Markov process with local interactions and particledependent rates which maps the TASEP distributions μ_t backwards in time. Under the backwards process, particles jump to the left, and the dynamics can be viewed as a ver sion of the discretespace Hammersley process. Combined with the forward TASEP evolution, this leads to a stationary Markov dynamics preserving μ_t which in turn brings new identities for expectations with respect to μ_t. Based on a joint work with Axel Saenz. 
Condensed Matter Seminar Thursday, November 14, 2019 3:30 PM Physics Building, Room 313 
ABSTRACT:
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Condensed Matter Seminar Thursday, August 22, 2019 2:30 PM Physics Building, Room 204 
"Twodimensional magnetism and spintronics"Adam Wei Tsen , University of Waterloo [Host: Seunghun Lee]
ABSTRACT:
The recent discoveries of ferromagnetism in single atomic layers have opened a new avenue for twodimensional (2D) materials research. Not only do they raise fundamental questions regarding the requirements for longrange magnetic order in lowdimensional systems, but they also provide a new platform for the development of spintronic devices. In this talk, I will present a series of studies on the family of layered ferromagnetic semiconductors, CrX_{3} (X = I, Br, Cl), in the atomically thin limit. By incorporating these materials as tunnel barriers between graphene electrodes, we are able to achieve extremely large tunnel magnetoresistance as well as robust memritive switching that is tunable with magnetic field. Tunneling spectroscopy further allows for direct observation of their spin wave excitations, or magnons, from which we are able to derive a simple microscopic Hamiltonian for all three spin systems. These results show that strong exchange anisotropy is not necessary to stabilize ferromagnetism in the monolayer limit. 
Condensed Matter Seminar Thursday, May 30, 2019 11:00 AM Physics Building, Room 313 
ABSTRACT:
I shall review the history of Sagnac interferometers and give a geometric description of light rays propagation in flexible optical fibers in Minkowski space. Based on joint works with Amos Ori and Oded Kenneth. 
Condensed Matter Seminar Tuesday, May 28, 2019 11:00 AM Physics Building, Room 313 
"Tunnelinginduced restoration of classical degeneracy in quantum kagome ice"Professor YingJer Kao , National Taiwan University [Host: GiaWei Chern]
ABSTRACT:
Quantum effect is expected to dictate the behavior of physical systems at low temperature. For quantum magnets with geometrical frustration, quantum fluctuation usually lifts the macroscopic classical degeneracy, and exotic quantum states emerge. However, how different types of quantum processes entangle wave functions in a constrained Hilbert space is not well understood. Here, we study the topological entanglement entropy (TEE) and the thermal entropy of a quantum ice model on a geometrically frustrated kagome lattice. We find that the system does not show a Z2 topological order down to extremely low temperature, yet continues to behave like a classical kagome ice with finite residual entropy. Our theoretical analysis indicates an intricate competition of offdiagonal and diagonal quantum processes leading to the quasidegeneracy of states and effectively, the classical degeneracy is restored. 
Condensed Matter Seminar Thursday, May 16, 2019 11:00 AM Physics Building, Room 313 
"Exciton polarons in twodimensional organicinorganic hybrid perovskites"Professor Carlos Silva , Georgia Tech [Host: Seunghun Lee]
ABSTRACT:
Owing to both electronic and dielectric confinement effects, twodimensional organicinorganic hybrid perovskites sustain strongly bound excitons at room temperature. In this seminar, we demonstrate that there are nonnegligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic countercation. In these systems, organic cations not only serve as spacers between slabs consisting of cornersharing metalhalide octahedra, but also determine lattice structure by inducing varying degree of distortion of the octahedra via the organicinorganic interactions. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in singlelayer organicinorganic hybrid perovskites, based on the twodimensional Wannier formalism. We include four distinct excitonic states separated by 35±5 meV, and additional vibronic progressions. Intriguingly, the associated HuangRhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the line shape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Furthermore, by means of highresolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various lowfrequency (≲50 cm^{−1}) optical phonons involving motion in the lead iodide layers. This supports our conclusion that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. Excitonic correlations (exciton and biexciton binding energies) and exciton dynamics (e.g. uni and bimolecular population decay mechanisms, pure dephasing processes, excitationinduced dephasing, etc.) reflect the polar solvationlike processes induced by organic cation components of the hybrid lattice in a broad structural space. I will address how ultrafast nonlinear spectroscopies yield deep insight on the multiparticle properties in compelx semiconductor materials.

Condensed Matter Seminar Thursday, May 9, 2019 11:00 AM Physics Building, Room 313 
ABSTRACT:
Through string theory, people found interesting relations in particle theory. For example, KawaiLewellenTye (KLT) relation relates the scattering amplitudes of QCD and gravity. However, these kinds of relation are completely mysterious from the point view of Quantum Field Theory since the gravity Lagrangian seems totally unrelated to the YangMills Lagrangian. On the other hand, these kinds of relations are nevertheless true and can be checked by computing the amplitudes using Feynman diagrams order by order. Thus the Feynman diagrammatic expansion does not capture everything of interest, there are still hidden relations between different field theories. Worldline approach, born as a first quantized approach to calculate amplitudes, shares a lot of similarities to string theory. In this talk, I will show how worldline approach works and how it helps shed some light on the problems we are interested in. I will also discuss the subtlety and limitation of the approach and the possibility of generalizing it to "worldgraph approach".

Condensed Matter Seminar Thursday, May 2, 2019 3:30 PM Physics Building, Room 313 
"Thermoelectric transport properties of topological BiSb cryogenic materials"Xixiao Hu , University of Virginia  Physics [Host: Joe Poon]
ABSTRACT:
BiSb alloys have shown promising thermoelectric (TE) properties at cryogenic temperature (<200 K). Over six decades, the figure of merit zT of ntype polycrystalline BiSb has plateaued at ~0.4, while its ptype counterpart has remained even lower at ~0.1. We have studied the TE properties of meltspun and spark plasma sintered (SPS) BiSb alloys. We obtained a zT of 0.55 @100150 K for ntype undoped Bi_{85}Sb_{15 }based on a low thermal conductivity 1.5 W/(m*K) measured with the hotdisk method. For ptype BiSb, doping effects of Ge, Sn, and Pb were investigated. A high doping level of Ge and a high doping efficiency of Pb were obtained with the help of a lowtemperature SPS processing. The transport properties (resistivity and Seebeck coefficient) of ntype undoped and ptype doped Bi_{85}Sb_{15} were analyzed using the twoband effective mass model within the Boltzmann transport theory. A band gap decreasing phenomenon was observed which poses challenges to the improvement of ptype BiSb’s zT. 
Condensed Matter Seminar Wednesday, May 1, 2019 1:00 PM Physics Building, Room 314 
"Topological phases with twofold spatial antiunitary symmetries"Meng Hua , University of Virginia  Physics [Host: Jeffrey Teo]
ABSTRACT:
An interesting theme in topological materials has been classification and prediction of symmetry protected topological(SPT) phases. Despite the AltlandZirnbauer(AZ) classification under timereversal symmetry, particlehole symmetry and chiral symmetry, a system can also be invariant under a combined symmetry composed by two distinct operations. In this talk I will discuss the classification of nodal topological phases under twofold spatial antiunitary symmetries. We also generalize SPT phases to nonHermitian system with twofold spatial antiunitary symmetries and give an example of dissipative topological superconductors. 
Condensed Matter Seminar Friday, April 26, 2019 1:00 PM Physics Building, Room 205 
"The threebody problem: periodic solutions, topological classification"Milovan Suvakov , Institute of Physics Belgrade [Host: Marija Vucelja ]
ABSTRACT:
The threebody problem dates back to the 1680s. Isaac Newton had
https://scholar.google.com/citations?hl=en&user=dEJ0ThoAAAAJ&view_op=list_works&sortby=pubdate 
Condensed Matter Seminar Thursday, April 25, 2019 11:00 AM Physics Building, Room 313 
"Magnetic Skyrmions on a racetrack"Hamed Vakili , University of Virginia  Physics [Host: Avik Ghosh]
ABSTRACT:
Skyrmions are topologically protected magnetic quasiparticles. An isolate skyrmion is a metastable state of ferromagnet. The metastable state of skyrmions have a finite lifetime at non zero temperature which depends on energy barrier and attempt frequency. I will talk about how we are trying to calculate lifetime of skyrmion in candidate Heuslers compounds. Materials with different symmetry groups can support different kind of skyrmions (Bloch, Neel, Antiskyrmion). We will see how this different types of symmetries can be used to control movements of a skyrmion. Also, we will look at how presence of point defects can effect dynamics of skyrmion, either for movement or nucleation. The ultimate goal is to figure out a compact analytical model for describing skyrmion movement and critical spin current needed for nucleation.

Condensed Matter Seminar Wednesday, April 24, 2019 10:00 AM Physics Building, Room 313 
"Spin dynamics in two distinct types of classical spin liquids "Preetha Saha , University of Virginia  Physics [Host: GiaWei Chern]
ABSTRACT:
Unconventional magnetic states such as spin liquids and spin glasses continue to attract the interest of researchers in magnetism.These materials retain their magnetic disorder even at zero temperatures. We study two different cases of frustrated systems 1)In the case of Kitaevtype models frustration originates from highly anisotropic exchange interactions. We report a new classical spin liquid in which the collective flux degrees of freedom break the translation symmetry of the honeycomb lattice. This exotic phase exists in frustrated spinorbit magnets where a dominant offdiagonal exchange, the socalled Γ term, results in a macroscopic groundstate degeneracy at the classical level [1]. We show that this phase transition actually corresponds to plaquette ordering of hexagonal fluxes. We also study the dynamical behavior of fluxes. 2) We study the deterministic spin precession dynamics using energy conserving LandauLifshitz equation on a geometrically frustrated magnet. The lattice constitutes of a triangular arrangement of bipyramids with classical antiferromagnetic Heisenberg interaction. Such a lattice structure is realized in frustrated SrCr9Ga129pO19 [SCGO(p)] compounds [2]. Monte Carlo simulations are used to thermalize the system, which is then used as the initial state for the dynamical studies. We explore the temperature, wave vector and frequency dependence in the dynamical structure factor and the corresponding time dependent correlation functions of the model. Dynamics simulations is further used to estimate the extent to which transport of spin excitations in the lattice conform with phenomenological concept of spin diffusion [1]I. Rousochatzakis and N. B. Perkins Phys. Rev. Lett. 118, 147204 (2017). [2]T. Arimori and H. Kawamura J. Phys. Soc. Jpn. 70, 3695 (2001) 
Condensed Matter Seminar Thursday, April 18, 2019 11:00 AM Physics Building, Room 313 
"Introduction to Rivanna"UVa's Advanced Research Computing Services , University of Virginia [Host: Bryan Wright]
ABSTRACT:
Members of UVa's Advanced Research Computing Services group will be presenting the second of two information/Q&A sessions about rivanna (UVa's supercomputing cluster) on Thursday, April 4 at 11am in Physics 313. Rivanna is a 7,000core cluster with more than a petabyte of storage. It includes a subset of GPUequipped nodes. Please drop in if you have any interest in highperformance computing. Slides from the talk can be found here: http://galileo.phys.virginia.edu/compfac/faq/IntroductionToRivanna.pdf

Condensed Matter Seminar Thursday, April 4, 2019 11:00 AM Physics Building, Room 313 Special Presentation 
"Quantum gas microscopy of manybody dynamics in FermiHubbard and Ising systems"Peter Schauss , University of Virginia  Physics [Host: Dmytro Pesin]
ABSTRACT:
The ability to probe and manipulate cold atoms in optical lattices at the atomic level using quantum gas microscopes enables quantitative studies of quantum manybody dynamics. While there are many welldeveloped theoretical tools to study manybody quantum systems in equilibrium, gaining insight into dynamics is challenging with available techniques. Approximate methods need to be benchmarked, creating an urgent need for measurements in experimental model systems. In this talk, I will discuss two such measurements. 
Condensed Matter Seminar Thursday, March 28, 2019 11:00 AM Physics Building, Room 313 
"MetalInsulator Transition in Phase Change Material Ge2Sb2Se5xTe55x"Zhenyang Xu , University of Virginia  Physics [Host: Despina Louca]
ABSTRACT:
Ge2Sb2Te5 (GST225) is a phase change material which has wide use in fabricating random access memories. With different quenching process, the GST225 could have three different phases: an amorphous phase, an intermediate cubic phase, a crystalline hexagonal phase. The fast transition between amorphous and crystalline phase makes GST225 an ideal material for RAM with fast speed. In our project, the Sedoped GST225 materials are grown and studied. At x=0.9 in liquid nitrogen quenched samples, we observe a phase transition from crystalline to amorphous. The transport measurement also confirm that there are metalinsulator transition happen for both furnace cooled samples and liquid N2 quenched samples at this limit. A tentative hypothesis is proposed to explain this metalinsulator transition. 
Condensed Matter Seminar Thursday, March 21, 2019 11:00 AM Physics Building, Room 313 
"Walks, tiles, and zippers: exact holographic tensor networks for Motzkin spin chains"Rafael Alexander , UNM [Host: Israel Klich]
ABSTRACT:
The study of lowdimensional quantum systems has proven to be a particularly fertile field for discovering novel types of quantum matter. The tensor network's utility in studying short range correlated states in 1D have been thoroughly investigated. Yet, despite the large number of works investigating these networks and their relations to physical models, examples of exact correspondence between the ground state of a quantum critical system and an appropriate scaleinvariant tensor network have eluded us so far. Here we show that the features of the quantumcritical Motzkin model can be faithfully captured by an analytic tensor network that exactly represents the ground state of the physical Hamiltonian. In particular, our network offers a twodimensional representation of this state by a correspondence between walks and a type of tiling of a square lattice. We discuss connections to renormalization and holography. 
Condensed Matter Seminar Wednesday, March 13, 2019 11:00 AM Physics Building, Room 313 Special Condensed Matter Seminar 
"Thermodynamic properties of disordered unconventional superconductors with competing interactions"Maxim Dzero , Kent State [Host: Israel Klich]
ABSTRACT:
A topic of interplay between disorder and competing electronic phases in multiband superconductors have recently got renewed interest in the context of ironbased superconductors. In my talk I will present a theory of disordered unconventional superconductor with competing magnetic order. My discussion will be based on the results obtained for on a twoband model with quasitwodimensional Fermi surfaces, which allows for the coexistence region in the phase diagram between magnetic and superconducting states in the presence of intraband and interband scattering induced by doping. Within the quasiclassical approximation I will present the analysis of the quasiclassical Eilenberger’s equations which include weak external magnetic field. I will demonstrate that disorder has a crucial effect on the temperature dependence of the magnetic penetration depth as well as critical current, which is especially pronounced in the coexistence phase. 
Condensed Matter Seminar Thursday, February 14, 2019 11:00 AM Physics Building, Room 313 
"Fractionalized excitations towards a nonAbelian phase in a Kitaev honeycomb magnet"Arnab Banerjee , Oak Ridge National Laboratory [Host: Bellave Shivaram]
ABSTRACT:
The Kitaev model on a honeycomb lattice predicts a special quantum spin liquid (QSL) ground state with excitations resembling Majorana Fermions and gauge flux excitations. These emergent features are exciting prospects to both basic physics and applications towards a lossless technology for quantum qubits. In this talk, I will describe our recent range of experiments on the magnetic Mott insulator alphaRuCl3 which has honeycomb layers held together with weak vanderWaals interactions. A strong spinorbit coupling and an octahedral crystal field makes the Kitaev interactions arguably the leading order term in the Hamiltonian. Prominently, despite a longrange ordered ground state, our neutron scattering measurements reveal a continuum of fractionalized excitations resembling predictions from Majorana Fermions, confirming that the material is proximate to a QSL. In a 8T magnetic field the longrange order vanishes and the continuum becomes gapped, supporting a state where a direct evidence of nonAbelian excitations can be measured. I will describe the present and future endeavors that may help to stabilize the coherent quantum excitations allow a better understanding of the underlying physics, as well as experiments to complete the understanding of the phase diagram of this material. 
Condensed Matter Seminar Thursday, November 8, 2018 11:00 AM Physics Building, Room 313 
"Competing orders in a quantum spin model with longrange interactions"Erhai Zhao , George Mason University [Host: Bellave Shivaram]
ABSTRACT:
Quantum spin liquids evade longrange magnetic order down to absolute zero temperature. These anarchic, yet highly entangled states break no symmetry but have remarkable properties such as fractional excitations. In this talk, I will first give an example of spin liquid using a compass model relevant to recently discovered honeycomb antiferromagnet NaNi2BiO6. Then I will introduce a new model, the dipolar Heisenberg model, motivated by recent experiments on artificial manyspin systems based on interacting dipoles. I will argue that longrange magnetic order can be suppressed by simply tuning the direction of the dipoles using an external field. The classical, semiclassical, and quantum phase diagram of this frustrated spin model will be presented to show an extended region where the ground state is a quantum paramagnet. By comparing to DMRG, I will argue that it is likely a quantum spin liquid. 
Condensed Matter Seminar Thursday, October 18, 2018 11:00 AM Physics Building, Room 313 
"Transport in Strongly Correlated 2D Electron Fluids"Alex Levchenko , University of WisconsinMadison [Host: Dmytro Pesin]
ABSTRACT:
In this talk I plan to overview measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities with an emphasis on magnetoresistance and drag resistance. As many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium we will concentrate on physics associated with strong correlation effects and develop hydrodynamic theory of transport. We will apply these ideas to composite fermions of quantum Hall bilayers in hydrodynamic regime. 
Condensed Matter Seminar Thursday, October 11, 2018 11:00 AM Physics Building, Room 313 
"Toward the next quantum revolution: controlling physical systems and taming decoherence"Ed Barnes , Virginia Tech [Host: Israel Klich]
ABSTRACT:
Recent years have witnessed enormous progress toward harnessing the power of quantum mechanics and integrating it into novel technologies capable of performing tasks far beyond presentday capabilities. Future technologies such as quantum computing, sensing and communication demand the ability to control microscopic quantum systems with unprecedented accuracy. This task is particularly daunting due to unwanted and unavoidable interactions with noisy environments that destroy quantum information in a process known as decoherence. I will present recent progress in understanding and modeling the effects of multiple noise sources on the evolution of a quantum bit and show how this can be used to develop new ways to slow down decoherence. I will then describe a new general theory for dynamically combatting decoherence by driving quantum bits in such a way that noise effects destructively interfere and cancel out, enabling the high level of control needed to realize quantum information technologies. 
Condensed Matter Seminar Thursday, September 27, 2018 11:00 AM Physics Building, Room 313 
""A materialsdriven approach to the novel topological states of matter""Nirmal Ghimire , George Mason University [Host: Bellave Shivaram]
ABSTRACT:
Materials in condensed matter have recently been testbeds for several exotic particles, predicted but never realized, in high energy physics. The examples are skyrmions observed in magnetic textures. Weyl fermions in the low energy electronic excitations of Weyl semimetals and Majorana fermions in topological superconductors. These discoveries have not only allowed access to the fundamental physics of the rare particles but also driven large interest in the application of such exotic states to future technologies such as spin based electronics and quantum computation. Discoveries of topological states in materials have largely benefited from the precision of the electronic structure calculations in the weakly correlated systems. In the first part of this talk, I will discuss our resent results on two such predicted materials – 1) NbAs, one of the first generation Weyl semimetals [13] and 2) Pd_{3}Pb, a novel topological material hosting multiple Dirac points and surface states [4]. While calculations are pretty accurate in weakly correlated systems, the topological states in presence of strong electron correlations are still not well understood. As such, materials can take a lead in this field. In the second part of the talk, I will briefly highlight our recent efforts in this area, driven by specific materials design criteria. As an illustration, I will discuss our study on the chirallattice antiferromagnet CoNb_{3}S_{6} that has topological character in the electronic band structure, and manifests an unusually large anomalous Hall effect [5].
[1] N. J. Ghimire et al. J. Phys.: Condens. Matter 27, 152201 (2015). [2] Y. Luo et al. Phys. Rev. B 92, 205134 (2015) [3] P. J. W. Moll et al., Nat. Communs. 7, 12492 (2016). [4] N. J. Ghimire et al., Phys. Rev. Materials 2, 081201(R) (2018) [5] N. J. Ghimire et al., Nat. Communs. 9, 3280 (2018)

Condensed Matter Seminar Thursday, September 20, 2018 11:00 AM Physics Building, Room 313 
"The mother of all states of the kagome quantum antiferromagnet"Hitesh J. Changlani , Florida State University [Host: Bellave Shivaram]
ABSTRACT:
Strongly correlated systems provide a fertile ground for discovering exotic states of matter, such as those with topologically nontrivial properties. Among these are geometrically frustrated magnets, which harbor spin liquid phases with fractional excitations. On the experimental front, this has motivated the search for new low dimensional quantum materials and on the theoretical front, this area of research has led to analytical and numerical advances in the study of quantum manybody systems.
I present aspects of our theoretical and numerical work in the area of frustrated magnetism, focusing on the frustrated kagome geometry, which has seen a flurry of research activity owing to several nearideal material realizations. On the theoretical front, the kagome problem has a rich history and poses multiple theoretical puzzles which continue to baffle the community. First, I present a study of the spin1 antiferromagnet, where our numerical calculations indicate that the ground state is a trimerized valence bond (simplex) solid with a spin gap [1], contrary to previous proposals. I show evidence from recent experiments that support our findings but also pose new questions. The second part of the talk follows from an unexpected outcome of my general investigations in the area for the wellstudied spin1/2 case [2]. I explain the existence of an exactly solvable point in the XXZHeisenberg model for the ratio of Ising to transverse coupling $J_z/J=1/2$ [3]. This point in the phase diagram, previously unreported in the literature, has "threecoloring" states as its exact quantum ground states and is macroscopically degenerate. It exists for all magnetizations and is the origin or "mother" of many of the observed phases of the kagome antiferromagnet. I revisit aspects of the contentious and experimentally relevant Heisenberg case and discuss its relationship to the newly discovered point [3,4].
[3] H. J. Changlani, D. Kochkov, K. Kumar, B. K. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018). [4] H. J. Changlani, S. Pujari, C.M. Chung, B. K. Clark, under preparation. 
Condensed Matter Seminar Friday, August 31, 2018 3:30 PM Physics Building, Room 204 Special Condensed Matter Seminar 
"Gravitational Weyl Anomalies in 1+1Dimensional Lifshitz Field Theories"Amr Ahmadain , UVADepartment of Physics [Host: Israel Klich ]
ABSTRACT:
In this talk, I will explain the notion of anisotropic gravitational Weyl anomalies in Lifshitz field theories and their potential applications. Anomalies are quantum violations of classical symmetries, in this case under a nonrelativistic (nonLorentzinvariant) symmetry group. More specifically, I will focus on the only anomaly found in 1+1dimensional Lifshitz field theories that arise after coupling to NewtonCartan geometry with torsion, and it's relation to the Lorentz (diffeomorphism) anomaly in the quantum effective action of a 1+1 CFT. At the heart of this discussion, I will emphasize the role of temporal torsion in generating this anomaly and show how such an anomaly can be derived from a 2+1dimensional Weylinvariant ChernSimons HoravaLifshitz theory of gravity. I will finally comment briefly on how the 1+1 Lifshitz Weyl anomaly can potentially be connected to thermal Hall transport and some other potential applications. 
Condensed Matter Seminar Thursday, May 3, 2018 10:00 AM Physics Building, Room 313 
"MagneticField Induced Weyl Semimetal from WannierFunctionbased TightBinding Model"Kyungwha Park , Virginia Tech [Host: Jeffrey Teo]
ABSTRACT:
Weyl semimetals (WSMs) have a threedimensional (3D) bulk band structure in which the conduction and valence bands meet at discrete points, i.e. Weyl nodes. Projections of Weyl points with opposite chirality can be connected by Fermi arcs at a surface. Topological Dirac semimetals (DSMs) have 3D Dirac points which can be viewed as superimposed copies of Weyl points and are stabilized by rotational symmetry. When an external magnetic field is applied to a DSM, Dirac points can be separated into multiple Weyl points and so a WSM phase can be driven. DSMs and WSMs have received a lot of attention because of the chiral anomaly and novel magnetotransport signatures. We develop a tightbinding model based on Wannier functions directly from density functional theory (DFT) calculations for a topological DSM Na3Bi. We add spinorbit coupling and Zeeman terms in the tightbinding model. Upon magnetic field along the rotational axis, we find that each Dirac node splits into two single Weyl nodes and two double Weyl nodes with opposite chirality, in contrast to common belief. Our calculations also reveal an interesting evolution of Fermiarc surface states and other topological surface states as a function of chemical potential in the presence of the external magnetic field.

Condensed Matter Seminar Thursday, May 3, 2018 3:30 PM Physics Building, Room 204 
ABSTRACT:
We discuss significant events in the recent Renaissance triggered by the enigmatic and elusive, but rich stochastic nonlinear PDE of Kardar, Parisi & Zhang,^1 a celebrated equation whose reach far exceeds its grasp, touching such diverse phenomena as nonequilibrium stochastic growth, optimal paths in illcondensed matter, as well as the extremal statistics of random matrices & increasing subsequences in random permutations.
1. J. Stat. Phys. 160, 794 (2015). 
Condensed Matter Seminar Thursday, April 26, 2018 11:00 AM Physics Building, Room 313 
"Frustrated Kondo chains and glassy magnetic phases on the pyrochlore lattice"Jing Luo , UVADepartment of Physics [Host: GiaWei Chern]
ABSTRACT:
We present an extensive numerical study of a new type of frustrated itinerant magnetism on the pyrochlore lattice. In this theory, the pyrochlore magnet can be viewed as a crosslinking network of Kondo or doubleexchange chains. Contrary to models based on Mott insulators, this itinerant magnetism approach provides a natural explanation for several spin and orbital superstructures observed on the pyrochlore lattice. Through extensive Monte Carlo simulations, we obtain the phase diagrams at two representative electron filling fractions $n = 1/2$ and 2/3. In particular, we show that an intriguing glassy magnetic state characterized by ordering wavevectors $\mathbf q = \langle \frac{1}{3},\frac{1}{3}, 1\rangle$ gives a rather satisfactory description of the low temperature phase recently observed in spinel~GeFe$_2$O$_4$. 
Condensed Matter Seminar Tuesday, April 24, 2018 11:00 AM Physics Building, Room 313 
"Crystal Structures and Photoluminescence of a TwoDimensional Perovskite"Depei Zhang , UVA  Department of Physics [Host: Seunghun Lee]
ABSTRACT:
Arguably the biggest challenge of the highefficiency perovskite solar cells, such as CH_{3}NH_{3}PbI_{3} and CH(NH_{2})_{2}PbI_{3}, is their device instability. A recent study of 2D perovskite compounds, butylammonium methylammonium lead iodide perovskite, [CH_{3}(CH_{2})_{3}NH_{3}]_{2}(CH_{3}NH_{3})_{n1}Pb_{n}I_{3n+1}, proposed a solution to this problem. This class of materials shows a maximum photovoltaic efficiency of 12.52%, without any obvious degradation over thousands of hours under standard light illumination and humidity test. This talk focuses on the study of temperaturedependent crystal structures, along with the photovoltaic properties of the 2D 1layer (n = 1) perovskite material. We have performed elastic and inelastic neutron scattering, Raman scattering, and photoluminescence measurements on a powder sample of the 1layer system ([CH_{3}(CH_{2})_{3}NH_{3}]_{2}PbI_{4}). Our analysis of the data illuminates the evolution of the lattice structure, rotational and vibrational dynamics with temperature, and their connection to the charge carrier lifetime of the solar cell will be discussed. 
Condensed Matter Seminar Thursday, April 19, 2018 11:00 AM Physics Building, Room 313 
"Opportunities in Quantum Materials Research using Neutrons"Clarina Dela Cruz , Oak Ridge National Laboratory [Host: Despina Louca]
ABSTRACT:
Quantum materials will arguably be the key materials to push forward the forefront energy relevant technologies of the future. Having two powerful neutron sources at the Oak Ridge National Laboratory, enables us to be positioned strongly to use neutron scattering in unveiling the structure of, and dynamics in quantum systems that lead to fundamental understanding and control of quantum phenomena such as coherence, entanglement and novel emergent states. quantum critical phenomena among others, with a range of correlation strength in them. With the increasing progress in instrumentation, the instruments at ORNL are able to study systems in extreme environments of ultralow temperatures, high magnetic field as well as high pressure. In this talk, I will give several examples of quantum materials problems where neutron scattering played a crucial role with some prospects for possible studies in molecular magnets in particular. 
Condensed Matter Seminar Thursday, April 12, 2018 11:00 AM Physics Building, Room 313 
"New Directions in Theoretical Studies of High Tc Superconductors"Adriana Moreo , University of Tennessee [Host: Despina Louca]
ABSTRACT:
The discovery of high critical temperature superconductivity in ironbased pnictides and chalcogenides brought to the forefront the need to develop efficient theoretical procedures to treat multiorbital models of interacting electrons. Among the many challenges, we need to clarify the role that the orbital degree of freedom plays in pairing and how its interaction with magnetic and lattice degrees of freedom leads to the stabilization of exotic phases such as the nematic state. Theoretical studies in the strong and weak coupling limits cannot address the physically relevant intermediate regime, with a mixture of itinerant and localized degrees of freedom. Traditional numerical methods, such as Lanczos or quantum Monte Carlo, have either a too rapidly growing Hilbert space with increasing size or sign problems. For this reason, it is necessary to develop new models and techniques, and also better focus on systems where both experiments and accurate theory can be used in combination to reach a real understandingof iron pairing tendencies. Examples of recent advances along these directions that will be discussed in this talk include: 
Condensed Matter Seminar Thursday, March 29, 2018 11:00 AM Physics Building, Room 313 
"Local structure and the JahnTeller effect in TiSe2xTex charge density waves"Aaron Wegner , UVA Department of Physics [Host: Despina Louca]
ABSTRACT:
Transition metal dichalcogenide (TMDC) materials exhibit a wide variety of interesting physical phenomena. This diverse family of materials forms a quasitwo dimensional layered hexagonal structure of XMX sandwiches (M= Ti, Mo, Hf, W, etc.., X= S, Se, Te) that, depending on composition, may be semiconducting, metallic, or superconducting and many undergo charge density wave transitions. As the materials are layered and can be exfoliated, interest in the TMDCs has increased due to the search for graphenelike materials and the importance of thin film applications. One particularly interesting material is TiSe_{2}, which forms a prototypical commensurate CDW that occurs in the vicinity of superconductivity. The origin of this CDW phase is controversial and has alternatively been attributed to exciton condensation or several possible JahnTeller type mechanisms. I will discuss how neutron scattering and local structure refinements give insight into the effect of the lattice on CDW formation in TiSe_{2} and the doping series in which Te is substituted for Se. 
Condensed Matter Seminar Thursday, March 1, 2018 11:00 AM Physics Building, Room 313 
"Phase transitions and mode coupling in PbZrO3 and Zrrich PbZr1xTixO3 solid solutions"Sergey Vakhrushev , Ioffe Institute, St. Petersburg, Russia [Host: Despina Louca]
ABSTRACT:
(PbZrO3)1x(PbTiO3)x (PZT) solid solutions probably represent the most studied group of functional dielectrics materials. For many years the main efforts were devote to the study of the PZT compounds at the morphotropic boundary region (x≈0.5). However recently Zrrich (x<0.06) compounds attracted new attention due to their potential for the electric energy storage and electricaloric application. Beside possible application related interest these crystals demonstrate extremely reach phase diagram including antiferroelectric, ferroelectric and incommensurate phases. In my presentation I would like to concentrate on the dynamical features related to the phase transitions in the Zrrich PZT (including PbZrO3 itself) and in the PbHfO3. In the papers [1,2] we demonstrated that the antiferroelectric phase transition In PbZrO3 with order parameter described by the wavevector q_{AFE}=(¼ ¼ 0) can be considered as a missed incommensurate transition with some arbitrary wavevector, corresponding to the flat part of the dispersion curve of the TA mode. Later in PbHfO_{3} and PbZrO_{3} at high pressure the minima at the TA dispersion curves were found [3,4], resulting in the realization of the incommensurate phases. Extremely complicated diffraction pattern is observed in the PZr_{1x}Ti_{x}O_{3} crystals with x<0.06. In the intermediate phase between the paraelectric and antiferroelectric phases incommensurate phase is sometime observed similar to that in the PbZrO_{3} under high presuure. And in addition complicated system of the satellite peaks in the vicinity of the q_{M}=(½ ½ 0) including first order and second order satellites exists. In addition to the satellite peaks near the Mpoints we found second order satellites near the main Bragg peaks. Observed diffraction pattern can be fully described by the incommensurate structure determined by 2 wavevectors from the same star: q_{1}=(0.5+δ 0.5δ δ) and q_{2} =(0.5δ δ 0.5+ δ). Combination of the q1 and q2 describes all observed superstructure peaks. Creation of the true incommensurate phase can be attributed to the mode softening not at q_{M}, but at a position shifted from the zone boundary. Such unusual soft mode can be described in terms of the coupling of 2 modes in the vicinity of Mpoint, namely TA mode and oxygen tilt mode. Such coupling is forbidden at q_{M} but became allowed aside of it. Proposed model provides qualitative agreement with the results of the inelastic and diffuse Xray scattering measurements
References

Condensed Matter Seminar Thursday, February 1, 2018 11:00 AM Physics Building, Room 313 
"Searching the origin of the knee of cosmic ray spectrum LHAASO experiment"Cunfeng Feng , Shandong University [Host: Xiaochao Zheng]
ABSTRACT:
The curve of cosmic ray energy spectrum is turned around PeV region, which is called "the knee" of the cosmic ray spectrum. The origin of the PeV knee of cosmic ray spectrum remains a puzzle since its first discovery near 70 years ago. Searching for the origin of the knee is one of main aims of LHAASO experiment, a hybrid cosmic ray observatory, which is building on Haizi mountain, south of China. In this talk, the puzzle of the knee of cosmic ray spectrum will be introduced briefly together with the LHAASO experiment introduction. Then I will focus on the technique of scintillator detector of LHAASO and the photomultiplier tube used in this detector. The PMT test bench of Shandong University will also will be introduced. 
Condensed Matter Seminar Thursday, January 25, 2018 3:30 PM Physics Building, Room 204 Special Nuclear Seminar 
"Interface symmetry and nonhelical states in topological insulatorsemiconductor heterostructures"Ilya Vekhter , Louisiana State University [Host: Utpal Chatterjee]
ABSTRACT:
Heterostructures combining topological and nontopological materials constitute the next frontier in the effort to incorporate topological insulators (TIs) into functional electronic devices. I show that the properties of the interface states appearing at the planar boundary between a topologicallytrivial semiconductor (SE) and a TI are qualitatively different from those at the vacuum surface, and are controlled by the symmetry of the interface. In contrast to the wellstudied helical Dirac surface states, SETI interface states exhibit elliptical contours of constant energy and complex spin textures with broken helicity. Experimental signatures include out of plane spin accumulation under a transport current and the opening of a spectral gap that depends on the direction of an applied inplane magnetic field. I will also discuss how symmetry breaking at the interface controls proximityinduced superconductivity of the TI surface state. 
Condensed Matter Seminar Thursday, November 16, 2017 11:00 AM Physics Building, Room 313 
ABSTRACT:
Carbon is one of the most fascinating elements in the periodic table. It not only forms the basis of all life on the Earth but also it is important to technology. The unique properties of carbon emerge from its ability to form diverse sp^{n} (1 < n < 3) bonds. Until 1960’s graphite with sp^{2} and diamond with sp^{3 }bonding were the most common forms of carbon known. The discovery of onedimensional (1D) chainlike polymer called “carbyne” in 1960 and later zerodimensional (0D) carbon fullerenes, 1D carbon nanotube, and twodimensional (2D) graphene, all with novel properties characteristic of their reduced dimensionality and size, has ushered a new era in carbon science. In recent years many new metastable forms of carbon exhibiting a mixture sp^{1},^{ }sp^{2} and/or sp^{3} bonding pattern have also emerged. In this talk I will focus on the carbon allotropes that have been studied in our group^{17}. These include functionalized C_{60 }fullerenes for hydrogen storage^{1, 2}, semihydrogenated graphene for metalfree ferromagnet^{3}, metalorganic complexes with large electron affinity^{4}, 3D metallic carbon made of hybridized sp^{2} and sp^{3} bonded atoms^{5}, a Cairotilling inspired quasi2D pentagraphene made of only carbon pentagons,^{6} and its thermal conductivity^{7}. All calculations have been carried out using gradient corrected density functional theory. Thermodynamic stability of the above carbon allotropes is confirmed by total energy calculations as well as quantum molecular dynamics. Potential applications of some of these carbon allotropes will be discussed.

Condensed Matter Seminar Thursday, November 9, 2017 11:00 AM Physics Building, Room 313 
"Realspace condensation in mass transport models: statics, dynamics, and large deviations"Ori Hirschberg [Host: Israel Klich ]
ABSTRACT:
The formation of traffic jams on highways, the clustering of particles in shaken granular gases, and the emergence of macroscopicallylinked hubs in complex networks are all examples of realspace condensation. This realspace analogue of BoseEinstein condensation is rather ubiquitous in nonequilibrium systems. In this talk, I shall present some of the insights into this phenomenon garnered from the study of prototypical toy models. After reviewing static properties of the condensation phase transition, I shall focus on two unexpected features recently discovered: (1) Spatial correlations, which generically exist in driven systems, may give rise to a collective motion of the condensate through the system. Using simplified models, the mechanism behind this motion is explained and shown to be rather robust. (2) Rare fluctuations with extremely atypical currents may lead to condensate formation in systems that otherwise do not condense. I will present microscopic and macroscopic approaches to analyze this novel scenario of condensation. 
Condensed Matter Seminar Thursday, November 2, 2017 11:00 AM Physics Building, Room 313 
"Dissipation bounds on farfromequilibrium fluctuations"Jordan Horowitz , MIT [Host: Marija Vucelja]
ABSTRACT:
Near equilibrium, linear response theory has proven to be a powerful tool. At its core is the fluctuationdissipation theorem, which dictates that the variance of small fluctuations is intimately related to dissipation. However, far from equilibrium no such equality exists. I will show that arbitrarily far from equilibrium dissipation still plays a dominant role in shaping fluctuations, both small and large, through some novel inequalities. In particular, I will discuss the thermodynamic uncertainty relation and its variants, which are universal nonequilibrium constraints between the variance of fluctuations and dissipation. These predictions offer general design principles for engineering artificial and natural nanodevices under energy restrictions. 
Condensed Matter Seminar Thursday, October 26, 2017 11:00 AM Physics Building, Room 313 
"Skyrmion Lattices in Random and Ordered Potential Landscapes"Charles Reichhardt , Los Alamos National Lab [Host: GiaWei Chern]
ABSTRACT:
Since the initial discovery of skyrmion lattices in chiral magnets [1], there has been a tremendous growth in this field as an increasing number of compounds are found to have extended regions of stable skyrmion lattices [2] even close to room temperature [3]. These systems have significant promise for applications due to their size scale and the low currents or drives needed to move the skyrmions [4]. Another interesting aspect of skyrmions is that the equations of motion have significant nondissipative terms or a Magnus effect which makes them unique in terms of collective driven dynamics as compared to other systems such as vortex lattices in typeII superconductors, sliding charge density waves, and frictional systems. We examine the driven dynamics of skyrmions interacting with random and periodic substrate potentials using both continuum based modelling and particle based simulations. In clean systems we examine the range in which skyrmion motion can be explored as a function of the magnetic field and current and show that there can be a currentinduced creation or destruction of skyrmions. In systems with random pinning we find that there is a finite depinning threshold and that the Hall angle shows a strong dependence on the disorder strength. We also show that features in the transport curves correlate with different types of skyrmion flow regimes including a skyrmion glass depinning/skyrmion plastic flow region as well as a transition to a dynamically reordered skyrmioncrystal at higher drives. We find that increasing the Magnus term produces a low depinning threshold which is due to a combination of skyrmions forming complex orbits within the pinning sites and skyrmionskyrmion scattering effects. If the skyrmions are moving over a periodic substrate, with increasing drive the Hall angle changes in quantized steps which correspond to periodic trajectories of the skyrmion that lock to symmetry directions of the substrate potential. 
Condensed Matter Seminar Thursday, October 19, 2017 11:00 AM Physics Building, Room 313 
"Topological mechanics and hidden symmetry in rigid origami"Bryan Chen , University of Pennsylvania [Host: Jeffrey Teo]
ABSTRACT:
The study of topological invariants in band theory has led to many insights into the behavior of electrons in insulators and superconductors. In 2013, Kane and Lubensky pointed out that certain "isostatic" mechanical systems can also admit topological boundary modes [1]. This has led to the design and realization of several families of "topological mechanical metamaterials". In my talk I will introduce the "topological polarization" of Kane and Lubensky and then explain how it can be realized in certain mechanical structures, called rigid origami and kirigami, which consist of rigid plates joined by hinges meeting at vertices [2]. Mysteriously, we found in [2] that triangulated origami structures always seem to be unpolarizable, that is, despite the lack of any apparent symmetry, all of these structures have a vanishing polarization invariant. I will describe recent work with Zeb Rocklin (Georgia Tech), Louis Theran (St. Andrews) and Chris Santangelo (UMass Amherst) which explains this via a "motion to stress" correspondence, that generalizes the 19thcentury MaxwellCremona correspondence in several directions.
[1] C.L. Kane and T.C. Lubensky, Nat Phys 10, 39–45 (2014). [2] B.G. Chen, B. Liu, A.A. Evans, J. Paulose, I. Cohen, V. Vitelli, and C. Santangelo, Phys Rev Lett 116, 135501 (2016). 
Condensed Matter Seminar Thursday, October 5, 2017 11:00 AM Physics Building, Room 313 
"Geometric theory of nonlocal transport in metals"Dmytro Pesin , University of Utah [Host: GiaWei Chern]
ABSTRACT:
I will discuss the topological and geometric aspects of optical 
Condensed Matter Seminar Thursday, September 28, 2017 11:00 AM Physics Building, Room 313 
"Structure and dynamics of hydrogen in nanocrystalline palladium"Maiko Kofu , JPARC, Japan [Host: Despina Louca]
ABSTRACT:
The behavior of hydrogen in metals has attracted much attention in fundamental and applied research areas. Palladium hydride (PdHx) is a typical metalhydrogen system and has been studied for many decades. Pd has remarkable abilities to absorb plenty of H atoms and the H atoms are highly mobile in the Pd lattice. It is interesting to examine how the properties are changed as the particle size is reduced to a nanometerscale. We have investigated structure, diffusion and vibrational dynamics by means of neutron scattering techniques for both bulk and nanocrystalline PdHx with a diameter of 8nm. Neutron diffraction work on nanocrystalline sample demonstrated that some of hydrogen atoms are accommodated at the tetrahedral (T) sites. This is in contrast to bulk PdHx with octahedral (O) occupation. In quasielastic scattering measurements, we found an additional fast diffusion process with a small potential barrier in nanocrystalline PdHx. Furthermore, our inelastic scattering works revealed that nanocrystalline PdHx exhibits two distinct vibrational excitations; one resembles that observed in bulk PdHx and the other is the excitations appeared at higher energies. The additional diffusion process and vibrational states are attributed to the H atoms at T sites near the surface of nanoparticles. The potential shape around the T site near the surface will be discussed in the seminar. 
Condensed Matter Seminar Thursday, September 7, 2017 11:00 AM Physics Building, Room 313 
"SUBATOMIC STATES IN CONDENSED MATTER AND MAGNETORESISTANCE OF SUPERCONDUCTORS"Boris I. Ivlev , Landau Institute and Instituto de Fisica, UASLP [Host: Genya Kolomeisky]
ABSTRACT:
In the recent experiments, PRB 94, 054504 (2016), the unusual oscillatory magnetore sistance of superconductors was discovered with a periodicity essentially independent on magnetic ﬁeld direction and even material parameters. The nearly universal period points to a subatomic mechanism of the phenomenon. This mechanism is related to formation inside samples of subatomically thin (10−11cm size) threads in the form of rings of the interatomic radius. Electron states of rings go over into conduction electrons which carry the same spin imbalance in energy as the ring. The imbalance occurs due to the spin interaction with the orbital momentum of the ring. The conductivity neat Tc is determined by ﬂuctuating Cooper pairs consisting of electrons with shifted energies. Due to diﬀerent angular momenta of rings these energies periodically depend on magnetic ﬁeld resulting in the observed oscil latory magnetoresistance. Calculated universal positions of peaks (n + 1)∆H (∆H '"" 0.18T and n = 0, 1, 2, ...) on the R(H) curve are in a good agreement with measurements. 
Condensed Matter Seminar Thursday, April 27, 2017 11:00 AM Physics Building, Room 313 
"Surfaces and Slabs of Fractional Topological Insulators"Alexander Sirota , UVADepartment of Physics [Host: Jeffrey Teo]
ABSTRACT:
Fractional topological insulators (FTI) are electronic topological phases in (3 + 1) dimensions enriched by time reversal (TR) and charge U(1) conservation symmetries. We focus on the simplest series of fermionic FTI, whose bulk quasiparticles consist of decon ned partons that carry fractional electric charges in integral units of e = e=(2n + 1) and couple to a discrete Z2n+1 gauge theory. We propose massive symmetry preserving or breaking FTI surface states. Combining the longranged entangled bulk with these topological surface states, we deduce the novel topological order of quasi(2 + 1) dimensional FTI slabs as well as their corresponding edge conformal eld theories. We will also describe some past work on coupled wire constructions of Weyl/Dirac semimetals, and some new work about mechanical topology and conformal field theory. 
Condensed Matter Seminar Wednesday, April 26, 2017 5:00 PM Physics Building, Room 313 
"Unusual magnetic order in some frustrated spin systems: "Bhanu Mahanti , Michigan State University [Host: Utpal Chatterjee]
ABSTRACT:
Classical and quantum spin systems have played an enormously important role in the study of magnetism. They provide excellent models to understand the ground state magnetic structure and low energy excitations of magnetic systems. In addition, they provide a rich arena for studying the physics of interacting manybody systems. In this talk I will discuss a very specific problem, namely the observation of an unusual ordering of spins, the so called uudd states, in quasi onedimensional Heisenberg spin chains and anisotropic twodimensional Heisenberg spin systems. Theoretical work to understand this and some related issues using both model spin Hamiltonians with competing interactions, exchange anisotropy, biquadratic exchange etc. and ab initio electronic structure calculations, will be discussed. 
Condensed Matter Seminar Thursday, April 13, 2017 11:00 AM Physics Building, Room 313 
"Symmetry preserving gapping of Topological Weyl and Dirac semimetals"Syed Raza , UVA Physics Department [Host: Jeffrey Teo]
ABSTRACT:
Topological Weyl and Dirac semimetals in three dimensions have gapless (massless) Weyl fermions. These semimetals were predicted theoretically in 2011 (Vishwanath et al, Burkov and Balents) and have been experimentally discovered recently in TaAs, TaP, NbAs, NbP (Weng et al., Huang et al., Shekhar et al., 2015). Although these gapless systems can be gapped (given mass) trivially by symmetry breaking terms, a more interesting problem is if they can be gapped without breaking symmetries? We show that you can indeed do this by introducing many body interactions. We also show that this symmetrypreserving gapping gives pointlike and linelike anyonic excitations in the gapped bulk. 
Condensed Matter Seminar Thursday, April 13, 2017 3:30 PM Physics Building, Room 204 
"Understanding glassy systems using population annealing"Jon Machta , University of Massachusetts [Host: Marija Vucelja]
ABSTRACT:
Population annealing is a sequential Monte Carlo method for studying the equilibrium properties of glassy systems characterized by rough free energy landscapes. In this talk I will introduce two glassy systems, the Ising spin glass and a glassforming binary mixture of hard spheres. I will describe population annealing and some of its useful features, and then present results from simulations of these glassy systems. 
Condensed Matter Seminar Thursday, April 6, 2017 11:00 AM Physics Building, Room 313 
"Understanding and Controlling Organic Molecule and Metal Organic Framework Crystallization "Gaurav Giri , University of Virginia, Chemical Engineering [Host: Seunghun Lee]
ABSTRACT:
Small organic molecules have had a dramatic impact on our health and daily life over the past century. Small molecule pharmaceuticals have increased human lifespans, and organic pigments have expanded in use in textiles and displays. This impact is set to accelerate in the near future, as small molecules are explored for novel applications such as organic electronics, or metalorganic frameworks for chemical separations, catalysis and sensing. One of the major barriers for using small organic molecules for new applications is the limited understanding we possess on how molecules aggregate together to form different crystal habits and phases. Different crystal structures and morphologies can have wildly varying physical, chemical and physiological properties. Thus, if we do not control the crystallization organic molecules, we cannot predict its behavior for the aforementioned applications. Understanding the crystallization process can also help form metastable phases. These metastable phases can be more useful than the equilibrium phase for many applications. Metastable phases permit tunable optical bandgap for optoelectronics, control over pore size and shape in metal organic frameworks (MOF), and increased bioavailability in pharmaceuticals. General methods used to create metastable phases, like confinement or rapid cooling, require small length scales and extreme rates of heat and mass transfer. Moreover, these processes need precise control to get reliable results. This talk will focus on flow coating and microfluidic methods of controlling organic molecule and MOF crystallization characteristics, and the use of these materials for various applications. 
Condensed Matter Seminar Thursday, March 30, 2017 11:00 AM Physics Building, Room 313 
"Energetics of a heat engine: a molecular dynamics simulation study"Mulugeta Bekele , Addis Ababa University, Ethiopia [Host: Bellave Shivaram]
ABSTRACT:
I first consider an elevator as a system that lifts a load from a ground floor to a top floor by consuming an input energy. I ask: what amount of load enables the system to perform maximum amount of power? Can I design a mode of operation where I can utilize good enough amount of the input energy better than that of energy utilized at maximum power? I will then go over to a molecular dynamics simulation study of a heat engine where the working substance is a real gas and try to address similar questions as the elevator system. 
Condensed Matter Seminar Friday, March 24, 2017 9:00 AM Physics Building, Room 205 
"The Haldane phase in spin chains with SU(N) symmetry"Abhishek Roy , Institute for Theoretical Physics, University of Cologne [Host: Jeffrey Teo]
ABSTRACT:
Spin chains with PSU(n) symmetry are known to have symmetry protected topological phases distinguished by boundary states. We determine the parent Hamiltonians of some of these phases at the AKLT point. In the adjoint representation we use a graphical method that produces expressions for arbitrary N. Finally we discuss the fate of the Haldane conjecture. 
Condensed Matter Seminar Thursday, March 23, 2017 11:00 AM Physics Building, Room 313 
"Dynamics of KitaevHeisenberg model on a Honeycomb lattice in the classical limit"Anjana Samarakoon , UVADepartment of Physics [Host: Seunghun Lee]
ABSTRACT:
Quantum spin liquids (QSLs) have achieved great interest in both theoretical and experimental condensed matter physics due to their remarkable topological properties. Among many different candidates, the Kitaev model on the honeycomb lattice is a 2D prototypical QSL which can be experimentally studied in materials based on iridium or ruthenium.. Here we study the spin1/2 Kitaev model using classical MonteCarlo and semiclassical spin dynamics of classical spins on a honeycomb lattice. Both real and reciprocal space pictures highlighting the differences and similarities of the results to the linear spin wave theory will be discussed in terms dispersion relations of the pureKitaev limit and beyond. Interestingly, this technique could capture some of the salient features of the exact quantum solution of the Kitaev model, such as features resembling the Majoranalike mode comparable to the Kitaev energy, which is spectrally narrowed compared to the quantum result, can be explained by magnon excitations on fluctuating onedimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of a magnon to Majorana fermions. The calculations will be directly compared with our neutron scattering data on αRuCl3 which is a prime candidate for experimental realization of Kitaev physics. 
Condensed Matter Seminar Thursday, February 23, 2017 11:00 AM Physics Building, Room 313 
"Between topological strings and topological phases"Jeffrey Teo , UVA  Department of Physics [Host: Eugene Kolomeisky]
ABSTRACT:
Topological phases in two and three dimensions can be theoretically constructed by coupledwire models whose fundamental constituents are electronic channels along strings. On the other hand the collective topological phases support further fractionalized emergent quasistring excitations or defects such as flux vortices. In this talk I will describe topological superconductors and Dirac (or Weyl) semimetals using coupledwire models, and discuss the fractional behavior of emergent topological strings. 
Condensed Matter Seminar Thursday, February 2, 2017 3:30 PM Physics Building, Room 204 
"Resonant inelastic Xray scattering response of the Kitaev spin liquid"Gabor Halasz , KITP [Host: Marija Vucelja]
ABSTRACT:
We propose that resonant inelastic Xray scattering (RIXS) is an effective probe to detect spinliquid character in potential material incarnations of the Kitaev spin liquid (such as the honeycomb iridates and ruthenium chloride). Calculating the exact RIXS response of the Kitaev honeycomb model, we find that the fundamental RIXS channels, the spinconserving (SC) and the nonspinconserving (NSC) ones, can probe the fractionalized excitations of the Kitaev spin liquid separately. In particular, SC RIXS picks up the gapless Majorana excitations with a pronounced momentum dispersion, while NSC RIXS creates immobile flux excitations, thereby rendering the response weakly momentum dependent. 
Condensed Matter Seminar Thursday, January 19, 2017 11:00 AM Physics Building, Room 204 
"Formation probabilities, Postmeasurement entanglement entropy and Casimir effect "Mohammed Ali Rajabpour , Fluminense Federal University [Host: Israel Klich ]
ABSTRACT:
I will first introduce formation probability as a quantity which can determine the universality class of a quantum critical system. In other words, by calculating this quantity one can find the central charge and critical exponents of the quantum system. We will show that calculating this quantity boils down to finding Casimir energy of two needles. Then we will briefly talk about Shannon mutual information as another quantity which can play similar role. Finally, we will introduce postmeasurement entanglement entropy as a tripartite measure of entanglement. We will show that this quantity is related to the Casimir energy of needles on Riemann surfaces and can be calculated exactly for conformal field theories. 
Condensed Matter Seminar Tuesday, January 17, 2017 11:00 AM Physics Building, Room 313 
"Spheres form strings, and a swimmer from a spring"Daphne Klotsa , University of North Carolina at Chapel Hill [Host: Marija Vucelja]
ABSTRACT:
Rigid spherical particles in oscillating fluid flows form interesting patterns as a result of fluid mediated interactions. Here, through both experiments and simulations, we show that two spheres under horizontal vibration align themselves at right angles to the oscillation and sit with a gap between them, which scales in a nonclassical way with the boundary layer thickness. A large number of spherical particles form strings perpendicular to the direction of oscillation. Investigating the details of the interactions we find that the driving force is the nonlinear hydrodynamic effect of steady streaming. We then design a simple swimmer (twospheresandaspring) that utilizes steady streaming in order to propel itself and discuss the nature of the transition at the onset of swimming as the Reynolds number gradually increases. We discuss implications and connections to biological systems, motility, and collective behavior of swimmers. 
Condensed Matter Seminar Thursday, December 1, 2016 11:00 AM Physics Building, Room 313 
"OPTICS IN FLATLAND: RAMAN SPECTROSCOPIC PROBES OF STRUCTURAL SYMMETRY"Patrick Vora , George Mason University [Host: Utpal Chatterjee]
ABSTRACT:
The progress of modern technology is increasingly driven by the development and evaluation of novel materials. Layered twodimensional (2D) materials, collectively referred to as van der Waals solids, are receiving intense interest in the community due to their unconventional and diverse electronic behaviors. Transition metal dichalcogenides (TMDs) are a class of 2D material following the basic chemical formula MX2, where M = (Mo, W, Nb, Re, …) and X = (S, Se, Te, …). Varying the chemical composition allows access to semiconducting, semimetallic, superconducting, or magnetic behaviors in the 2D limit. Of recent interest are telluride based TMDs such as MoTe2 and WTe2 which are predicted exhibit controllable structural phase transitions appropriate for phase change memory applications as well as topologically protected and spin polarized electronic states. In this colloquium, I will present our efforts to understand the temperaturedependent optical properties of MoTe2 and MoxW1xTe2 using temperaturedependent and polarizationresolved Raman spectroscopy. We have used this technique to identify the anharmonic contributions to the optical phonon modes in bulk MoTe2 occupying the distorted orthorhombic (Td) lattice structure. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature however, below 100 K we observe nonlinear frequency shifts in some modes. We show that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near 250 K that correlate well with a structural phase transition. We also explore the compositiondependent optical properties of MoxW1xTe2 alloys. Our observations identify signatures of the hexagonal (H), monoclinc (1T’) and Td structural phases. Polarizationresolved Raman measurements allow for the assignment of all vibrational modes as well as the evolution of mode symmetry and frequency with composition. We discover a previously unobserved WTe2 mode as well as a Ramanforbidden MoTe2 mode that is activated by compositional disorder. The primary WTe2 Raman peak is asymmetric for x <= 0.1, and is well fit by the spatial correlation model. From these fits, we extract the spatial phonon correlation length which serves as an indirect measure of the WTe2 domain size. Our study is foundational for future studies of MoxW1xTe2 and provides new insights into the impact of disorder in transition metal dichalcogenides. 
Condensed Matter Seminar Thursday, November 17, 2016 11:00 AM Physics Building, Room 313 
"Conductance of a superconducting Coulomb blockade nanowire at finite temperature"ChingKai Chiu , University of Maryland [Host: Jeffrey Teo]
ABSTRACT:
By applying a magnetic field, a superconducting proximity nanowire in the presence of spinorbital coupling can pass through topological phase transition and possesses Majorana bound states on the ends. One of the promising platforms to detect the Majorana modes is a coulomb blockade island by measuring its twoterminal conductance (S. M. Albrecht et al., Nature (London) 531, 206 (2016)). Here, we study the transportation of a single electron across the superconducting Coulomb blockade nanowire at finite temperature to obtain the generic conductance equation. By considering all possible scenarios that Majorana modes appear in the nanowire, we compute the nanowire conductance as the magnetic field and the gate voltage of the nanowire vary. The oscillation behavior of the conductance peak is temperature independent and the oscillation amplitude of the conductance peak spacings increases as the magnetic field increases. 
Condensed Matter Seminar Friday, November 11, 2016 3:30 PM Physics Building, Room 204 
"Interacting helical liquids in bilayer graphene"Chaoxing Liu , Penn State University [Host: Jeffrey Teo]
ABSTRACT:
A grand challenge in the field of topological physics is to understand the role of interaction and to realize interacting topological phases in realistic materials. In this talk, we will discuss the possible realization of interacting topological states in bilayer graphene under a strong magnetic field. We start from a fermonic twochannel quantum spin Hall state with two copies of helical edge states, which have been demonstrated experimentally in bilayer graphene. By introducing interaction into two channel helical liquids, we demonstrate that all the fermion degrees are gapped out and only one bosonic mode remains, thus yielding a bosonic version of topological insulator, dubbed “bosonic symmetry protected topological state”. Physically, the two dual boson fields of this bosonic mode carry charge2e and spin1, respectively, due to the helical nature. Thus, we dubbed them “bosonic helical liquids”. We further study the transport of a quantum point contact for bosonic helical liquids and compare them to fermonic twochannel helical liquids. A novel charge insulator/spin conductor phase is identified in the weak repulsive interaction regime for bosonic helical liquids while charge insulator/spin insulator or charge conductor/spin conductor phase is present for fermonic twochannel helical liquids. Thus, a quantum point contact experiment will allow us to identify the bosonic symmetry protected topological states unambiguously. Similar physics can also emerge in topological mirror Kondo insulators, such as SmB6.
Reference: [1] Bilayer Graphene as a platform for Bosonic Symmetry Protected Topological States, Zhen Bi, Ruixing Zhang, YiZhuang You, Andrea Young, Leon Balents, ChaoXing Liu, Cenke Xu, arXiv:1602.03190v1, 2016 [2] Interacting topological phases in thin films of topological mirror Kondo insulators, RuiXing Zhang, Cenke Xu, ChaoXing Liu, arXiv: 1607.06073, 2016 [3] Fingerprints of bosonic symmetry protected topological state in a quantum point contact, Ruixing Zhang, Chaoxing Liu, arxiv: 1610.01236, 2016 
Condensed Matter Seminar Thursday, November 10, 2016 11:00 AM Physics Building, Room 313 
"Spin and orbital interactions in complex oxide heterostructures and transient states probed by xrays"Mark Dean , Brookhaven National Lab [Host: Israel Klich ]
ABSTRACT:
The spin and orbital degrees of freedom play a crucial role in determining the remarkable properties of transition metal oxide materials. In this talk, I will describe how resonant inelastic xray scattering (RIXS) opens up important new possibilities for measuring these degrees of freedom even in challenging cases such a heterostructures and transient states [1]. This includes determining how orbitals are modified within LaNiO3based heterostructures [2] and characterizing the spin behavior within the ultrafast transient state of photodoped Sr2IrO4 [3]. References 1. M. P. M. Dean et al., Nature Materials 11, 850 (2012); M. P. M. Dean et al., Nature Materials 12, 1019–1023 (2013) 2. G. Fabbris et al., Phys. Rev. Lett. 117, 147401 (2016) 3. M. P. M. Dean et al., Nature Materials 15, 601605 (2016) 
Condensed Matter Seminar Thursday, October 27, 2016 11:00 AM Physics Building, Room 313 
"Firstprinciples predictions of thermodynamically stable twodimensional electrides"Mina Yoon , Center for Nanophase Materials Sciences, Oak Ridge National Laboratory [Host: Seunghun Lee]
ABSTRACT:
Twodimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. Experimentally, however, 2D electrides have been only found in a couple of layered nitrides and carbides. Here, we report new thermodynamically stable alkalineearth based 2D electrides by using a firstprinciples global structure optimization method, phonon spectrum analysis, and molecular dynamics simulation. The method was applied to binary compounds consisting of alkalineearth elements as cations and group VA, VIA, or VIIA nonmetal elements as anions. We revealed that the stability of the layered 2D electride structure is closely related to the cation/anion size ratio; stable 2D electrides possess a sufficiently large cation/anion size ratio to minimize electrostatic energy among cations, anions, and anionic electrons. Our work demonstrates a new avenue to the discovery of thermodynamically stable 2D electrides beyond the material database and provides new insight into the principles of electride design. 
Condensed Matter Seminar Monday, October 24, 2016 11:00 AM Physics Building, Room 313 
"The Causes of Metastability and Their Effects on Transition Times"Katie Newhall , University of North Carolina at Chapel Hill [Host: Marija Vucelja]
ABSTRACT:
Many experimental systems can spend extended periods of time relative to their natural time scale in localized regions of phase space, transiting infrequently between them. This display of metastability can arise in stochastically driven systems due to the presence of large energy barriers, or in deterministic systems due to the presence of narrow passages in phase space. To investigate metastability in these different cases, I take a Langevin equation and determine the effects of small damping, small noise, and dimensionality on the dynamics and mean transition time. Of particular interest is what happens in the infinite dimensional limit, a stochastic partial differential equation, and the question of what ensemble this system appears to sample over time. Both analytical and numerical results will be presented. 
Condensed Matter Seminar Thursday, October 20, 2016 11:00 AM Physics Building, Room 313 
"Anomalous cooling and heating â the Mpemba effect and its inverse"Oren Raz , University of Maryland [Host: Israel Klich ]
ABSTRACT:
Under certain conditions, it takes a shorter time to cool a hot system than to cool the same system initiated at a lower temperature. This phenomenon – the “Mpemba Effect” – is well known in water, and has recently been observed in other systems as well. However, there is no single generic mechanism that explains this counterintuitive behavior. Using the theoretical framework of nonequilibrium thermodynamics, we present a widely applicable mechanism for this effect, derive a sufficient condition for its appearance in Markovian dynamics, and predict an inverse Mpemba effect in heating: under proper conditions, a cold system can heat up faster than the same system initiated at a higher temperature. Our results suggest that it should be possible to observe the Mpemba effect and its inverse in a variety of systems, where they have never been demonstrated before. 
Condensed Matter Seminar Thursday, October 13, 2016 11:00 AM Physics Building, Room 313 
"Multiferroics by design with frustrated molecular magnets"Yoshitomo Kamiya , RIKEN [Host: GiaWei Chern]
ABSTRACT:
Geometric frustration in Mott insulators permits perturbative electron fluctuations controlled by local spin configurations [1]. An equilateral triangle (“trimer”) of spins with S = 1/2 is the simplest example, in which lowenergy degrees of freedom consist of builtin magnetic and electric dipoles arising from the frustrated exchange interaction. Such trimers can be weakly coupled to make multiferroics by design [2]. An organic molecular magnet known as TNN [3], with three S = 1/2 nitronyl nitroxide radicals in a perfect C3 symmetric arrangement, is an ideal building block as demonstrated by recent experiments on a single crystal comprising TNN and CH3CN. The fascinating thermodynamic phase diagram of this molecular crystal, TNN·CH3CN, is in excellent agreement with our theory, which predicts multiferroic behavior and strong magnetoelectric effects arising from an interplay between magnetic and orbital degrees of freedom [4]. Our study thus opens up new avenues for designing multiferroic materials using frustrated molecular magnets.
References: [1] L. N. Bulaevskii, C. D. Batista, M. V. Mostovoy, and D. I. Khomskii, Phys. Rev. B 78, 024402 (2008). [2] Y. Kamiya and C. D. Batista, Phys. Rev. Lett. 108, 097202 (2012). [3] Y. Nakano et al., Polyhedron 24, 2147 (2005). [4] Y. Kamiya et al., in preparation.

Condensed Matter Seminar Thursday, October 6, 2016 11:00 AM Physics Building, Room 313 
"Exploring metalinsulator transitions with optical microscopy and spectroscopy"Mumtaz Qazilbash , William and Mary [Host: Utpal Chatterjee]
ABSTRACT:
The study of metalinsulator, structural and magnetic phase transitions in materials with strongly interacting electrons is a challenging frontier of condensed matter physics. The challenge is to disentangle the contributions of charge, lattice, spin and orbital degrees of freedom to phase transitions. I will report on the optical properties of two materials that undergo thermallyinduced metalinsulator transitions accompanied by structural and/or magnetic instabilities: vanadium dioxide (VO2) and the manganite La0.67Sr0.33MnO3. Infrared microspectroscopy and microellipsometry measurements on crystals of VO2 reveal that its insulating phases are MottHubbard insulators, not Peierls insulators. Scanning nearfield infrared microscopy (SNIM) allows us to directly image nanoscale metallic puddles that appear at the onset of the firstorder metalinsulator transition (MIT) in VO2 films. We find that the patterns of metallic domains are reproducible upon repeated thermal cycles across the MIT and point to the important role of imperfections in films. In contrast to VO2, recent SNIM data shows time dependence of nearfield infrared amplitude in a La0.67Sr0.33MnO3 film which we attribute to fluctuating conductivity in the vicinity of its second order metalinsulator transition. 
Condensed Matter Seminar Thursday, September 15, 2016 11:00 AM Physics Building, Room 313 
"Topological Phase Transitions in Linenodal Superconductors"Gil Young Cho , Advanced Institute of Science and Technology, Korea [Host: Jeffrey Teo]
ABSTRACT:
Fathoming interplay between symmetry and topology of manyelectron wavefunctions has deepened understanding of quantum many body systems, especially after the discovery of topological insulators. Topology of electron wavefunctions enforces and protects emergent gapless excitations, and symmetry is intrinsically tied to the topological protection in a certain class. Namely, unless the symmetry is broken, the topological nature is intact. We show novel interplay phenomena between symmetry and topology in topological phase transitions associated with linenodal superconductors. The interplay may induce an exotic universality class in sharp contrast to that of the phenomenological LandauGinzburg theory. Hyperscaling violation and emergent relativistic scaling are main characteristics, and the interplay even induces unusually large quantum critical region. We propose characteristic experimental signatures around the phase transitions in three spatial dimensions, for example, a linear phase boundary in a temperaturetuning parameter phasediagram. 
Condensed Matter Seminar Thursday, September 8, 2016 11:00 AM Physics Building, Room 313 
"Gutzwiller paradigm and its first ab initio implementation in molecular and lattice systems"Jun Liu , Ames Lab [Host: GiaWei Chern]
ABSTRACT:
Predicting materialspecific physical properties is always a fascinating goal of physics research. The current stateoftheart methods for doing this are the density function theory (DFT) and its derivatives for lattice systems and Quantum Chemistry approaches for molecular systems. However, their performance is hindered when being applied to strongly correlated systems either due to their intrinsic limitations of being only valid in weakly correlated systems or not truly ab initio, or limited by system and basis set sizes. Thus it is highly expected to come up with new ideas to help tackle this research goal. In this talk, I will discuss the possibility of having such an alternative line of thought based on Gutzwiller wavefunction and effective KohnSham Hamiltonian, and introduce an implementation of this idea in our recent work. Discussion on alternative implementations, generality to different phases and interactions will be provided. 
Condensed Matter Seminar Tuesday, May 17, 2016 2:00 PM Physics Building, Room 313 
ABSTRACT:
A measurement of charge asymmetry in BetheHeitler (BH) pair production using the High Intensity GammaRay Source at the TriUniversities Nuclear Laboratory (TUNL) has been approved and is undergoing preparation. These preparations include theoretical computation, support and target construction, as well as detector testing and simulation. A GEANT4 simulation has been created in order to optimize the experimental setup, to make predictions of statistics, and to identify systematic uncertainties. Following a brief introduction about the physics and the experimental preparations, the following will be discussed: 1) relationship of the simulation to the BH experiment and their difference, 2) basic simulation process for the experiment, 3) simulation results for various virtual detectors, 4) results including the charge asymmetry computed from simulations, 5) unresolved questions regarding the simulation results. 
Condensed Matter Seminar Thursday, April 28, 2016 1:00 PM Physics Building, Room 313 Nuclear Seminar 
"Aging and memory effects in the spin jam states of densely populated frustrated magnets"Anjana Samarakoon , UVA  Department of Physics
ABSTRACT:
Defects and randomness has been largely studied as the key mechanism of glassiness find in a dilute magnetic system. Even though the same argument has also been made to explain the spin glass like properties in dense frustrated magnets, the existence of a glassy state arise intrinsically from a defect free spin system, far from the conventional dilute limit with different mechanisms such as quantum fluctuations and topological features, has been theoretically proposed recently. We have studied field effects on zerofield cooled and field cooled susceptibility bifurcation and memory effects below freezing transition, of three different densely populated frustrated magnets which glassy states we call spin jam, and a conventional dilute spin glass. Our data show common behaviors among the spin jam states, which is distinct from that of the conventional spin glass. We have also performed both Neutron scattering experiments and Monte Carlo simulations to understand the nature of their energy landscapes. 
Condensed Matter Seminar Tuesday, April 26, 2016 1:00 PM Physics Building, Room 313 
"Critical mechanical structures: topology and entropy"Xiaoming Mao , University of Michigan [Host: Marija Vucelja]
ABSTRACT:
Critical mechanical structures are structures at the verge of mechanical instability. These structures are characterized by their floppy modes, which are deformations costing little energy. On the one hand, numerous interesting phenomena in soft matter are governed by the physics of critical mechanical structures, because they capture the critical state between solid and liquid. On the other hand, the design of mechanical metamaterials (i.e., engineered materials that gain their unusual mechanical properties, such as negative Poisson's ratio, from their structures) often rely on floppy modes to realize novel properties, and the floppy modes in this situation are called "mechanisms". This talk focuses on the intersection between the research of soft matter and mechanical metamaterials. I will propose a new design principle, for mechanical metamaterials that are transformable between states with dramatically different properties. These different properties are highly robust because they are topologically protected. Then I will discuss entropic effects on floppy modes (mechanisms), the interplay of which with topology leads to fascinating phenomena that need be considered when machines and metamaterials are made at small scales. 
Condensed Matter Seminar Thursday, April 21, 2016 1:00 PM Physics Building, Room 313 
"SpinOrbit Coupled Double Perovskites: Topology and Magnetism"Arun Paramekanti , University of Toronto [Host: Israel Klich ]
ABSTRACT:
Double perovskites are a broad class of transition metal oxides which exhibit a wide range of phenomena  high temperature ferromagnetism, halfmetallicity, and Mott insulators with geometrically frustrated magnetism. I will discuss how spinorbit coupling leads to new physics in these systems. Specifically, I will discuss how ultrathin double perovskite films can support Chern bands and quantum anomalous Hall insulators. Turning to spin dynamics, I will present our work on understanding neutron scattering experiments in halfmetallic double perovskites as well as bulk iridiumbased Mott insulators where we argue for dominant Kitaev exchange interactions. 
Condensed Matter Seminar Thursday, April 14, 2016 1:00 PM Physics Building, Room 313 
"Experimental study of the 3He and neutron spin structure at low Q2 using a polarized 3He target"Nguyen Ton , UVA Department of Physics
ABSTRACT:
I will report on the analysis status of the Jefferson Lab (JLab) Hall A E97110 experiment. The experiment performed a precise measurement of the neutron spin structure functions at low Q2 by using a polarized 3He target as an effective polarized neutron target. The goals of the experiment are to make a benchmark test of Chiral Perturbation Theory calculations and to check the GerasimovDrellHearn (GDH) sum rule by extrapolating the integral to the real photon point. The data were taken in two experimental run periods. The first period covered the lowest Q2 points but with a defective equipment which complicate the data analysis. The second period covered higher Q2 point, with a properly working equipment. The raw asymmetry analysis and elastic carbon cross section measurement will be discussed for the fi run period along with the future plans for the analysis. I will also discuss the progress and the tests in the polarized 3He Target Lab at JLab. Results of Pulse NMR (nuclear magnetic resonance) will be presented. 
Condensed Matter Seminar Thursday, April 14, 2016 3:30 PM Physics Building, Room 204 Nuclear Seminar 
"Spin1/2 Heisenberg antiferromagnet in one dimension"Yasu Takano , University of Florida [Host: Seunghun Lee]
ABSTRACT:
The onedimensional spin1/2 Heisenberg model is one of the few exactly solvable models in physics. When the interaction is antiferromagnetic, the ground state of this model is a TomonagaLuttinger liquid, in which dynamic and static properties are inextricably linked. Lowenergy excitations are spinons, which are fermions, instead of bosonic magnons, with a unique gapless dispersion. These and other properties of the model have been extensively studied since the pioneering work by Bethe, published in 1931. This seminar describes our recent experiment that puts some of the theoretical predictions to tests. 
Condensed Matter Seminar Thursday, April 7, 2016 1:00 PM Physics Building, Room 313 
"Simulating frustrated itinerant magnets at the mesoscale"Kipton Barros , LANL [Host: GiaWei Chern]
ABSTRACT:
Models of classical fields coupled to itinerant fermions appear commonly in condensed matter. We present an efficient technique to sample these classical fields. Instead of expensive direct diagonalization, we use the Kernel Polynomial Method (KPM) to stochastically estimate electron states and relevant observables. Our extension of KPM significantly improves stochastic convergence by leveraging locality, e.g. spatial decay of 2point correlations. A GPU/MPI implementation enables practical treatment of tens of thousands of lattice sites. To demonstrate the method, we study complex spin textures in the Kondo lattice model. We observe mesoscale chiral domain coarsening and Z2 vortex dynamics. Other emergent phenomena includes metastable skyrmions and a chiral stripe phase that arises due to instability of standard helical order. Finally, we discuss a generic spin liquid state that may explain the experimentally observed resistivity minima in compounds with large local magnetic moments, e.g. the pyrochlore oxides Pr2Ir2O7, Nd2Ir2O7 under pressure, and RInCu4 (R=Gd, Dy, Ho, Er and Tm). 
Condensed Matter Seminar Thursday, March 24, 2016 1:00 PM Physics Building, Room 313 
"Gauging and Orbifolding the topological phases in (2+1) and (3+1) dimensions"Xiao Chen , University of Illinois at UrbanaChampaign [Host: Jeffrey Teo]
ABSTRACT:
Topological phases of matter are commonly equipped with global symmetries, such as electricmagnetic duality in gauge theories and bilayer symmetry in fractional quantum Hall states. Gauging these symmetries into local dynamical ones is one way of obtaining exotic phases from conventional systems. We first study this using the bulkboundary correspondence in the (2+1) dimensional topological phases and orbifolding the (1+1) dimensional edge described by a conformal field theory (CFT). Our procedure puts twisted boundary conditions into the partition function, and predicts the fusion, spin and braiding behavior of anyonic excitations after gauging. We demonstrate this for the twofoldsymmetric D(Z_N ) quantum double model, SU(3)_1, and the S_3symmetric SO(8)_1 state. Later on, we generalize this idea to (3+1) dimensional topological phases and study the bulkboundary correspondence there 
Condensed Matter Seminar Friday, March 18, 2016 1:00 PM Physics Building, Room 313 
"Complex Magnetic Phase Diagrams of Models for Iron Based Superconductors"Elbio Dagotto , University of Tennessee/Oak Ridge National Laboratory [Host: Despina Louca]
ABSTRACT:
This presentation will start with a brief introduction to ironbased 
Condensed Matter Seminar Thursday, March 3, 2016 1:00 PM Physics Building, Room 313 
"Jamming and glassy behavior in dense biological tissues"Max Dapeng Bi , Rockefeller University [Host: Marija Vucelja]
ABSTRACT:
Cells must move through tissues in many important biological processes, including embryonic development, cancer metastasis, and wound healing. Often these tissues are dense and a cell's motion is strongly constrained by its neighbors, leading to glassy dynamics. Although there is a densitydriven glass transition in particlebased models for active matter, these cannot explain liquidtosolid transitions in confluent tissues, where there are no gaps between cells and the packing fraction remains fixed and equal to unity. I will demonstrate the existence of a new type of rigidity transition that occurs in confluent tissue monolayers at constant density. The onset of rigidity is governed by a model parameter that encodes singlecell properties such as cellcell adhesion and cortical tension. I will also introduce a new model that simultaneously captures polarized cell motility and multicellular interactions in a confluent tissue and identify a glassy transition line that originates at the critical point of the rigidity transition. This work suggests an experimentally accessible structural order parameter that specifies the entire transition surface separating fluid tissues and solid tissues. Finally I will present my collaboration work with the Fredberg group (Harvard School of Public Health), where these predictions have been successfully tested in bronchial epithelial cells from asthma patients, explaining pathologies in lung epithelia. 
Condensed Matter Seminar Thursday, February 25, 2016 1:00 PM Physics Building, Room 313 
"Phase Diagram and Quantum Order by Disorder in the Kitaev K1K2 Honeycomb Model"Natalia Perkins , University of Minnesota [Host: GiaWei Chern]
ABSTRACT:
The search for experimentally tangible scenarios of spin liquids as topologically ordered quantum states of matter is one of the most vibrant subfields of contemporary condensed matter research. Honeycomb iridates and related materials have originally been suggested as possible candidates for hosting a spinliquid state. Intriguingly, no quantum paramagnetic ground state has been discovered so far in these materials, posing fundamental challenges to determining an accurate underlying microscopic spin model. In our study we show that the secondneighbor Kitaev coupling is an important ingredient to such a microscopic description for the strong spinorbit transitionmetal oxide Na2IrO3. We analyze the K1K2 model from a variety of methodological perspectives. As a coherent picture emerges from the investigation, the K1K2 model allows us to explain the onset of zigzag magnetic order that is also found experimentally. Furthermore, we find that the K1K2 model is a suitable minimal description for resolving the substantially different nature of quantum and thermal fluctuations originating from such Kitaev couplings. 
Condensed Matter Seminar Thursday, February 18, 2016 1:00 PM Physics Building, Room 313 
"Resonant inelastic xray scattering as a probe of band structure"Marton KanaszNagy , Harvard University [Host: Israel Klich ]
ABSTRACT:
Resonant inelastic xray scattering (RIXS) has emerged as a powerful tool for studying hightemperature superconducting materials. In the talk, I will show how a theory based on noninteracting quasiparticles can describe recent experimental data on optimally doped and overdoped YBa2Cu3O6+x [Minola et al., Phys. Rev. Lett. 114, 217003 (2015)]. Surprisingly, the RIXS signal is qualitatively different from those measured using a different cuprate material, Bi2Sr2CuO6+x [Guarise et al., Nat. Commun. 5, 5760 (2014)], a feature originally attributed to collective magnetic excitations. I will demonstrate that this discrepancy can be explained by the sensitivity of RIXS to details of the band structures of these materials, especially at energies well above the Fermi surface. This energy range is inaccessible to traditionally used band structure probes, such as angleresolved photemisson spectroscopy, making RIXS a powerful band structure probe, potentially applicable to a wide range of materials. 
Condensed Matter Seminar Thursday, February 4, 2016 12:30 PM Physics Building, Room 313 
"Extracting Hidden Hierarchies in Complex Spatial Networks"Carl Modes , Rockefeller University [Host: Marija Vucelja]
ABSTRACT:
Natural and manmade transport webs are frequently dominated by dense sets of nested cycles. The architecture of these networks  the topology and edge weights  determines how efficiently the networks perform their function. Yet, the set of tools that can characterize such a weighted cyclerich architecture in a physically relevant, mathematically compact way is sparse. In order to fill this void, this seminar presents a new characterization that rests on an abstraction of the physical `tiling' in the case of a two dimensional network to an effective tiling of an abstract surface in space that the network may be thought to sit in. Generically these abstract surfaces are richer than the plane and upon sequential removal of the weakest links by edge weight, neighboring tiles merge and a tree characterizing this merging process results. The properties of this characteristic tree can provide the physical and topological data required to describe the architecture of the network and to build physical models. This new algorithm can be used for automated phenotypic characterization of any weighted network whose structure is dominated by cycles, such as mammalian vasculature in the organs, the root networks of clonal colonies like quaking aspen, and the force networks in jammed granular matter. http://www.tcm.phy.cam.ac.uk/~cdm36/

Condensed Matter Seminar Thursday, January 21, 2016 12:30 PM Physics Building, Room 313 
"Local probe investigation of interfaces in twodimensional materials"Prof. Chenggang Tao , Virginia Tech [Host: Utpal Chatterjee]
ABSTRACT:
Emerging twodimensional (2D) materials, such as graphene and atomically thin transition metal dichalcogenides, have been the subject of intense research efforts for their fascinating properties and potential applications in future electronic and optical devices. The interfaces in these 2D materials, including domain boundaries and edges, strongly govern the electronic and magnetic behavior and can potentially host new states. On the other hand these interfaces are more susceptible to thermal fluctuation and external stimuli that enable mass displacement and generate disorder. In this talk I will present our scanning tunneling microscopy (STM) and spectroscopy (STS) explorations of edges of few layered MoS_{2} nanostructures with unique structural and electronic properties and show how step edges on TiSe_{2} surfaces change dynamically due to electrical fields. I will also discuss temperature evolution of quasi1D C_{60} nanostructures on graphene. 
Condensed Matter Seminar Thursday, December 3, 2015 11:00 AM Physics Building, Room 313 
"Magnetic orderings of XYpyrochores"Prof. Haidong Zhou , University of Tennessee [Host: Seunghun Lee]
ABSTRACT:
Due to the geometrically frustrated lattice and effective spin1/2, the XY pyrochlores Er_{2}Ti_{2}O_{7} and Yb_{2}Ti_{2}O_{7} exhibit exotic magnetic ground states related to quantum spin fluctuations. In this talk, we presented our recent studies on the magnetic orderings of new XY pyrochlores Er_{2}Ge_{2}O_{7} and Yb_{2}Ge_{2}O_{7}. Then we tried to unify the magnetic orderings of all studied XY pyrochlores Er_{2}B_{2}O_{7} and Yb_{2}B_{2}O_{7} (B = Sn, Ti, and Ge) through the chemical pressure effects based on the theoretically proposed phase diagram. 
Condensed Matter Seminar Thursday, November 12, 2015 11:00 AM Physics Building, Room 313 
"Topological superconductivity in Metal/QuantumSpinIce heterostructure"Prof. EunAh Kim , Cornell University [Host: Seunghun Lee]
ABSTRACT:
Achieving predictive power is the central problem faced by the field of unconventional superconductivity. One long standing proposal by Anderson predicts that a quantum spin liquid(QSL) will give way to a superconductor upon doping. However, to the best of our knowledge no QSL has been successfully doped into becoming a superconductor. In this talk, I will discuss our proposal of a conceptually new framework. We propose to exploit spin entanglement in QSL to drive superconductivity without doping and hence destroying QSL: a heterostructure consisting of a QSL and a metal. In this new proposal, the conduction electrons in metal will "borrow" the spin correlation in QSL to pair leaving QSL itself intact. To aid materialization of the proposed setup we focus on using quantum spin ice as the QSL layer and establish the guideline for finding a suitable compound to be grown on a QSL substrate. I will present our prediction for a topological superconductivity in one such setup: Y2Sn2xSbxO7 grown on the (111) surface of Pr2Zr2O7. The predicted order parameter symmetry is analogous to that of 2D superfluid He3B phase and the realization of the proposal will amount to the first solidstate realization of such pairing state. 
Condensed Matter Seminar Thursday, November 5, 2015 11:00 AM Physics Building, Room 313 
"Bulk Thermoelectric and Thermodynamic Probes of nonAbelian Anyons in Topological States of Matter"Kun Yang , FSU [Host: Israel Klich ]
ABSTRACT:
Topological states of matter support quasiparticle excitations with fractional charge and possibly exotic statistics of the nonAbelian type, known as nonAbelian anyons. Most current experimental attempts to reveal such exotic statistics focus on interference involving edge transport. After a brief introduction of topological states (mostly in the context of fractional quantum Hall effect) in general, in this talk we will discuss how one can reveal the nonAbelian quasiparticle statistics using bulk probes. We show that bulk thermopower is a promising way to detect their nonAbelian nature, and measure the quantum dimension (a key parameter that quantifies nonAbelian statistics) of these anyons. This method is particularly effective in the Corbino geometry. We also demonstrate a novel cooling effect associated with them. We discuss application of these ideas to the specific candidate system of fractional quantum Hall liquid at filling factor 5/2, and topological insulatorsuperconductor hybrid systems. Some of the predicted behavior has been observed in recent experiments, which will also be discussed. 
Condensed Matter Seminar Thursday, October 29, 2015 11:00 AM Physics Building, Room 313 
"Phononics in liquids and supercritical fluids"Dima Bolmatov , Brookhaven National Lab [Host: Genya Kolomeisky]
ABSTRACT:
Abstract: While physicists have a good theoretical understanding of the heat capacity of both solids and gases, a general theory of the heat capacity of liquids has always remained elusive. Apart from being an awkward hole in our knowledge of condensedmatter physics, heat capacity – the amount of heat needed to change a substance's temperature by a certain amount – is a technologically relevant quantity that it would be nice to be able to predict^{1}. I will introduce a phononbased approach to liquids^{2} and supercritical fluids^{3}, and describe its thermodynamics in terms of phonon excitations. I will show that the effective Hamiltonian^{4} has a transverse phononic gaps and explain their evolution with temperature variations. I will explain how the introduced formalism covers the Debye theory of solids, the phonon theory of liquids, and thermodynamic limits such as the DelongPetit and the ideal gas thermodynamic limits. The experimental evidence for the new thermodynamic boundary (the Frenkel line) on the pressuretemperature phase diagram will be demonstrated^{5}. Finally, I will discuss the phonon propagation and localization effects in liquids above and below the Frenkel line, and outline new directions towards phonon band gaps engineering and sound manipulation. 
Condensed Matter Seminar Thursday, October 15, 2015 11:00 AM Physics Building, Room 313 
"Quantum dissection of a covalent bond with the entanglement spectrum"Norman Tubman , UIUC [Host: Israel Klich]
ABSTRACT:
Theoretical analysis of molecular bonding is quite successful in many regards, but leaves open many mysteries, even for simple diatomic molecules. Such mysteries have been highlighted in several recent theoretical studies in which there have been claims of a new type of bond identified in the carbon dimer. These studies have been quite controversial. To address these claims directly from the quantum mechanical wave function, we developed a new approach to describe chemical bonds from ideas originating in the field of quantum entanglement. This approach is based on breaking up a many body wave function into real space pieces, which is formally known as the entanglement spectrum. We are able to address the controversial C2 molecule with these tools, and demonstrate its bonding properties, which includes an inverted fourth bond. The ideas considered in this work provide a significantly more detailed picture of bonding than allowed by previous techniques. 
Condensed Matter Seminar Thursday, October 8, 2015 11:00 AM Physics Building, Room 313* 
ABSTRACT:
Entanglement is a quantum correlation which does not appear classically, and it serves as a resource for quantum technologies such as quantum computing. The area law says that the amount of entanglement between a subsystem and the rest of the system is proportional to the area of the boundary of the subsystem and not its volume. A system that obeys an area law can be simulated more efficiently than an arbitrary quantum system, and an area lawprovides useful information about the lowenergy physics of the system. It was widely believed that the area law could not be violated by more than a logarithmic factor (e.g. based on critical systems and ideas from conformal field theory) in the system’s size. We introduce a class of exactly solvable onedimensional models which we can prove have exponentially more entanglement than previously expected, and violate the area law by a square root factor. We also prove that the gap closes as n^{c}, where c \ge 2, which rules out conformal field theories as the continuum limit of these models. It is our hope that the mathematical techniques introduce herein will be of use for solving other problems. (Joint work with Peter Shor).
References: Phys. Rev. Lett. 109, 207202 http://arxiv.org/abs/1408.1657

Condensed Matter Seminar Thursday, October 1, 2015 3:30 PM Physics Building, Room 204 
"Topological line nodes in Ca3P2 and other semimetals"ChingKai Chiu , University of Maryland [Host: Jeffrey Teo]
ABSTRACT:
In an ordinary threedimensional metal the Fermi surface forms a twodimensional closed sheet separating the filled from the empty states. Topological semimetals, on the other hand, can exhibit protected onedimensional Fermi lines or zerodimensional Fermi points, which arise due to an intricate interplay between symmetry and topology of the electronic wavefunctions. Here, we study how reflection symmetry, timereversal symmetry, and inversion symmetry leads to the topological protection of line nodes in threedimensional semimetals. We derive the Z and Z2type invariants that guarantee the stability of the line nodes and lead to the appearance of protected surfaces states. As a representative example of a topological semimetal with line nodes, we consider Ca3P2 and discuss the topological properties of its Fermi line in terms of a tightbinding model, derived from ab initio DFT calculations. We show that due to a bulkboundary correspondence, Ca3P2 displays nearly dispersionless surface states, which take the shape of a drumhead. These topological surface states give rise to the charge polarization. 
Condensed Matter Seminar Thursday, September 3, 2015 3:30 PM Physics Building, Room 204 
"Magnetic Nematicity in the Hidden Ordered Material URu2Si2"Peter Riseborough , Temple University [Host: Bellave Shivaram]
ABSTRACT:
The compound URu_{2}Si_{2} is a heavyfermion compound with a linear T term in the low temperatures electronic specific heat with a coefficient of 155 mJ/mole/K^{2}. At T=1.5 K the system undergoes a transition to a superconducting state but exhibits another transition at the higher temperature of T=17.5 K. The 17.5 K transition is marked by a large lambdalike anomaly in the specific heat and was originally thought to be a secondorder transition to a spin density wave state. The change in entropy associated with the specific heat anomaly is of the order of 0.3 kB ln(2). Also, at the transition, the system loses 90% of the carriers and develops a gap of the order of 7 meV over about 60% of the Fermisurface. However, neutron scattering experiments have demonstrated that spin density wave order is absent below the 17.5 K transition and, likewise, xray scattering experiments have indicated the absence of charge or orbital density wave ordering. Despite over 30 years of intensive study, the nature of the ordering is still unknown. Hence, the transition is now known as the Hidden Order transition. We propose that the 17.5 K transition is due to a combined spin and orbital ordering which can only be probed by measurements that are both charge and spin sensitive. We have developed a model in which the Hund’s rule exchange interaction stabilizes this novel state. Furthermore, we show that the material breaks spinrotational invariance at low temperatures, but does not develop a static magnetic moment. In particular, the model shows that the magnetic susceptibility becomes anisotropic below the ordering temperature. Our calculations are compared with the results of the magnetic torque measurements of Okazaki et al. that found the fourfold symmetry of the magnetic susceptibility in the ab plane is broken as the susceptibility develops a twofold axis below the transition. 
Condensed Matter Seminar Thursday, April 23, 2015 3:30 PM Physics Building, Room 204 
"Inplane Charge Fluctuations in Bismuth Sulfide Superconductors"Anushika Athauda , University of Virginia [Host: Despina Louca] 
Condensed Matter Seminar Friday, April 17, 2015 11:00 AM Physics Building, Room 313 
"Entanglement entropy from thermodynamic entropy"Mohammad Magrebi , JQI Maryland/NIST [Host: Israel Klich]
ABSTRACT:
In recent years, entanglement has become a main frontier with applications across several fields in physics. Nevertheless, simple conceptual pictures and practical ways to compute the entanglement, especially in manybody systems, have remained elusive. In this talk, I will consider a simple setup where a dispersive medium becomes entangled with zeropoint fluctuations in the vacuum. I show that the entanglement entropy of the material body with the vacuum can be viewed as the classical thermodynamic entropy in one higher dimension. Such mapping allows us to immediately verify the quantum version of the strong subadditivity property. As a byproduct of our formalism, I show that the entanglement finds a simple pictorial interpretation in terms of phantom polymers. 
Condensed Matter Seminar Thursday, April 16, 2015 3:30 PM Physics Building, Room 204 
"Length Scale Dependent of Thermal Conductivity of SiGe Alloys"Long Chen , University of Virginia [Host: Joe Poon] 
Condensed Matter Seminar Tuesday, April 7, 2015 3:30 PM Physics Building, Room 204 
ABSTRACT:
A Skyrmion is a topological configuration in which local spins wrap around the unit sphere for an integer number of times. After decades of theoretical discussions in high energy physics, it has been recently observed in a series of noncentrosymmetric chiral magnets. Several experiments by neutron scattering or transmission electron microscopy confirm the presence of skyrmions in a crystalline state at a finite window of magnetic field and temperature. Skyrmions show various novel properties inherent to its topological nature, such as topological Hall effect, topological stability, and ultralow critical current for movement, which offer the skyrmion promising prospects for next generation spintronic devices and information storage. In this talk, I will explain the physical origin of skyrmions in chiral magnets, and discuss their dynamics under electric current or temperature gradient, where an emergent electromagnetism plays an important role. Furthermore, several routes to single skyrmions will be presented, and new chiral magnet materials are discussed. 
Condensed Matter Seminar Thursday, April 2, 2015 3:30 PM Physics Building, Room 204 
"Micromachined probes for the study of quantum fluids"Yoonseok Lee , University of Florida [Host: Seunghun Lee]
ABSTRACT:
In the superfluid phases of liquid 3He with pwave spin triplet pairing, all of the continuous symmetry except for the translational symmetry are broken, exhibiting extraordinarily rich physical phenomena. In particular, the surface scattering in unconventional superfluids/superconductors induces quasiparticle subgap bound states spatially localized near the surface within the coherence length xo, called the Andreev surface bound states (ASBS). This generic nature of the unconventional order parameter combined with the exotic symmetries in the superfluid phases of 3He lends an unfathomable source of fascinating physical phenomena predicted to exist in confined geometry: crystalline superfluid phase, Majorana fermionic excitations, and helical spin current in the Bphase, and the edge current and the chiral edge state in the Aphase, many of which are of topological origin. We have developed micromachined probes or microelectromechanical system (MEMS) devices for this purpose. In this talk, we will discuss the design and the operation of the device, and the results obtained using these devices in air, liquid 3He and also in liquid 4He in a wide range of temperature down to submillikelvin range. Our work demonstrates great potential of the device in a wide range of experiments in quantum fluids. 
Condensed Matter Seminar Thursday, March 26, 2015 3:30 PM Physics Building, Room 204 
"Magnetotransport and domain wall scattering in the epitaxial L10 MnAl thin films"Linqiang Luo , University of Virginia [Host: Stu Wolf]
ABSTRACT:
L1_{0} MnAl films with perpendicular magnetic anisotropy were synthesized by a bias target ion beam deposition technique. XRD results showed that MnAl thin film was expitaxially grew on Cr buffered MgO(001) substrate with tetragonal distortion c/a ration ~1.3. A Cr seeding layer optimized the magnetic anisotropy and saturation magnetization. The magnetotransport(MR) properties of MnAl were investigated by using a Hall bar structure. From 320K to 175K, the MR curve peaked exactly at coercivity field and was suggested to be a manifestation of domain wall(DW) scattering. Mazelike strip domains were observed by MFM after demagnetization. The difference of inplane and outofplane resistance showed evidence of domain effect. The quantitative analysis for domain density in comparison with that of resistance implied the contribution of domain wall scattering. Below 150K, the effect of DW scattering on the MR was very small compared to the Lorentz MR. 
Condensed Matter Seminar Wednesday, March 18, 2015 3:30 PM Physics Building, Room 204 
"Problems in Human Motion Planning"Brian Skinner , Argonne National Laboratory [Host: Genya Kolomeisky]
ABSTRACT:
Moving through a denselypopulated environment can be surprisingly hard, owing to the problem of congestion. Learning to deal with congestion in crowds and in networks is a longstanding and urgentlystudied problem, one that can be equally well described at the level of dense, correlated matter or at the level of gametheoretical decision making. In this talk I describe two related problems associated with motion planning in congested environments. In the first part I consider a description of pedestrian crowds as denselypacked repulsive particles, and I address the question: what is the form of the pedestrianpedestrian interaction law? In the second part of the talk I examine a simple model of a traffic network and study how inefficiency in the traffic flow arises from "selfish" decisionmaking. Analysis of the model reveals a surprising connection between Nash equilibria from game theory and percolative phase transitions from statistical physics. 
Condensed Matter Seminar Thursday, February 26, 2015 3:30 PM Physics Building, Room 204 
ABSTRACT:
Bending of doublestranded DNA plays an essential role in genome structure and function. However, the energetic cost of sharp DNA bending is difficult to determine. In this talk, I will explain how we study the energetics of sharp DNA bending using small DNA loops. Using singlemolecule Fluorescence Resonance Energy Transfer (FRET), we measure the lifetime of a DNA loop as a function of loop size. Above a critical loop size, the loop lifetime changes with loop size in a manner consistent with elastic bending stress, but below it, becomes less sensitive to loop size, indicative of DNA softening. Our result is in quantitative agreement with the kinkable wormlike chain model. Time permitting, I will also present a new method to compute the equilibrium distribution of forces exerted by a semiflexible polymer loop. 
Condensed Matter Seminar Thursday, February 12, 2015 3:30 PM Physics Building, Room 204 
ABSTRACT:
I will describe how to define a proper RG flow in the space of tensor networks, with applications to the evaluation of classical partition functions, euclidean path integrals, and overlaps of tensor network states. 
Condensed Matter Seminar Wednesday, February 11, 2015 3:30 PM Physics Building, Room 204 
"Quantum quenches in 2D via arrays of coupled chains"Andrew James , University College of London [Host: Israel Klich]
ABSTRACT:
Matrix product state (MPS) methods, while highly effective when applied to the study of quantum systems in 1D, stumble in higher dimensions due to the 'area law' growth of entanglement entropy. This growth of entanglement can be mitigated in 2D by studying anistropic systems composed of coupled integrable chains, because the required `area' is reduced. As a specific example I will describe the implementation of the time evolving block decimation algorithm to study quantum quenches in a system of coupled quantum Ising chains. 
Condensed Matter Seminar Thursday, February 5, 2015 3:30 PM Physics Building, Room 204 
ABSTRACT:
A metamagnet (MM) is characterized by the sharp rise of the lowtemperature magnetization at a critical field B_{c}. Other MM properties that become singular at T=0 and B= B_{c} include specific heats, magnetostriction, and sound velocities. Metamagnetism occurs in single molecules as well as in solids, and is not a phase transition in the classical sense; however, distinctive MM features are also seen at finite T for all B. We will present new data on macromolecules and heavyfermion metals, such as UPt_{3}, and show how these systems are well described by a simple twolevels model, which appears to be applicable to all MMs, when suitably extended. 
Condensed Matter Seminar Thursday, January 29, 2015 3:30 PM Physics Building, Room 204 
"Uniaxial Pressure on Strongly Correlated Materials"Rena Zieve , University of California, Davis [Host: Bellave Shivaram]
ABSTRACT:
We describe a heliumactivated bellows cell for measurements under uniaxial pressure. The apparatus can be useful for various reasons: monitoring asymmetric behavior with pressure applied along different axes; breaking the symmetry of a tetragonal crystal; changing pressure at low temperature to cross pressureinduced phase transitions directly; and making smaller pressure adjustments than possible with typical hydrostatic pressure arrangements. Several families of unconventional superconductors, including highTc materials, the 115 family, and iron pnictides and chalcogenides, have a layered structure and notably anisotropic behavior, making them good candidates for uniaxial pressure measurements. We present data on 115 and pnictide materials highlighting the influence of geometry on their phase diagrams. 
Condensed Matter Seminar Thursday, January 15, 2015 3:30 PM Physics Building, Room 204 
"Optimal number of terms in QED series and its consequence in condensed matter implementations of QED"Genya Kolomeisky , University of Virginia [Host: Oscar Rondon]
ABSTRACT:
In 1952 Dyson put forward a simple and powerful argument indicating that the perturbative expansions of QED are asymptotic. His argument can be related to Chandrasekhar's limit on the mass of a star for stability against gravitational collapse. Combining these two arguments we estimate the optimal number of terms of the QED series to be approximately 5000. For condensed matter manifestations of QED in narrow bandgap semiconductors and Weyl semimetals the optimal number of terms is around 80 while in graphene the utility of the perturbation theory is severely limited. 
Condensed Matter Seminar Thursday, December 4, 2014 3:30 PM Physics Building, Room 204 Joint Nuclear/Condensed Matter Physics Seminar 
"Anyonics: Designing exotic circuitry with nonAbelian anyons"Kiril Shtengel , University of California, Riverside [Host: Israel Klich]
ABSTRACT:
NonAbelian anyons are widely sought for the exotic fundamental physics they harbour as well as for their possible applications for quantum information processing. Currently, there are numerous blueprints for stabilizing the simplest type of nonAbelian anyon, a Majorana zero energy mode bound to a vortex or a domain wall. One such candidate system, a socalled "Majorana wire" can be made by judiciously interfacing readily available materials; the experimental evidence for the viability of this approach is presently emerging. Following this idea, we introduce a device fabricated from conventional fractional quantum Hall states, swave superconductors and insulators with strong spinorbit coupling. Similarly to a Majorana wire, the ends of our “quantum wire” would bind "parafermions", exotic nonAbelian anyons which can be viewed as fractionalized Majorana zero modes. I will briefly discuss their properties and describe how such parafermions can be used to construct new and potentially useful circuit elements which include current and voltage mirrors, transistors for fractional charge currents and "flux capacitors". 
Condensed Matter Seminar Thursday, November 20, 2014 3:30 PM Physics Building, Room 204 
"Impact of electronic nematicity on the unconventional superconductivity of the iron pnictides"Rafael Fernandes , University of Minnesota [Host: Despina Louca]
ABSTRACT:
Five years after their discovery, much of the interest in the iron pnictides remains in understanding not only their hightemperature superconducting phase, but also the nature of their normal state. In this context, recent experiments have provided strong evidence for the existence of an unusual correlated state in the phase diagram of these materials, dubbed electronic nematic. Below the nematic transition temperature, the tetragonal symmetry of the system is broken down to orthorhombic not by lattice vibrations, but by electronic degrees of freedom. However, two questions remain open: What is the origin of this nematic state? What is its relationship to the superconducting state? In this talk we will explore these two issues via a microscopic electronic model in which the nematic instability is caused by magnetic fluctuations arising from a degenerate ground state. A key consequence of this model is that lattice fluctuations and magnetic fluctuations are not independent. Instead, they follow a simple scaling relation, which we will show to be satisfied by elastic modulus and NMR experimental data. We will also demonstrate that, in general, nematic order competes with the unconventional signchanging s^{+} superconducting state, although they may coexist under certain conditions. When the s^{+} instability is in close competition with a dwave instability – as it has been suggested in several iron pnictides – we will show that nematic and superconducting degrees of freedom are strongly coupled. As a result, not only T_{c} can be significantly enhanced by nematic order, but also nematicity itself can be used as a diagnostic tool to search for more exotic superconducting states – such as states that spontaneously break timereversal or tetragonal symmetries. 
Condensed Matter Seminar Monday, November 17, 2014 3:30 PM Physics Building, Room 204 
"Negative Casimir Entropies in Nanoparticle Interactions"Kimball Milton , University of Oklahoma [Host: Israel Klich]
ABSTRACT:
Negative entropy has been known in Casimir systems for some time. For example, it can occur between parallel metallic plates modeled by a realistic Drude permittivity. Less well known is that negative entropy can occur purely geometrically, say between a perfectly conducting sphere and a conducting plate. The latter effect is most pronounced in the dipole approximation, which occurs when the size of the sphere is small compared to the separation between the sphere and the plate. Therefore, here we examine cases where negative entropy can occur between two electrically and magnetically polarizable nanoparticles or atoms, which need not be isotropic, and between such a small object and a conducting plate. Negative entropy can occur even between two perfectly conducting spheres, between two electrically polarizable nanoparticles if there is sufficient anisotropy, between a perfectly conducting sphere and a Drude sphere, and between a sufficiently anisotropic electrically polarizable nanoparticle and a transverse magnetic conducting plate. 
Condensed Matter Seminar Thursday, November 6, 2014 3:30 PM Physics Building, Room 204 
"Spin dynamics in a rareearth based single molecule magnet"Maiko Kofu , Institute of Solid State Physics, University of Tokyo [Host: Despina Louca]
ABSTRACT:
Singlemolecule magnets (SMMs) are a class of metalorganic compounds in which each constituent molecule, containing magnetic atoms, possesses a giant and isolated resultant spin. Given that the giant spin exhibits easyaxis magnetic anisotropy, the magnetization reversal between the ground states is hindered by the potential barrier, yielding a slow magnetic relaxation that is characteristic of SMMs. In the beginning of SMM researches, SMMs containing multiple transition metal atoms such as Mn, Fe, and Ni, have been intensively studied. Recently, however, a new series of rareearth based SMMs attracts much attention. In this talk, I will show our recent results on Tbbased SMMs. We have investigated the energy scheme by inelastic neutron scattering (INS) and the relaxation phenomena by quasielastic neutron scattering (QENS) measurements. The mechanism of magnetization reversal through quantum tunneling in the Tbbased SMM will be discussed. 
Condensed Matter Seminar Monday, October 13, 2014 3:30 PM Physics Building, Room 204 
"Hybrid inorganicorganic nanomaterials as building blocks for designer solids and their applications in optoelectronic devices"Joshua Choi , University of Virginia [Host: Seunghun Lee]
ABSTRACT:
Tunable optoelectronic properties of hybrid inorganicorganic nanomaterials provide unique opportunities for science and technology. The degree of quantum confinement and collective electronic interaction in these materials can be readily tuned by controlling their synthesis and selfassembly processes. This enables fabrication of 'designer solids' with programmable optoelectronic properties tailored for specific scientific studies and technological applications. In this talk, I will discuss colloidal quantum dots and metalorganic perovskites, two of the most promising building blocks for designer solids. Both material systems exhibit intriguing properties tunable by design while looking set to revolutionize the field of solution processed optoelectronic devices. 
Condensed Matter Seminar Thursday, September 18, 2014 3:30 PM Physics Building, Room 204 
"Amplification, entanglement and storage of microwave radiation using superconducting circuits"JeanDamien Pillet , Columbia University [Host: Tom Gallagher]
ABSTRACT:
Microwave signals have appeared during the last decade as a powerful and versatile platform to investigate a wide variety of quantum phenomena, from fundamental quantum optics to more oriented researches toward quantum information processing. This specific place originates from the fact that microwave signals can on one hand be precisely tailored and controlled with standard commercial electronics. On the other hand they can easily be processed at the single photon level in the quantum regime by superconducting circuits cooled down to dilution fridge temperatures thanks to a unique component, the Josephson junction, an intrinsic non dissipative lumped nonlinear inductor.
I will present how we designed and built a superconducting circuit, based on a Josephson ring modulator (JRM), a ring of 4 Josephson junctions in a Wheatstone bridge configuration, allowing nondegenerate three wavemixing. I will show that, when pumped at the appropriate frequency, this single circuit behaves as a tunable beam splitter with frequency conversion, a quantum limited amplifier or an EPR states generator. Using frequency conversion, we demonstrate on demand capture, storage and release of microwave radiations with approx. 80% catching efficiency and about 30 storage operations per memory lifetime. We then demonstrate entanglement generation between a propagating microwave mode and a localized mode in the cavity. 
Condensed Matter Seminar Thursday, April 24, 2014 3:30 PM Physics Building, Room 204 
"Effect of surface disorder on transport in topological insulators"Kun Woo Kim , Caltech [Host: Israel Klich]
ABSTRACT:
What is the effect of surfaceonly disorder on the electronic states of a 3d TI? The layers in the clean bulk parallel to surface probe the surface impurities as they hop in and out of the surface layer. A recursive treatment of the impurity effects is made possible through successive elimination of the lattice layer by layer. This leads to nonlinear renormalization group flow of an effective surface impurity potential. We found an exact mapping between the recursion relation and Schrodinger equation along the layers, therefore the modified self energy due to surface impurity could be simply obtained from the transfer matrix method. As a concrete example of 2d topological insulator, we found the exact expression of onlayer self energy for a clean system and an asymptotic expression that captures a general behavior of layers deep in the bulk.

Condensed Matter Seminar Wednesday, April 23, 2014 11:00 AM Physics Building, Room 210 
ABSTRACT:
An unusual manifestation of Mott physics dependent on strong spinorbit interactions has recently been identified in a growing number of classes of 5d transition metal oxides built from Ir^{4+} ions. Instead of the naively expected increased itinerancy of these iridates due to the larger orbital extent of their 5d valence electrons, the interplay between the amplified relativistic spinorbit interaction (intrinsic to large Z iridium cations) and their residual onsite Coulomb interaction U, conspires to stabilize a novel class of spinorbit assisted Mott insulators with a proposed J_{eff}=1/2 ground state wavefunction. The identification of this novel spinorbit Mott state has been the focus of recent interest due to its potential of hosting a variety of new phases driven by correlated electron phenomena (such as high temperature superconductivity or enhanced ferroic behavior) in a strongly spinorbit coupled setting. Currently, however, there remains very little understanding of how spinorbit Mott phases respond to carrier doping and, more specifically, how relevant U remains for the charge carriers of a spinorbit Mott phase once the bandwidth is increased. Here I will present our groupâs recent experimental work exploring carrier doping and the resulting electronic phase behavior in one such spinorbit driven Mott material, Sr_{3}Ir_{2}O_{7}, with the ultimate goal of determining the relevance of U and electron correlation effects within the doped systemâs ground state. Our results reveal the stabilization of an electronically phase separated ground state in Bsite doped Sr_{3}Ir_{2}O_{7}, suggestive of an extended regime of localization of inplane doped carriers within the spinorbit Mott phase. This results in a percolative metaltoinsulator transition with a novel, global, antiferromagnetic order. The electronic response of Bsite doping in Sr_{3}Ir_{2}O_{7} will then be compared with recent results exploring Asite doping of electrons into the system and the resulting electronic phase diagrams discussed.

Condensed Matter Seminar Thursday, April 17, 2014 3:30 PM Physics Building, Room 204 
"The internal structure of a vortex and its exited states, in a two dimensional superfluid"Oded Agam , Hebrew University [Host: Israel Klich]
ABSTRACT:
The motion of quantum vortex in a twodimensional
spinless superfluid is analyzed within Popov's hydrodynamic description. In the long healing length limit (where a large number of particles are inside the vortex core) the superfluid dynamics is determined by saddle points of Popov's action, which allows for weak solutions of the GrossPitaevskii equation. The resulting equations are solved for a vortex moving with respect to the superfluid. It is found that the vortex core is reconstructed in a nonanalytic way. The response of the vortex to applied force produces an anomalously large dipole moment of the vortex and, as a result, the spectrum associated with the vortex motion exhibits narrow resonances lying within the phonon part of the spectrum, contrary to traditional view.

Condensed Matter Seminar Thursday, April 10, 2014 3:30 PM Physics Building, Room 204 
"Matrix Product States for the Fractional Quantum Hall Effect"Roger Mong , Caltech [Host: Paul Fendley]
ABSTRACT:
The quantum Hall effect is an exotic phenomena found in nature. At fractional filling, these phases support fractional charge excitations with anyonic braiding statistics, which may be suitable for topological quantum computation. I will discuss the recent progress on studying these phases using matrix product states (MPSs) and the densitymatrixrenormalization group (DMRG) technique. On one hand the MPS structure for quantum Hall reveals deep connection between quantum entanglement, conformal field theory, and topological field theory. Pragmatically, an understanding of the MPS structure allows us to perform efficient DMRG simulations for physical quantum Hall systems. There, the key advance is to reliably get the ground state wavefunction from a microscopic Hamiltonian, and to be able to identify the braiding statistics of the excitations solely from the ground state. Finally, I will also discuss various applications of these numerical methods. 
Condensed Matter Seminar Monday, February 24, 2014 3:30 PM Physics Building, Room 204 
"From Topological Insulators to Majorana Fermions"Fan Zhang , University of Pennsylvania [Host: Paul Fendley]
ABSTRACT:
The discovery of topological insulators has created a revolution in condensed matter science that has far ranging implications over coming decades. I will introduce a simplest way to understand various topological phases that can fit into an elegant periodic table, and apply these ideas to semimetals, insulators, and superconductors. In the case for a timereversalinvariant topological superconductor, a Majorana Kramers pair may appear on the boundary and induces unprecedented fractional Josephson effects. These effects indicate the existence of a periodic building for topological phases, with the aforementioned table being its ground floor. Strong connections will be made to ongoing experiments and related topics.

Condensed Matter Seminar Thursday, February 20, 2014 3:30 PM Physics Building, Room 204 
"Exotic Zero Energy Modes at Topological Defects in Crystalline Superconductors"Jeffrey Teo , University of Illinois at UrbanaChampaign [Host: Paul Fendley]
ABSTRACT:
Majorana bound states (MBS) are zero energy pointlike excitations that support robust nonlocal storage of quantum information and nonabelian unitary operations. These exotic properties are essential requirements of a topological quantum computer. MBS were predicted to be present at magnetic flux vortices in spinless chiral pwave superconductors. We show that MBS can also arise at ``gravitational" vortices in the form of lattice defects, such as dislocations, disclinations and corners, in a wider class of topological crystalline superconductors. We completely classify these BCS superconductors in two dimensions with discrete crystalline symmetries, and propose a criterion that determines the existence of MBS at a defect. This predicts the appearance of MBS at lattice defects in superconducting strontium ruthenate, regardless of its controversial pairing nature, and also doped graphene or silicene, which are single layers of carbon or silicon.

Condensed Matter Seminar Monday, February 17, 2014 3:30 PM Physics Building, Room 204 
"The BulkEdge Correspondence in Abelian Fractional Quantum Hall States"Michael Mulligan , Station Q/UCSB [Host: Paul Fendley]
ABSTRACT:
It is commonly assumed that a given bulk quantum Hall state and its low energy edge excitations are in onetoone correspondence. In this talk, I will explain, contrary to this conventional wisdom, how a given bulk state may host multiple, distinct edge phases. I will describe a few surprising examples of this phenomenon and discuss experimental consequences.

Condensed Matter Seminar Thursday, February 6, 2014 3:30 PM Physics Building, Room 204 
"Controlling Atomic Movement on the Nanoscale"Sinisa Coh , University of California, Berkeley [Host: Paul Fendley]
ABSTRACT:
Some of the grand challenges in nanoscience are the ability to control movement of atoms either to propel nanometersized machines, or to synthesize novel electronic devices and materials. To that end, electrical current can be used to move a wide range of metals (Fe, Cu, W, In, Ga) along the outside and inside of a carbon nanotube. In this talk I will present a peculiar mechanism in which these metals move. For example, this mechanism allows an iron nanocrystal to pass through a constriction in the carbon nanotube with a smaller crosssectional area than the nanocrystal itself. Remarkably, while passing through a constriction, the nanocrystal remains largely solid and crystalline and the carbon nanotube is unaffected. This behavior is accounted for by a pattern of iron atom motion and rearrangement on the surface of the nanocrystal. The nanocrystal motion can be described with a model whose parameters are nearly independent of the nanocrystal length, area, temperature, and electromigration force magnitude. I will also discuss implications of this work on synthesis of nanocomposite materials, and on the stability of carbonbased electronic devices.
More details can be found in these publications:
Phys. Rev. Lett. 110, 185901 (2013)
Phys. Rev. B 88, 045424 (2013) 
Condensed Matter Seminar Monday, February 3, 2014 3:30 PM Physics Building, Room 204 
"Casimir forces and torques. from quantum fluctuations to self assembly"Raul EsquivelSirvent , Universidad Nacional AutÃ³noma de MÃ©xico [Host: Israel Klich] 
Condensed Matter Seminar Friday, January 24, 2014 2:30 PM Physics Building, Room 313 
"Simulating condensed matter systems with tensor network states and discovery of algebraic decoherence"Thomas Barthel , LudwigMaximiliansUniversitÃ¤t Munich [Host: Paul Fendley]
ABSTRACT:
The nonlocality of quantum manybody systems can be quantified by entanglement measures. Studying the scaling behavior of such measures, one finds that the entanglement in most states of interest (occurring in nature) is far below the theoretical maximum. Hence, it is possible to describe such systems with a reduced set of effective degrees of freedom. This is exploited in simulation techniques based on socalled tensor network states (MPS, PEPS, or MERA). I will describe how this approach can be employed to simulate systems of all particle statistics in order to study ground states, thermal states, and nonequilibrium phenomena. Besides explaining the main ideas, I will highlight some applications.
The second part of the talk focuses on an application to the decoherence in systems that are coupled to an environment. Until our recent study, it was assumed that, as long as the environment is memoryless (i.e. Markovian), the temporal coherence decay is always exponential  to a degree that this behavior was synonymously associated with decoherence. However, the situation can change if the system itself is a manybody system. For the open spin1/2 XXZ model, we have discovered that the interplay between dissipation and internal interactions can lead to a divergence of the decohernce time. The quantum coherence then decays according to a power law. To complement the quasiexact numerical simulation, I will explain the result on the basis of a perturbative treatment. 
Condensed Matter Seminar Thursday, January 23, 2014 3:30 PM Physics Building, Room 204 
"Equilibration and Thermalization in Quantum ManyBody Systems"Alioscia Hamma , Tsinghua University [Host: Paul Fendley]
ABSTRACT:
A closed quantum mechanical manybody system evolves unitarily. Therefore, in a strict sense, it should not equilibrate or thermalize. However, thermalization occurs, as it is revealed in experiments in ultra cold atom gases with very long coherence times. How do equilibration and thermalization occur then? Even if equilibration cannot happen at the level of the wavefunction, it may happen in the expectation value of typical observables, as Von Neumann first pointed out. Recently, it has been understood that equilibration and thermalization are due to the typicality of Entanglement. In this talk, I will review some of the recent theoretical progress made in this field. Typically, quantum states are very entangled, and this implies that locally they look like thermal states. A longstanding problem is the meaning and role of integrability for thermalization. Recent novel results show that the structure of Entanglement reveals the connection between nonintegrability, irreversibility, and thermalization. Finally, I will describe how some exotic states of matter also show exotic ways of equlibrating, which is very promising for quantum information processing.

Condensed Matter Seminar Tuesday, January 21, 2014 3:30 PM Physics Building, Room 204 
"What would you do with the exact functional? Probing the limits of density functional theory (DFT)"Miles Stoudenmire , Perimeter Institute [Host: Paul Fendley]
ABSTRACT:
Density functional theory (DFT) is a very successful method for computing properties of realistic manyelectron systems applied across condensed matter and materials physics. It is based on exact principles, but using it in practice requires approximations. Unfortunately, present approximations have systematic drawbacks, such as failing to produce accurate charge gaps for strongly correlated systems or predicting spurious magnetic order.
To understand DFTâs potential and limitations, we have extended the powerful density matrix renormalization group (DMRG) technique to solve onedimensional continuum electron systems with realistic interactions. Such systems are also interesting in their own right (in the context of cold atom experiment, for example). With our ability to solve these systems essentially exactly, we have even implemented the exact functional at the heart of DFT. I will discuss our results on computing gaps within DFT (both exact and approximate) and a recent proof from our group that KohnSham DFT always converges when using the exact functional. We find that while some drawbacks of DFT can be blamed on approximations, other limitations are fundamental. Yet our work suggests that great progress is possible for applying DFT to strongly correlated systems. 
Condensed Matter Seminar Thursday, January 16, 2014 3:30 PM Physics Building, Room 204 
ABSTRACT:
The name Spin Ice refers to a class of materials where rareearth magnetic moments reside on a pyrochlore lattice of corner sharing tetrahedron. Since their discovery in 1997 by Harris and colleagues, these materials (such as
Dy2Ti2O7 and Ho2Ti2O7) have been a mainstay of classical frustrated magnetism research. The analogy to ice arises because the magnetic groundstate is actually a degenerate manifold of equalenergy states, which leads to the same groundstate entropy as the protonpositional disorder in ice water, calculated by Linus Pauling. The highlyconstrained spin ice groundstate is an example of a classical "spin liquid", which harbors a fascinating array of phenomena, such as emergent monopole excitations and topological winding number sectors. In this talk, I will give a basic introduction to the physics of spin ice from the perspective of Monte Carlo simulations, including the necessity of considering dipolar interactions, its behavior in magnetic fields, and topological order in ice systems with reduced dimensionality.

Condensed Matter Seminar Thursday, December 12, 2013 3:30 PM Physics Building, Room 313 
ABSTRACT:
I will discuss the physics of mixing and elasticity on random graphs. The results I will show are important step toward understanding on the macroscopic properties of amorphous solids, spin glasses and alike and are also closely related to fields of interest of computer science. The first part of the talk is about elasticity. The physical properties of amorphous solids are far less understood than those of crystalline solids. The analysis of these systems is complicated due to disorder and vastly different interaction strengths present in these materials. I will look at spectral properties of random elastic networks and argue in which sense they provide a good toymodel of disordered solids. Using the Cavity method, a sort of BethePeierls iterative method, I will derive the analytical expressions for the spectral density of such graphs. I will conclude this part by pointing out implications of these results on the physics of amorphous solids. Next I plan to talk about a new type of numerics for mixing on random graphs. These results are about Markov Chain Monte Carlo algorithms and have potential applications in studying granular materials, colloids, protein folding, etc. Most implementations of these algorithms use Markov Chains that obey detailed balance, even though this not a necessary requirement for converging to a steady state. I plan to overview several examples that utilize irreversible Markov Chains, where violating detailed balance has improved the convergence rate. Finally I will pose some open questions and discuss attempts to use nonequilibrium dynamics for efficient sampling.

Condensed Matter Seminar Thursday, December 5, 2013 3:30 PM Physics Building, Room 204 
"Full counting statistics for matrix product states"Yifei Shi , University of Virginia [Host: Israel Klich] 
Condensed Matter Seminar Thursday, November 21, 2013 3:30 PM Physics Building, Room 204 
"Theory of Resonant XRay Scattering with Applications to highTc Cuprates"David Benjamin , Harvard University [Host: Israel Klich]
ABSTRACT:
Resonant xray scattering has recently become an exceptionally powerful and versatile technique. However, its theoretical foundation lags behind experiments. In particular, experiments on conducting phases are often interpreted in terms of local models based on Mott insulators. I will discuss a method that handles arbitrary band structures and includes the effect of the positivelycharged core hole. Next, I will compare our results to experiments on conducting and charge density wave cuprate phases and show how simple quasiparticle physics yields signatures previously considered evidence of more exotic behavior.

Condensed Matter Seminar Thursday, November 7, 2013 3:30 PM Physics Building, Room 204 
"Studying Topological Insulators via time reversal symmetry breaking and proximity effect"Roni Ilan , UC Berkeley [Host: Israel Klich]
ABSTRACT:
Topological Insulators (TI) have been at the focus of immense theoretical and experimental studies in the past few years. They represent accessible topological phases, where time reversal symmetry plays a key role. These systems provide a platform for the emergence of many exciting physical phenomena, such as helical transport in one and two dimensions, physics of Dirac fermions and the formation of Majorana fermion states. While the theoretical understanding of these materials progresses, it is crucial that theorists make concrete proposals for simple and informative ways to detect all the properties attributed to them. In this talk I will describe our proposals for transport experiments, the most common way to probe physics in condensed matter systems. I will present a few interesting questions concerning the current challenges transport experiments in TI are facing, discuss which physical phenomena are yet to be confirmed, and suggest two experimental schemes meant to detect the physics of the surface states in both two and three dimensional TI.

Condensed Matter Seminar Thursday, October 31, 2013 3:30 PM Physics Building, Room 204 
"Microcavity ExcitonPolariton Condensates Physics and Applications"Na Young Kim , Stanford University [Host: Seunghun Lee]
ABSTRACT:
Microcavity excitonpolaritons are hybrid lightmatter quasiparticles as an admixture of cavity photons and quantum well excitons. The inherent lightmatter duality provides experimental advantages to form coherent condensates at high temperatures (e.g. 4 K in GaAs and room temperature in GaN materials), and to access the
dynamics of excitonpolaritons.
I will first discuss the characteristics of excitonpolariton condensates with emphasis on their intrinsic opendissipative nature. I will present excitonpolaritonlattice systems, where we explore the nonzero momentum condensate order. We envision that the polaritonlattice systems would serve as a solidstate platform to investigate strongly correlated materials. Finally, I will show our recent progress on electrically pumped excitonpolariton coherent matter waves towards the development of novel coherent light sources operating at low threshold powers and at high temperatures. 
Condensed Matter Seminar Thursday, October 24, 2013 3:30 PM Physics Building, Room 204 
"The Interplay of Correlation, ElectronPhonon Coupling, Magnetism, and Superconductivity in High Tc Superconductors"Zhiping Yin , Rutgers University [Host: Joe Poon]
ABSTRACT:
Strong electronic correlation lies at the heart of modern condensed matter and material physics. An adequate treatment of the electronic correlation beyond standard band theory is a key to understand many unusual physical properties of strongly correlated materials such as Mott insulators and unconventional superconductors. In this talk, I will show how electronic correlation leads to dramatic modifications of the band theory predictions for the electronic structures in general, and the electronphonon coupling and spin dynamics in particular in high temperature superconductors. I will demonstrate that firstprinciples electronic structure methods GW and dynamical mean field theory (combined with density functional theory) can account for the nonlocal and local electronic correlations, respectively, beyond standard band theory, and are useful tools to understand the microscopic mechanisms of high temperature superconductivity in (Ba,K)BiO3 and ironbased superconductors. For the former, electronic correlation leads to a strong enhancement of the electronphonon coupling while for the latter, electronic correlation significantly strengthens the lowenergy spin excitation, both of which favor higher superconducting temperature. In the end, I will discuss how we may use these firstprinciples methods to accelerate the discoveries of new functional materials with desired properties.
References: 
Condensed Matter Seminar Thursday, October 17, 2013 3:30 PM Physics Building, Room 204 
"Gauge dynamics of kagome antiferromagnets"Michael Lawler , Binghamton University [Host: Israel Klich]
ABSTRACT:
How to describe the semiclassical dynamics of spins in kagome lattice Heisenberg antiferromagnetic insulators remains a unsolved problem. In essence, the largest term in their classical Hamiltonian merely selects out a low energy sector leaving a great freedom for them to roam around within it. To make progress on the problem, I have employed the "Dirac constraint" method used to study problems in constrained Hamiltonian mechanics. This method is excellent at identifying the "physical modes" of a system, particularly when there is an unexpected gauge redundancy. Remarkably, for the spin waves of kagome antiferromagnets in their low energy sector, I found an extreme reduction in their number with it growing at best with the size of the boundary of the system. We have also carried out numerical simulations on the Heisenberg model with additional DzyaloshinskiiMoriya interactions and found strong evidence for both, even though no specific restriction was made to the low energy sector. These results make significant progress on the solution to the problem of spin dynamics in kagome antiferromagnets and we hope similar progress could be made on a wide variety of other condensed matter systems.

Condensed Matter Seminar Thursday, September 26, 2013 3:30 PM Physics Building, Room 204 
"Metal insulator transitions in Ir oxides with strong SOC: a case of SrIrO_{3} and related compounds"Yoon H. Jeong , Pohang University of Science and Technology [Host: Seunghun Lee]
ABSTRACT:
Motivated by the rich physics anticipated from strong spin orbit coupling, we have investigated various 5d Ir oxides. We will first describe the general physics of Ir compounds arising from strong SOC, and then focus on the metal insulator transitions occurring in perovskite SrIrO_{3} and related compounds.

Condensed Matter Seminar Thursday, September 5, 2013 3:30 PM Physics Building, Room 204 
"Fock Parafermions and SelfDual Representations of the Braid Group"Emilio Cobanera , Leiden University [Host: Israel Klich]
ABSTRACT:
Because of potential relevance to topological quantum information processing, we introduce and study the selfdual family of representations of the braid group. Selfdual representations are physically motivated by strongcoupling/weakcoupling dualities in clock models and include as special cases the Majorana and Gaussian (metaplectic) representations. To show that selfdual representations admit a particle interpretation, we introduce and describe in second quantization a family of particle species with (p=2,3,dots) exclusion and ( heta=2pi/p) exchange statistics. We call these anyons Fock parafermions, because they are the particles naturally associated to the parafermionic zeroenergy modes potentially realizable in mesoscopic arrays of fractional topological insulators. Selfdual representations are local combinations of either parafermions or Fock parafermions, an important requisite for the potential physical implementation of (semi)topologically protected quantum logic gates. The secondquantization description of Fock parafermions entails the concept of Fock algebra, i.e., a Fock space endowed with a statistical multiplication that captures and logically correlates these anyons' exclusion and exchange statistics. As a consequence, normalordering remains a welldefined operation.

Condensed Matter Seminar Thursday, August 15, 2013 3:30 PM Physics Building, Room 204 
"Metabolic Imaging: A Novel Diagnostic Strategy for Hypertensive Heart"Min Zhong , University of Virginia [Host: Bijoy Kundu] 
Condensed Matter Seminar Thursday, April 25, 2013 3:30 PM Physics Building, Room 204 
"Z_2 spin liquid in the Heisenberg antiferromagnet on kagome"Oleg Tchernyshyov , Johns Hopkins University [Host: Paul Fendley]
ABSTRACT:
A spin liquid is a hypothetical quantum ground state of a frustrated Heisenberg magnet conjectured by P.W. Anderson in the 1970s. Its distinguishing features are (1) a lack of magnetic order and (2) a degeneracy that depends on the topology of the system. The status of spin liquids has been recently upgraded from purely speculative to plausible as these ground states were found in relatively simple twodimensional spin models on square and kagome lattices. Experiments with herbertsmithite yielded clues of spin excitations with fractional spin 1/2 in a kagome antiferromagnet.
I will present a phenomenological Z_2 lattice gauge theory that describes lowenergy properties of the spin liquid in a S=1/2 Heisenberg antiferromagnet on kagome. It reproduces many of the characteristic features observed in recent numerical studies of the model, including the groundstate degeneracy, response to quenched disorder, and spinon states near the system's edge. 
Condensed Matter Seminar Thursday, April 18, 2013 3:30 PM Physics Building, Room 204 
"Quantum Critical Behavior in a Resonant Level Coupled to a Dissipative Environment"Gleb Finkelstein , Duke University [Host: Israel Klich]
ABSTRACT:
We investigate tunneling through a resonant level embedded in a dissipative environment, which suppresses tunneling rates at low temperatures. Specifically, the resonant level is formed in a carbon nanotube quantum dot, and the dissipative environment is realized by fabricating resistive leads. For the symmetric coupling of the resonant level to the two leads, we find that the resonant peak reaches the unitary conductance e^2/h despite the presence of dissipative modes. Simultaneously, the width of the resonance tends to zero as a nontrivial power of temperature. We draw a connection between our system and a resonant tunneling in a Luttinger liquid and interpret the observed unitary resonance of vanishing width in terms of a quantum critical point (QCP). We further investigate an exotic state of electronic matter obtained by finetuning the system exactly to the QCP. Particularly striking is a quasilinear nonFermi liquid scattering rate found at the QCP, interpreted in terms of a Majorana resonant level model.

Condensed Matter Seminar Thursday, April 11, 2013 3:30 PM Physics Building, Room 204 
ABSTRACT:
An informal talk about the quantum adiabatic algorithm for solving optimization problems using a quantum computer, and about attempts to simulate this algorithm on classical computers using quantum Monte Carlo. Interestingly, the child's puzzle in this YouTube video
http://www.youtube.com/watch?v=dyWXPJSSRw8 will have direct applications to this problem.

Condensed Matter Seminar Wednesday, April 10, 2013 3:30 PM Physics Building, Room 313 Quantum Information Seminar 
"Magnetic ground states and excitations in vanadates, BaV10O15 and CoV2O4"Sachith Dissanayake , University of Virginia [Host: Seunghun Lee] 
Condensed Matter Seminar Thursday, April 4, 2013 3:30 PM Physics Building, Room 204 
"Transport Phenomena in CaVO3 and SrVO3 thin films"Mandy Gu , University of Virginia [Host: Stu Wolf] 
Condensed Matter Seminar Thursday, March 28, 2013 3:30 PM Physics Building, Room 204 
"What is an essential factor for determining Tc in Iron Pnictides? 
Condensed Matter Seminar Thursday, March 14, 2013 11:00 AM Physics Building, Room 204 
ABSTRACT:
Graphene possess unique properties for electronic and photonic applications, such as gate tunability, high carrier mobility, wideband optical absorption extending into terahertz regime and compatibility with silicon processing technologies. Research in graphene device arena has progressed at a rapid pace, propelled by constant discoveries and maturing appreciation of the underlying graphene physics, advancement in processing technologies and most importantly, innovations. In this talk, I will review some of the exciting latest developments in graphene electronics and photonics. I would then provide a device physics perspective of what I think are some current issues limiting their performances and new opportunities going forward. Throughout this talk, I will draw extensively upon theoretical and modeling studies of our inhouse experiments.
Here are some details of the talk. First, the internal and substrate polar optical phonons provide main energy dissipation pathway for optically excited carriers, and I will discuss our understanding of these energy loss channels and possibilities for more efficient graphene photodetectors and bolometers driven by hot electrons and phonons. Second, coupling of collective electronic excitations with these phonons were found leading to modified plasmon dispersions and losses, where longlived hybrid plasmonphonon coupled mode can be utilized in the terahertz to infrared spectrum for highly tunable plasmonic devices. Lastly, I will discuss how deformation and morphological structures found in large scale growth graphene can serve as dominant electronic scattering centers, compromising performance in highspeed electronic devices and the possibility of strain engineering for exploratory novel graphene electronics. 
Condensed Matter Seminar Thursday, February 14, 2013 3:30 PM Physics Building, Room 204 
"Toward a unified description of spin incoherent behavior at zero and finite temperatures"Adrian Feiguin , Northeastern University [Host: Israel Klich]
ABSTRACT:
While the basic theoretical understanding of spincharge separation in onedimension, known as "Luttinger liquid theory", has existed for some time, recently a previously unidentified regime of strongly interacting onedimensional systems at finite temperature came to light: The "spinincoherent Luttinger liquid" (SILL). This occurs when the temperature is larger than the characteristic spin energy scale. I will show that the spinincoherent state can be written exactly as a generalization of Ogata and Shiba's factorized wave function in an enlarged Hilbert space, using the socalled "thermofield formalism." Interestingly, this wavefunction can also describe the *groundstate* of other model Hamiltonians , such as tJ ladders, and the Kondo lattice. This allows us to develop a unified formalism to describe SILL physics both at zero, and finite temperatures.

Condensed Matter Seminar Tuesday, November 13, 2012 3:30 PM Physics Building, Room 204 
"Spinlattice coupling and anomalous magnetic excitations in the triangular lattice antiferromagnet Ag2CrO2"Masa Matsuda , Oak Ridge National Laboratory [Host: Seunghun Lee]
ABSTRACT:
Spinlattice coupling plays an important role in selecting the ground state in the geometrically frustrated magnets, since a small amount of structural distortion is sufficient to lift the ground state degeneracy and stabilize a longrange magnetic order. Ag2CrO2 consists of insulating triangular lattice planes of CrO2 (Cr3+ ion with S=3/2), which are separated by the metallic Ag2 layers. Interestingly, the electric transport in the Ag2 layer is strongly affected by the magnetism in the CrO2 layer. We found from their neutron diffraction experiments that a partially disordered state with 5 sublattices abruptly appears at TN=24 K, accompanied by a structural distortion [1]. The spinlattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of furtherneighbor interactions and temperature. The nonnegligible furtherneighbor interactions suggest the existence of the RKKY interaction mediated by the conduction electrons. We have also performed inelastic neutron scattering study of this material and found anomalous magnetic excitations, which cannot be explained simply by the linear spinwave theory. The exotic behaviors in Ag2CrO2 are noteworthy.
[1] M. Matsuda et al., PRB 85, 144407 (2012)

Condensed Matter Seminar Friday, November 9, 2012 2:30 PM Physics Building, Room 313 
"Nonabelian statistics of fractionalized Majorana Fermions"Netanel Lindner , Caltech [Host: Israel Klich]
ABSTRACT:
Edge states of topological insulators can be gapped out by introducing a coupling to a superconductor or by breaking time reversal symmetry. The interface between these two types of gapped regions hosts a Majorana zero mode. Here, we extend this idea to the case of edge
states of fractional quantum Hall states. We show that as more interfaces are introduced, the ground state degeneracy grows with a quantum dimension of a square root of an even integer, corresponding to a new family of nonabelian anyons. Topologically protected braiding of two anyons can be achieved by a sequence of operations on the ground state manifold. We show that the unitary matrices representing these operations form representations of the braid group
which are richer than that of Ising anyons. We discuss possible realizations of these ideas in experimentally accessible solid state systems.

Condensed Matter Seminar Thursday, November 8, 2012 3:30 PM Physics Building, Room 204 
"Unconventional 'optics' with pseudospins in graphene: Metamaterials, Klein Tunneling and Subthermal switching"Avik Ghosh , University of Virginia [Host: Bellave Shivaram]
ABSTRACT:
A major motivation for graphene based electronics lies in its photonlike bandstructure, which makes the electron effective mass vanishingly small and mobilities much larger than their silicon counterparts. In practice however, charge puddles wash out the Dirac points and produce quasiOhmic currentvoltage characteristics, making it hard to switch graphene electrons or to saturate their currents. Long channel devices saturate through remote optical phonon scattering, but are almost immediately compromised by bandtoband tunneling. One can
open a bandgap using quantization (e.g. nanoribbons), local strains, antidot arrays or transverse fields in bilayer geometries. But the
broken symmetry invariably increases the mass of the electrons and compromises mobility. This tradeoff seems fundamental.
The richness of graphene electronics lies not just in its photon like eigenspectrum, but in the symmetry of its eigenvalues, specifically, the pseudospins arising from its dimer basis sets. On the one hand quasimomentum conservation at a PN junction generates electronic analogues of Snell's law such as focusing, total internal reflection, and even negative index Veselago 'lensing'. On the other hand, the orthogonality of its pseudospins leads to Klein/antiKlein tunneling in mono/bilayer graphene, for which there seem to be experimental evidence in close agreement with atomistic models for current flow. By solving the LandauerKeldysh quantum kinetic equations, we show that such electron 'optics' and Klein tunneling can be used to design novel low power switches that can beat the Landauer Boltzmann thermal limit, including reconfigurable logic, metalinsulator transition switches, electron collimators and pseudospintronic analogs of electrooptic modulators. 
Condensed Matter Seminar Thursday, November 1, 2012 3:30 PM Physics Building, Room 204 
"Modal analysis of Casimir interactions"Francesco Intravaia , Los Alamos National Laboratory [Host: Israel Klich]
ABSTRACT:
While other numerical and analytic techniques exist, the modal expansion is one of the few that provides physical insight into the anatomy of the Casimir Effect. Although it is very simple in a small number of situations, this kind of analysis becomes quite involved when phenomena such as dissipation in the media or complicated geometries are taken into account for the calculation of the Casimir effect. Moreover, in this case, its connection and equivalence to other approaches becomes blurry and puzzling. The knowledge of the interplay between (quantum) thermodynamics, geometry, dissipation and mode decomposition can be important to understand the strength, the sign and, in general, the behavior of the Casimir force. Here, we investigate some aspects of the modal approach to Casimir forces, also presenting experimentally relevant examples.

Condensed Matter Seminar Thursday, October 25, 2012 3:30 PM Physics Building, Room 204 
"Quantum criticality and superconducting gap evolution in Febased superconductors"Kyuil Cho , The Ames Laboratory and Iowa State University [Host: Seunghun Lee]
ABSTRACT:
The quantum critical point (QCP), where the transitions occur at absolute zero temperature, usually induces a new phase, such as unconventional superconductivity in organic, cuprate, heavy fermion and Febased superconductors. Whether the quantum critical point still exists beneath the superconducting phase is a longstanding question, but very tricky task since most of physical and magnetic properties are useless inside of superconducting phase. Here we have measured the zerotemperature penetration depth in BaFe_{2}(As_{1x,}P_{x})_{2} by using tunnel diode resonator technique and identified the existence of the QCP beneath the superconducting dome [1]. In addition, the lowtemperature analysis of the penetration depth measurements revealed that the superconducting gap structure shows a universal domelike evolution as the dopant concentration increases. The details of experimental and theoretical consideration will be discussed.
[1] K. Hashimoto, Kyuil Cho, et al., Science 336, 1554 (2012). 
Condensed Matter Seminar Thursday, October 18, 2012 3:30 PM Physics Building, Room 204 
"Unconventional temperatureenhanced magnetism in Fe1.1Te"Igor Zaliznyak , Brookhaven National Laboratory [Host: Seunghun Lee]
ABSTRACT:
There are two common scenarios used to describe the magnetism in Febased superconductors.
In one, the magnetism originates from local atomic spins, while in the other it corresponds to a cooperative
spindensitywave instability (SDW) behavior of conduction electrons. Both assume clear partition into
localized electrons, giving rise to local spins, and itinerant ones, occupying welldefined, rigid conduction
bands. We have used inelastic neutron scattering to characterize both the static and the dynamic magnetism
in a crystal of Fe1.1Te, parent to Fe{1+y}Te{1+x}Se{x} family of superconductors [1]. In contrast to the simple
pictures, we find that localized spins and itinerant electrons are coupled together. In particular, we have
evaluated the effective magnetic moment by integrating both the elastic and inelastic magnetic scattering.
The effective spin per Fe at T = 10 K, in the antiferromagnetic phase, corresponds to S = 1, consistent
with the recent analyses that emphasize importance of Hundâs intraatomic exchange. However, it grows to
S =3/2 in the disordered phase, a result that presents a challenge to current theoretical models.

Condensed Matter Seminar Thursday, May 3, 2012 3:30 PM Physics Building, Room 204 
"Novel Semiconductor and Epitaxial Nanocomposite Materials for Energy Conversion and Optoelectronic Applications"Joshua M. O. Zide , University of Delaware [Host: ECE, MAE, & Physics]
ABSTRACT:
Advances in electronic materials (specifically, semiconductors and nanocomposites) enable new device technologies and improve the properties of existing technologies. In this talk, I will present efforts within my group on the growth of new materials by molecular beam epitaxy and the resulting advances in solar cells, thermoelectrics, and optoelectronics.
Specifically, I will discuss two material systems: (1) nanocomposites consisting of metallic nanoparticles (such as ErAs and TbAs) within IIIV semiconductors (such as InGaAs and GaAs), and (2) dilute bismuthide semiconductors in which bismuth is incorporated into IIIV materials to reduce the bandgap significantly, with unique band alignments that cannot be easily achieved in other materials. In these new materials, electronic, thermal, and optical properties can be quite different from those of conventional materials, with significant promise for applications in a variety of (opto)electronic devices. 
Condensed Matter Seminar Friday, April 20, 2012 2:00 PM , Room MEC 341 A Joint Seminar Sponsored by Electrical and Computer Engineering, Mechanical and Aerospace Engineering, and Physics 
"Transport Properties of VO_{2} Films near the MetalSemiconductor Transition"Salinporn Kittiwatanakul , University of Virginia [Host: Stu Wolf]
ABSTRACT:
Vanadium dioxide (VO_{2}) exhibits a metal semiconductor transition (MST) at 340 K. This transition is accompanied by the abrupt change in the electrical conductivity, optical transmittance and reflectance in infrared region, which can be used in the electronic devices such as temperature sensors and electric switches. In this study, Reactive Bias Target Ion Beam Deposition was used for epitaxial VO_{2} growth on TiO_{2} (100) substrates with fixed O_{2} flow rate at 5.0 sccm to study transport anisotropy, and for highly textured VO_{2} growth on cplane Al_{2}O_{3} substrates to study the effect of different O_{2} flow rates (4.56.0 sccm). The conductivity anisotropy ratio σc/σb of VO_{2}/TiO_{2} film was found to be ~41.5 at 300 K, much larger than that of single crystal VO_{2} and it is the largest among those previously reported. The temperature dependent anisotropy of the carrier concentration and the mobility is to be discussed. With XPS and XAS, the valence state of vanadium on different VO_{2}/cAl_{2}O_{3} films was investigated. As the O_{2} flow rate increases, the XRD results show decreasing lattice parameter, hence increasing compressive strain along baxis of monoclinic VO_{2}; the transport measurements also show the increasing transition temperature (T_{MST}) and the increasing change in resistivity associated with the strain. The correlation among the valence state, the strain and MST in VO_{2} will be discussed.

Condensed Matter Seminar Thursday, April 19, 2012 3:30 PM Physics Building, Room 204 
"Radiation Effect on Materials for Nanomagnetism and Spintronics Application"Tom Anuniwat , University of Virginia [Host: Stu Wolf] 
Condensed Matter Seminar Thursday, April 12, 2012 3:30 PM Physics Building, Room 204 
"Torsion and Viscosity in Condensed Matter Physics"Taylor Hughes , University of Illinois at UrbanaChampaign [Host: Israel Klich]
ABSTRACT:
In this talk I will review the common appearance of torsion in solids as well as some new developments. Torsion typically appears in condensed matter physics associated to topological defects known as dislocations. Now we are beginning to uncover new aspects of the coupling of torsion to materials. Recently, a dissipationless viscosity has been studied in the quantum Hall effect. I will connect this viscosity to a2+1d torsion ChernSimons term and discuss possible thought experiments in which this could be measured. If time permits, I will briefly discuss a new topological defect in 3+1d, a torsional skyrmion, which does not require a lattice deformation to exist in solids. If present, torsional skyrmions are likely to impact the propagation of electrons in materials with strong spinorbit coupling such as topological insulators and spinorbit coupled semiconductors.

Condensed Matter Seminar Thursday, April 5, 2012 3:30 PM Physics Building, Room 204 
"Magnetic excitation in the iron based superconductor, FeTe1xSex: fluctuation near maximal quantum criticality"Jooseop Lee , University of Virginia [Host: Seunghun Lee] 
Condensed Matter Seminar Tuesday, April 3, 2012 3:30 PM Physics Building, Room 204 
ABSTRACT:
The prediction and experimental discovery of topological band insulators and topological superconductors are recent examples of how topology can characterize phases of matter. In these examples, electronic interactions do not play a fundamental role. In this talk I shall discuss cases where interactions lead to new phases of matter of topological character. Specifically, I shall discuss fractional topological states in lattice models which occur when interacting electrons propagate on flattened Bloch bands with nonzero Chern number. Topologically ordered manyparticle states can emerge when these bands are partially filled, including a possible realization of the fractional quantum Hall effect without external magnetic fields. I shall also ponder on the possible practical applications, beyond metrology, that the quantized charge Hall effect might have if it could be realized at high temperatures and without external magnetic fields in strongly correlated materials.

Condensed Matter Seminar Thursday, March 22, 2012 3:30 PM Physics Building, Room 204 
"The topological insulator in a Fermi sea  sink or swim?"Doron Bergman , Caltech [Host: Israel Klich]
ABSTRACT:
In the flurry of experiments looking for topological insulator
materials, it has been found that some bulk metals very
close to topological insulator electronic states support the
same topological surface states that are the defining
characteristic of the topological insulator. First observed in
spinpolarized angle resolved photoemission spectroscopy
(ARPES) in Sb [D. Hsieh et al., Science 323, 919 (2009)],
the helical surface states in the metallic systems appear
to be robust to at least mild disorder. We present an
investigation of the nature of these "topological conductors"
 bulk metals with helical surface states. We explore how
the surface states can survive the insulator being turned
into a metal, in both clean and disordered systems. We
also explore magnetoelectric coupling phenomena in these
systems, which turn out to realize an analog of the intrinsic
anomalous Hall effect.

Condensed Matter Seminar Tuesday, February 7, 2012 3:30 PM Physics Building, Room 204 
"How to Get into Undergraduate Research"Bob Jones, Despina Louca, & Blaine Norum , University of Virginia [Host: Elton Ho]
ABSTRACT:
The purpose of this talk is to provide an opportunity for undergraduates to learn more about research in the Physics Department. Current and prospective physics majors are encouraged to participate. Topics to be discussed include: current researches in the department in different fields of interest, how research relates to the physics BS degree, how to search for research opportunities, and what professors expect from undergraduate researchers. Free pizza and drinks will be served.

Condensed Matter Seminar Thursday, December 1, 2011 5:45 PM Physics Building, Room 204 How to Get into Undergraduate Research 
"The Linear and Nonlinear Magnetic Response in Strongly Correlated Metals"Bellave Shivaram , University of Virginia [Host: Genya Kolomeisky]
ABSTRACT:
I will describe two phenomena from the world of heavy fermion physics  one very specific and one that appears general enough to apply to all strongly correlated metals. The metal URu2Si2 exhibits a magnetic transition whose order parameter has so far been "hidden". I will describe sensitive magnetization measurements that throw light on the special nature of this hidden order in this unique metal. The more general phenomenon pertains to our observations in UPt3 that appear to establish a universal rule for the behavior of the equilibrium nonlinear susceptibility in all strongly correlated electronic systems.

Condensed Matter Seminar Thursday, November 17, 2011 3:30 PM Physics Building, Room 204 
"Engineering Interface Magnetism via Defect Control in Complex Oxide Heterostructures"Chris Leighton , University of Minnesota [Host: Despina Louca]
ABSTRACT:
The remarkable functionality of complex oxides provides many opportunities for new physics and applications in oxide heterostructures. The manganite and cobaltite materials crystallizing in the perovskite structure provide excellent examples, being of interest in solid oxide fuel cells, catalysis, ferroelectric RAM, gas sensing, resistive switching memory, and oxide spintronics. However, the same delicate balance between phases that provides such diverse functionality also leads to a serious problem  the difficulty of maintaining desired properties close to the interface with other oxides. Although this problem appears universal, manifests itself in many ways, and presents a significant âroadblockâ to the development of heterostructured devices for oxide electronics, there is no clear consensus as to its origin, or even whether it is driven by electronic or chemical effects. In this work, using SrTiO3/La1xSrxCoO3 as a model system (i.e. a nonmagnetic semiconductor / ferromagnetic metal interface), we have determined the fundamental origin of the deterioration in interfacial transport and magnetic properties. The effect is found to be chemicallydriven, being due to a profound interaction between the strainstate and the formation of oxygen defects, which dominates the interface magnetism and transport. Most importantly, it is demonstrated that a full understanding of these effects enables design and synthesis of interfaces where the suppression in physical properties is dramatically reduced, or perhaps even eliminated.
Work supported by DOE and NSF.

Condensed Matter Seminar Thursday, November 10, 2011 3:30 PM Physics Building, Room 204 
"Resonant spin excitations in Iron Arsenide superconductors"Stephan Rosenkranz , Argonne National Lab [Host: Seunghun Lee]
ABSTRACT:
The iron pnictides have attracted great interest following the discovery of superconductivity up to 50K and because of their superficial similarities with the cuprate superconductors. In both systems, superconductivity emerges when longrange antiferromagnetic order is suppressed by doping or pressure. This indicates that spin correlations are intimately connected with superconductivity and may be involved in the pairing mechanism, in contrast to ordinary superconductors where the pairing is provided by phonons. Elastic and inelastic neutron scattering provides a most direct probe of both the longrange spin order as well as the strong spin fluctuations remaining when the longrange order has been suppressed, allowing detailed studies of how these spin correlations evolve as the superconducting state is approached and entered as a function of doping and temperature. Here, I will discuss what we have learned about the iron arsenides from such neutron scattering studies across the whole doping range. Particular focus will be given on the observation of a resonant spin excitation, how its sensitivity to the superconducting order parameter proves that superconductivity in these compounds is indeed unconventional, and how its doping dependence provides further clues to the origin of pairing in these compounds.

Condensed Matter Seminar Thursday, October 13, 2011 3:30 PM Physics Building, Room 204 
"Surface analysis: new approaches to access, data and analyses"Paul Pigram , Centre for Materials and Surface Science and Department of Physics, La Trobe University, Australia [Host: Joe Poon]
ABSTRACT:
Surface characterisation is critically important in the development and validation of advanced materials, tailored molecular structures at surfaces, sensors and screening devices, electronic structures and systems. XPS and Time of flightSecondary Ion Mass Spectroscopy
(ToFSIMS) are core laboratorybased surface analytical techniques.
Major challenges remain in creating preferably automated data interpretation tools to extract molecular information from these datasets. This presentation will discuss our approach to creating a virtual research laboratory encompassing immersive remote access to surface analysis instrumentation and beam lines at the Australian Synchrotron, instrument scheduling and secure access, data repository integration, metadata capture and data visualisation. The feasibility of integrating other national and international facilities is currently being explored.

Condensed Matter Seminar Thursday, October 6, 2011 3:30 PM Physics Building, Room 204 
"Febased superconductors at high magnetic fields"Alexander Gurevich , Old Dominion University [Host: Joe Poon]
ABSTRACT:
Low carrier densities and short coherence lengths in the recently discovered semimetallic Febased superconductors (FBS) can result in exotic behaviors at strong magnetic fields due to the interplay of multiband superconductivity, unconventional pairing symmetry and the Zeeman and orbital pairbreaking. I this talk I will give an overview of these interesting new materials, particularly their anomalous temperature dependencies of the upper critical field Hc2(T) which often extrapolate to very high Hc2 > 100 T at low temperatures. The materials features of FBS and the multiband s pairing symmetry with the sign change of the order parameter on different sheets of the Fermi surface can significantly increase Hc2(T) and facilitate the FuldeFerrelLarkinOvchinnikov (FFLO) transition to the state with a spatially modulated order parameter. Small shifts of the chemical potential upon doping of FBS can produce new small pockets of the Fermi surface, triggering the FFLO instability due to the Lifshitz transition.

Condensed Matter Seminar Monday, September 26, 2011 3:30 PM Physics Building, Room 204 
"Hidden order and disorder in a spin glass state of a geometrically frustrated magnet"Seunghun Lee , UVA [Host: Genya Kolomeisky]
ABSTRACT:
Frustration arises in magnetic materials when pairwise interactions between the magnetic moments cannot be satisfied simultaneously, leading to exotic properties such as spin liquid and spin ice at low temperatures. Some cases, however, exhibit spin glass behaviors, and it has been a theoretical and experimental challenge to understand whether or not the spin glass state is an intrinsic ground state for the materials. Here we have studied a quasitwo dimensional triangular lattice of bipyramids with dominant antiferromagnetic nearest neighbor interactions. We show that the magnetic problem can be mapped into a problem of two independent degrees of freedom, tricolor and binary sign: the color represents the director of the collinear spins of each bipyramid, while the binary sign represents spins within each bipyramid that are either parallel or antiparallel to the director. At the meanfield level, the tricolor orders long range in a 3 Ã 3 structure, while the binary sign has numerous degenerate ground states. When combined, the two degrees of freedom yield the collinear bipyramid spin ground states for the hybrid lattice. Infinite number of coplanar bipyramid spin states can be systematically generated by collective rotations of spins from the collinear states. For long range ordered collinear and coplanar states, we identify âpartial but extensiveâ spin zeroenergy modes of excitations that are qualitatively different from the âlocalâ zeroenergy excitations found in the spin liquid states of the pure twodimensional kagome and pure threedimensional pyrochlore lattices. We argue that due to the infinite ground state degeneracy and the unique characteristics of the zeroenergy excitations the ground state of the quasitwodimensional hybrid antiferromagnet becomes a spin glass.

Condensed Matter Seminar Thursday, September 8, 2011 3:30 PM Physics Building, Room 204 
"Material Development for Magnetic tunnel junctions in Spin Transfer Torque Random Access Memory (STTRAM)"Ms. Yishen Cui , University of Virginia [Host: Stu Wolf]
ABSTRACT:
Magnetic Tunnel Junction(MTJ) has been widely studied recently due to its potential to be applied in high density memory devices. The basic structure of MTJ is composed of two ferromagnetic layers(free layer and fixed layer) separated by an insulating barrier layer. By aligning the free layer (FL) magnetization parallel or antiparallel with the fixed layer magnetization, different resistance levels can be obtained in the junction. The most significant characteristic of STT writing technology that distinguishes it from the conventional one is that it enables the manipulation on magnetization of FL by local spin polarized current rather than an external field, leading to many advantages such as a high scalability and low energy cost. The main challenge for improving the performance of MTJs driven by STT remains in the reduction of the critical switching current density that required by the FL switching without sacrificing the thermal stability of MTJ bits. These two factors are closely related with the intrinsic properties of the material used in the ferromagnetic layers in MTJ. This talk will be focused on some ferromagnetic materials that can be potentially applied in STTMTJs.

Condensed Matter Seminar Thursday, April 28, 2011 3:30 PM Physics Building, Room 204 
"Computational Study of ultrashortpulse lasermetal interaction"Chengping Wu , UVa [Host: Genya Kolomeisky]
ABSTRACT:
Rapid progress in the development of accessible sources of short (pico and femtosecond) laser pulses opens up new opportunities for surface modification with high accuracy and spatial resolution. But the small size of the lasermodified region makes experimental characterization of laserinduced structural changes challenging and, at the same time, the fast and highly localized energy deposition in short pulse laser processing unavoidably creates the conditions of strong thermodynamic, electronic, and mechanical nonequilibrium, making the theoretical description of the structural transformations difficult. On the other hand, atomiclevel computer modeling has the ability to provide detailed information on the complex structural and phase transformations induced by short pulse laser irradiation and can assist in the advancement of laserdriven applications. In this presentation, I will talk a series simulation I have done or am doing, especially focus on the simulation âlaser irradiation on layered targets (Ag filmCu substrate)â.

Condensed Matter Seminar Thursday, April 21, 2011 3:30 PM Physics Building, Room 204 
ABSTRACT:
Amorphous rareearth alloys exhibit compensation phenomena involving a low saturation magnetization and high anisotropy field near the compensation temperature. However, the anisotropy of these amorphous solids is not well understood. At the same time, some of these materials also exhibit low Gilbert damping parameter that is preferable in spintorquetransferred devices. I will discuss the properties of the various magnetic films being investigated.

Condensed Matter Seminar Thursday, April 7, 2011 3:30 PM Physics Building, Room 204 
ABSTRACT:
Thermoelectric materials are of great interests because they can fulfill the direct conversion between thermal and electrical energy, in a environmentally clean way. Half Heusler compounds are well known electronic and magnetic materials over the last few decades. Our work concentrates on improving the thermoelectric properties of these compounds via optimization of the Seebeck coefficient and thermal conductivity.

Condensed Matter Seminar Thursday, March 31, 2011 3:30 PM Physics Building, Room 204 
"Neutron scattering study on the ironbased highTc superconductor systems"Wei Bao , Renmin University of China [Host: Seunghun Lee]
ABSTRACT:
Superconductors which conduct electric current without dissipation (zero resistance)
and expel magnetic field (perfect diamagnetism) were discovered 100 years ago in
metal at a few Kelvin above absolute zero. Stimulated by the discovery of cuprate
superconductors in doped Mott antiferromagnetic insulator, over the last two decades
research has been focused on discovery of unconventional superconductors of high
transition temperature (Tc) in magnetic materials. Ironbased laminar materials in
several related structure families with Tc as high as 56 K have generated much
excitement in the last three years, and new discovery continues to appear.
Using neutron scattering technique, we have determined crystal and magnetic structure
of several families of the new superconductors and the sample composition [15],
which provide solid foundation for further investigation on electronic structure and
processes. Structural and magnetic transitions have been investigated to yield
phasediagrams which show a rich variety of relationship between superconducting and
magnetic orders [6,7]. Such investigation also reveals the shortcoming of widely accepted
spindensitywave scenario and provides first experimental indication of important
role of the orbital order [2].
The symmetry of superconducting order parameter has strong signature in magnetic
excitation spectrum. We observed with inelastic neutron scattering method the
telltale spin resonance mode of the unconventional s+/ symmetry in the superconducting
state of the 11 superconductor [8]. The normal state was shown to exhibit the singlelobed
incommensurate excitation continuum of a typical itinerant antiferromagnet, in contrast
to spinwave cone of a localized antiferromagnet [8,9], supporting a Fermi liquid
description of the normal state.
[1] Y. Qiu, W. Bao, Q. Huang et al., Phys. Rev. Lett. 101, 257002 (2008).
[2] Q. Huang, Y. Qiu, W. Bao et al., Phys. Rev. Lett. 101, 257003 (2008); M. Kofu, Y. Qiu, W. Bao et al., New J. Phys. 11, 055001 (2009).
[3] W. Bao, Y. Qiu, Q. Huang et al., Phys. Rev. Lett. 102, 247001 (2009).
[4] W. Bao et al., arXiv:1102.0830 (2010).
[5] F. Ye et al., arXiv:1102.2882 (2010).
[6] H. Chen et al., Europhys. Lett. 85, 17006 (2009).
[7] W. Bao et al., arXiv:1102.3674 (2010).
[8] Y. Qiu, W. Bao, Y. Zhao et al., Phys. Rev. Lett. 103, 067008 (2009).
[9] D.N. Argyriou et al., Phys. Rev. B 81, 220503 (R) (2010).

Condensed Matter Seminar Tuesday, March 29, 2011 3:30 PM Physics Building, Room 313 
"Nonconventional Odd Denominator Fractional Quantum Hall States"Gabor Csathy , Purdue University [Host: Jongsoo Yoon]
ABSTRACT:
The fractional quantum Hall states developing in a twodimensional electron gas in the second Landau level appear to be different from those of the lowest Landau level and continue to challenge our understanding. The even denominator state at filling factor 5/2 is currently under intense scrutiny since it is believed to arise from an exotic pwave pairing of composite fermions described by the Pfaffian wavefunction which might support nonAbelian quasiparticles. An
equally interesting and related problem is the origin of the odd denominator states of the second Landau level. While at first sight these states could be part of the composite fermion hierarchy, several recent theoretical works suggest that some might be supporting generalized Pfaffianlike correlations. Recent progress in cooling electrons allowed us to observe a new fractional quantum Hall state at the filling factor 2+6/13. We find that energy gaps of the prominent 2+1/3 and 2+2/3 states are consistent with the values predicted by the free composite fermion model. However, the weaker 2+2/5 and 2+6/13 states deviate significantly, suggesting therefore that these states are of exotic origin.

Condensed Matter Seminar Thursday, March 17, 2011 3:30 PM Physics Building, Room 204 
"Quantum Phases of Dipolar Bosons"Carlos A. R. Sa de Melo , Georgia Institute of Technology [Host: Cass Sackett]
ABSTRACT:
An ensemble of heteronuclear (dipolar) molecules or Rydberg atoms can exhibit many interesting condensedmatterlike properties at ultralow temperatures because of the longrange range interaction due to their permanent dipole moments [1]. In this talk, I discuss possible quantum phases of dipolar bosons for optical lattices [2] and continuum [3] systems in the twodimensional regime. In the optical lattice case, several exotic low temperature phases can emerge out the dipolar superfluid. These phases include checkerboard supersolids, striped supersolids and collapsed. The emergence of a striped supersolid is particularly interesting because the anisotropy of the dipolar interaction can be controlled externally. In the case of dipolar bosons in continuum systems, I discuss the finite temperature phase diagram of purely repulsive interactions and show that for large dipolar repulsions a dipolar Wigner crystal appears at low temperatures and melts at intermediate temperatures into a dipolar hexatic fluid, before becoming a normal dipolar fluid at higher temperatures. Other exotic intermediate phases such as supersolid and hexatic superfluid (a melted supersolid) are possible. The experimental characterization of theses phases may be achieved via Bragg scattering techniques.
Refs:
[1] M. Iskin and C. A. R. Sa de Melo, Phys. Rev. Lett. 99, 110402 (2007).
[2] I. Danshita and C. A. R. Sa de Melo, Phys. Rev. Lett. 103, 225301 (2009).
[3] K. Mitra, C. J. Williams, and C. A. R. Sa de Melo, Arxiv 0903.4655v1.

Condensed Matter Seminar Thursday, March 3, 2011 3:30 PM Physics Building, Room 204 
"Universal conductance in quantum multiwire junctions"Armin Rahmani , Boston University [Host: Austen Lamacraft ]
ABSTRACT:
Using boundary conformal field theory, we relate the universal conductance tensors of quantum multiwire junctions to static correlation functions in a finite system. We then propose a general method for determining the conductance. Applying the method to a Yjunction of interacting quantum wires, we verify a theoretical prediction for the conductance of the chiral fixedpoint of the Yjunction and then calculate the thus far unknown conductance of its M fixed point with the timeindependent density matrix renormalization group method.

Condensed Matter Seminar Thursday, February 24, 2011 3:30 PM Physics Building, Room 204 
"Quantum Information Processing with Prethreshold Superconducting Qubits"Michael R. Geller , University of Georgia [Host: Eugene Kolomeisky ]
ABSTRACT:
I will discuss an alternative approach to quantum computation and simulation
that is ideally suited for today's subthresholdfidelity qubits, especially in
superconducting architectures. This approach makes use of the the ndimensional
singleexcitation subspace (SES) of a system of n tunably coupled qubits.
Although technically unscalable and inefficient in terms of the number of qubits
required, the SES approach allows ndimensional unitary operations to be
implemented in a single shot, without the need to decompose into gates. The real
power of this approach is probably in its application to quantum simulation,
and I will show that for a particular class of timedependent quantum simulation
problems a practical device that would vastly outperform classical machines
is within experimental reach.

Condensed Matter Seminar Thursday, February 17, 2011 3:30 PM Physics Building, Room 204 
"Photons as phonons: from cavity QED to quantum crystallization"Sarang Gopalakrishnan , UrbanaChampaign [Host: Austen Lamacraft]
ABSTRACT:
Atoms in a transversely pumped optical cavity coherently scatter light between the pumping laser and the cavity mode(s) with a positiondependent intensity and phase; thus, the optical dipole potential created and experienced by an atom depends on its position in the cavity. For a singlemode cavity containing many atoms, the optical potential felt by each atom depends on the positions of all the other atoms; in this sense, the cavity mediates an infiniterange interatomic interaction with an oscillating sign. In cavities with many degenerate modes, one can realize more local interactions; these give rise to the crystallization of a BoseEinstein condensate in a multimode cavity through a fluctuationdriven firstorder quantum phase transition. This transition is described by a theory involving a nested surface of lowlying excitations, and is accompanied by a condensation of photons into one or more cavity modes. The resulting ordered state is a âsupersolid,â i.e., a superfluid that spontaneously breaks a continuous translational symmetry. We discuss the nature of fluctuations near the transition and lowenergy excitations in the ordered state, and prospects for their experimental detection.

Condensed Matter Seminar Thursday, February 10, 2011 3:30 PM Physics Building, Room 204 
"Study of spin state transition in La _{2x }Ca _{ x }CoO _{ 4 } by using neutron and soft xray scattering"Kazumasa Horigane , Tohoku University [Host: Despina Louca]
ABSTRACT:
Layered cobalt oxides have attracted much attention due to their wide variety of magnetic, electrical and structural properties. The parent confound La _{ 2 } CoO _{ 4 } is an antiferromagnetic insulator with quite high T _{ N } =275K. Upon Sr or Ca doping, a checkerboard Co ^{ 2+ } Co ^{ 3+ } charge order is realized in La _{ 1.5 } Ca _{ 0.5 } CoO _{ 4 } and Sr _{ 2 } CoO _{ 4 } shows a ferromagnetic metal with T _{ c } =250K. The background of these various magnetic states is proposed the relationship of spin state transition from high spin state of Co ^{ 3+ } (S=3/2) to intermediate spin state (S=1) [1]. However, the spin state of Co ^{ 3+ } is now controversial because there is no direct evidence to observe the magnetic scatterings of Co ^{ 3+ } by using neutron experiment. One of the big problems in this system is searching for magnetic scattering in an elastic condition. In this study, we measured global spin dynamics (inelastic condition) in La _{ 1.5 } Ca _{ 0.5 } CoO _{ 4 } using the Fermi chopper spectrometer 4SEASONS in JPARC [2].
Figure 1 shows the magnetic excitation of La _{ 1.5 } Ca _{ 0.5 } CoO_{ 4 } below and above T _{ N }(=50K). Spinwave due to Co ^{ 2+ } spins is seen up to 16meV below T _{ N }. Another feature of the excitation spectrum is flat mode around 27meV. On the other hand, two magnetic modes seem to be merged into single mode above T_{ N }. These results can not be explained by simple spin wave theory, therefore we need to consider another parameters such as Co ^{ 3+ } spin or angular moment L. In this presentation, I will discuss the origin of this novel magnetic behavior based on the Co ^{ 3+ } spins and orbital angular moment L. In order to discuss whether there is a significant orbital angular moment or not in this system, I will show you the recent research of soft xray scattering experiment.
[1] Y. Moritomo, K. Migashi, K. Matsuda and A. Nakamura, Phys. Rev. B 55, 14725 (1997)
[2] M. Nakamura, R. Kajimoto, Y. Inamura, F. Mizuno, M. Fujita, T. Yokoo and M. Arai, JPSJ 78, 093002 (2009)

Condensed Matter Seminar Thursday, January 27, 2011 3:30 PM Physics Building, Room 204 
"Fractional statistics and beam splitters in quantum Hall systems"Smitha Vishveshwara , UIUC [Host: Israel Klich ]
ABSTRACT:
One of the most fascinating aspects of the two dimensional world is the possible existence of anyons, particles which obey 'fractional' statistics different from fermionic and bosonic statistics.
The quantum Hall system is predicted to host such entities. This talk will begin with an introduction to fractional particles in quantum Hall systems and anyons in magnetic fields. Correlations and signatures of these anyons
will be discussed. Finally, schemes for detecting such anyons will be proposed, including those that follow the principles of beam splitters found in the entirely different setting of quantum optics.

Condensed Matter Seminar Thursday, November 18, 2010 4:00 PM Physics Building, Room 204 
"Interlayer exchange couplings in Ga1xMnxAs/GaAs diluted ferromagnetic semiconductor multilayers"JaeHo Chung , Korea University [Host: SeungHun Lee]
ABSTRACT:
The spintronics technology requires materials that allow simultaneous control of the charge and the spin degrees of freedom. Recent efforts dedicated in search of such materials have led us to the successful fabrication of artificial ferromagnetic semiconductor GaMnAs, where doping of magnetic Mn2+ ions into insulating GaAs results in robust ferromagnetism and semiconducting property at the same time. In order to realize useful devices from magnetic semiconductors, however, it is important to be able to control not only the magnetic properties of individual layers but also interactions between them. Previously the interactions between ferromagnetic GaMnAs layers across nonmagnetic GaAs spacers have been observed to be ferromagnetic only. Since the RKKY interaction in metallic magnetic multilayers is known to produce exchange oscillations between ferromagnetic and antiferromagnetic (AFM) as a function of layer thickness, AFM interlayer exchange coupling has been expected to be attainable also in semiconductorbased multilayers by enhancing carrier concentrations in the spacers.
To test this idea, we have fabricated GaMnAsbased multilayers with different carrier concentrations and thicknesses in the spacers. Molecular beam epitaxy was used to deposit 10 periods of Ga0.97Mn0.03As/GaAs on GaAs (001) substrates. In order to increase the carrier concentration of selected samples, Be doping at the concentration of 1.2 x 1020 cm3 was introduced in the nonmagnetic spacer. Using polarized neutron reflectivity, we indeed observed and confirmed the occurrence of antiferromagnetic interlayer coupling in some of the samples with Bedoped spacers. Their field cycling behavior clearly indicated that the observed antiferromagnetic coupling is spontaneous and robust. All of these samples showed GMRlike transitions in magnetotransport measurements. In contrast, none of the samples without Bedoped spacers showed such behavior. These results indicate that the interlayer exchange couplings in GaMnAsbased ferromagnetic multilayers can be controlled via engineering of the spacer properties. 
Condensed Matter Seminar Thursday, November 11, 2010 4:00 PM Physics Building, Room 204 
ABSTRACT:
With the experimental discovery of topological insulators in
2 and 3 dimensions, there has emerged the theoretical problem of completely classifying the conditions under which such novel phases can exist. That is, given a spatial dimension and a set of generic symmetries (e.g. time reversal, particlehole symmetry), what "topological" phases does one obtain? A complete classification has been obtained for effectively noninteracting systems. Here we present an example of a one dimensional fermionic chain in which this simple noninteracting classification breaks down when one adds quartic interactions. We generalize our methods to outline a complete classification of gapped phases of all onedimensional systems (this is joint work with Alexei Kitaev).

Condensed Matter Seminar Thursday, September 30, 2010 4:00 PM Physics Building, Room 204 
"Structural defects and the onset of flow in glassy materials"Lisa Manning , Princeton Center for Theoretical Science [Host: Despina Louca]
ABSTRACT:
How do glassy materials begin to flow? Experiments and simulations have confirmed that flow occurs via localized rearrangements in most jammed solids at finite temperatures and strain rates, but in the past it has been difficult to identify structural defects, or soft spots, based on the underlying structure of the packing alone. We have developed a method to systematically identify a population of soft spots by analyzing the lowenergy vibrational modes of a softsphere packing. We show that the soft spots are structurally distinct from the remainder of the packing and that under quasistatic shear a soft spot from the population will accommodate deformation. Finally, we discuss how statistics of these soft spot populations could be used to test predictions of theoretical models for plastic flow.

Condensed Matter Seminar Thursday, September 23, 2010 3:30 PM Physics Building, Room 204 
ABSTRACT:
Frustrated magnets show interesting phenomena originating from the macroscopic ground state degeneracy. Exotic states can be chosen as the ground state from many possible candidates. Usually, geometrically frustrated systems consist of edge and cornershared triangles (triangular and kagomÃ© lattices, respectively) and tetrahedra (pyrochlore lattice). However, even unfrustrated lattice systems, such as square or honeycomb lattice systems, can have frustrating interactions in the presence of antiferromagnetic furtherneighbor interactions. We have recently studied two interesting frustrated systems using neutron scattering technique. One is the Crbased spinel with pyrochlore lattice and another is the honeycomb lattice system with competing interactions.
Crbased spinel compounds ACr2O4 (A=Mg, Zn, Cd, and Hg) are so far the best model systems for a network of cornersharing tetrahedrons with isotropic nearestneighbor antiferromagnetic interactions. The systems exhibit novel spinPeierls phase transitions from cubic spin liquid to noncubic NÃ©el states at low temperatures. They also show the magnetic fieldinduced halfmagnetization plateau states that are stable over a wide range of field. Using an elastic neutron scattering technique under magnetic field, we determined the magnetic structure in the halfmagnetization plateau phase in the spinel HgCr2O4 and CdCr2O4. The magnetic structure has a common cubic P4332 symmetry. This suggests that there is a universal field induced spinlattice coupling mechanism at work in the Crbased spinels. This is consistent with the theoretical prediction based on the simplest Hamiltonian for the spinlattice coupling with the nearestneighbor exchange interaction and an elastic energy term.
Bi3Mn4O12(NO3), in which the Mn4+ ions carry S=3/2, is the first honeycomb lattice system that shows no longrange magnetic order. Using neutron scattering technique, we determined that a shortrange antiferromagnetic correlation develops at low temperatures. Applying magnetic field, an interesting fieldinduced magnetic transition occurs, in which the shortrange order abruptly expands into a longrange order.

Condensed Matter Seminar Thursday, September 2, 2010 4:00 PM Physics Building, Room 204 
"Structural Investigations of Hydrides Studied by Neutron Scattering "Toyoto Sato , Tohoku University [Host: Despina Louca]
ABSTRACT:
Hydrides have been expected to be high gravimetric and volumetric density hydrogen storage materials [1]. Recently the hydrides also showed some interesting materials properties which were lithiumion conductor (ex. LiBH_4 ) [2], semiconductor (ex. SrAlSiH) [3], and so on. In order to understand the interesting properties, the structural investigations are one important research.
On the other hand, it is very difficult to detect hydrogen by xray because xray scattering occurs by interaction with the electrons.
However neutrons were scattered by atomic nuclei and it is possible to identify hydrogen (deuterium) atomic positions. In addition, dynamics of hydrides studied by inelastic neutron scattering is also one interesting study because inelastic scattering cross section of hydrogen (82.02
barns) is approximately ten times as large as those of the most other elements (less than 12 barns). Therefore, neutron scattering experiments of hydrides are essential for research and developments of the hydrides.
For the reason, a lot of structures and dynamics of hydrides have been elucidated by neutron scattering studies [4, 5].
In this seminar, I will show the elastic (structure) and inelastic
(dynamics) neutron scattering studies on CaAlH system that are an interesting hydrogen storage material [6].
[1] S. Orimo, Y. Nakamori, J.R. Eliseo, A. ZÃ¼ttel, C.M. Jensen, Chem.
Rev. *107*, 4111, (2007).
[2] M. Matsuo, Y. Nakamori, S. Orimo, H. Maekawa, H. Takamura, Appl.
Phys. Lett. *91*, 224103, (2007).
[3] T. Bjorling, D. NorÃ©us, K. Jansson, M. Andersson, E. Leonova, M.
EdÃ©n, U. HÃ¥lenius, U. HÃ¤ussermann, Angew. Chem. Int Ed *44*, 7269 *(2005)*
[4] B.C. Hauback, Z. Kristallogr. *223*, 636 (2008).
[5] S.F. Parker, Coord. Chem. Rev. *254*, 215 (2010).
[6] T. Sato, M.H. SÃ¸rby, K. Ikeda, S. Sato, B.C. Hauback, S. Orimo, J.
Alloys Compd. *487*, 472 (2009).

Condensed Matter Seminar Friday, May 28, 2010 2:00 PM Physics Building, Room 313 
"Physical basis for the spatial organization of DNA"BaeYeun Ha , University of Waterloo [Host: Seunghun Lee ]
ABSTRACT:
DNA is not only a passive storage of lifeâs information but also a fascinating physical object, which actively participates in many biological processes of vital importance (e.g., DNA replication and organization). In aqueous solution, DNA is highlynegatively charged. By themselves, DNA molecules would repel each other. Also, they are molecular springs: DNA strands resist bending, twisting, stretching, and confinement. In a living cell, however, DNA is tightly packed and organized into higherorder structures. Perhaps, the most intriguing âshowâ DNA molecules display is their spatial organization or segregation, while maintaining a high level of compaction. How can this be accomplished? Using a simple but biologyinspired model of DNA, I will present a physical basis for DNA organization and segregation, especially in rodshape bacteria.

Condensed Matter Seminar Tuesday, April 27, 2010 1:30 PM Physics Building, Room 313 
"Projection Hamiltonians for clustered quantum Hall wavefunctions"Thomas Jackson , Yale [Host: Israel Klich]
ABSTRACT:
One of the main measures for determining whether a trial wavefunction for the fractional quantum Hall effect is physically realistic is the properties of an associated projection Hamiltonian, an artificial fewbody Hamiltonian constructed so that the given wavefunction and edge excitations lie in the zeroenergy eigenspace. The results presented here address the general (and nontrivial) problem of finding a projection Hamiltonian for a quantum Hall state defined in terms of a conformal field theory.
We consider lowest Landau level wavefunctions for bosons subjected to a magnetic field in the plane; "clustered" wavefunctions are those which vanish when k+1 (but not necessarily k) particles are brought to the same point. We begin by studying the zeroenergy eigenstates of a projection Hamiltonian which forbids three particles to come together with relative angular momentum less than six and, in addition, forbids one of two linearlyindependent states of relative angular momentum six. The counting of edge excitations of this Hamiltonian agrees with the character formula for the N=1 superconformal Kac vacuum module at generic (i.e., irrational) central charge c, despite the fact that the densest (ground) state reproduces the appropriate superconformal amplitude for all c.
The irrationality of the edge theory implies that this Hamiltonian is gapless for all c. For particular c, we try to ``improve'' the Hamiltonian by adding additional terms (related to singular vectors in the modules), so as to obtain a rational theory. We consider specifically states whose wavefunctions are related to the M(3,p) series of Virasoro minimal models.

Condensed Matter Seminar Thursday, April 1, 2010 4:00 PM Physics Building, Room 204 
"Symmetry breaking in smectics and surface models of their singularities"Gareth Alexander , University of Pennsylvania [Host: Andrew James]
ABSTRACT:
The homotopy theory of topological defects in ordered media fails to completely characterize systems with broken translational symmetry. We argue that the problem can be understood in terms of the lack of rotational Goldstone modes in such systems and provide an alternate approach that correctly accounts for the interaction between translations and rotations. Dislocations are associated, as usual, with branch points in a phase field, whereas disclinations arise as critical points and singularities in the phase field. We introduce a threedimensional model for twodimensional smectics that clarifies the topology of disclinations and geometrically captures known results without the need to add compatibility conditions. Our work suggests natural generalizations of the twodimensional smectic theory to higher dimensions and to crystals.

Condensed Matter Seminar Thursday, March 25, 2010 4:00 PM Physics Building, Room 204 
"Electron itinerancy, orbital symmetry and itinerant spin fluctuations in Febased superconductors as revealed by soft xray spectroscopies."Norman Mannella , University of Tennessee  Knoxville [Host: SeungHun Lee]
ABSTRACT:
The discovery of superconductivity with critical temperature exceeding 55 K in the ironpnictides (FeSC) compounds has offered the community a new set of materials hosting high temperature superconductivity [1]. In this talk, I will discuss some of our most important results obtained with complementary soft xray spectroscopies such as core level and Angle Resolved Photoemission (ARPES) and xray absorption (XAS) on different families of FeSC compounds. Results concerning two main aspects particularly important for the physics of the FeSC materials will be discussed, namely 1) the bonding topology/orbital symmetry, and 2) the presence of spin fluctuations. The symmetry of the orbitals and their hybridization play a central role in the interplay between magnetism and superconductivity, and in the nature of the superconducting pairing. I will discuss the symmetry and topology of the occupied and unoccupied orbitals by presenting results of polarizationdependent ARPES and elementspecific XAS measurements of the electronic structure in the normal state of BaFe _{ 2 } As _{ 2 } and BaFe _{ 1.8 } Co _{ 0.2 } As _{ 2 } single crystals. I will then focus on discussing how the presence of exchange multiplets in the Fe 3s photoemission spectra in different FeSC materials are indicative of the presence of fluctuating spin moments on the Fe sites. Due to extremely fast time scales involved, the detection of magnetic fluctuations by means of magnetic probes has so far remained elusive. Our experiment provides a strong test case for the occurring of itinerant magnetic fluctuations, whose detection has been made possible by the extremely fast time scales proper of the photoemission process. The Fermi surface topology revealed by ARPES experiments in different FeSC compounds and its possible relation to the presence of magnetic fluctuation will also be discussed.
[1] Y. Kamihara, et al., J. Am. Chem. Soc. 130, 3296 (2008).

Condensed Matter Seminar Thursday, March 4, 2010 4:00 PM Physics Building, Room 204 
"Orbital orders and orbital order driven quantum criticality"Zohar Nussinov , Washington University at St. Luis [Host: Israel Klich ]
ABSTRACT:
I will briefly review the rudiments of orbital order physics and then
discuss several more recent results. Orbitals are described by SU(2)
operators but unlike spins the orbitals live in real space and thus their
interactions are highly frustrated. It will be shown that spatial orders
of electronic orbital states in a crystal can be triggered by thermal
fluctuations alone (contrary to earlier lore, no zero point quantum
fluctuations are necessary to stabilize orders). It will be further shown
how symmetry considerations alone give rise to dimensional reduction as
well as topological order and selection rules that may be experimentally
tested from scattering measurements. I will illustrate that in addition to
charge and spin order driven quantum critical points, Orbital Order Driven
Quantum Criticality, as a matter of principle, also occur in the simplest
orbital Hamiltonians. New predicted orbital nematic orders will
be demonstrated in some of the best known orbital
dependent Hamiltonians.

Condensed Matter Seminar Thursday, February 25, 2010 4:00 PM Physics Building, Room 204 
"Effect of order parameter fluctuations on the spectral density in dwave superconductors"Maxim Khodas , Brookhaven [Host: Israel Klich ]
ABSTRACT:
My talk contains two related parts.
In the first part I will discuss the spectral density in 2D dwave superconductors in the regime of strong quantum fluctuations.
I will start with the motivation based on Angular Resolved Photo Emission Experiments.
The theoretical model is then presented to capture the effects of phase fluctuations without specifying the (unknown) paring mechanism.
I discuss the solution of the model and give results for the spectral function.
In the second part I will discuss the Fermi Surface Reconstruction and underlying mechanism related to the antiferromagnetic fluctuations.
I will show how the pockets obtained on the mean field level are modified by the fluctuations of the staggered magnetization.
Finally, I would conclude with a short summary.

Condensed Matter Seminar Thursday, February 18, 2010 4:00 PM Physics Building, Room 204 
"Orbital ordering in CaV _{ 2 } O _{ 4 } : A neutron scattering study"Oliver Pieper , Helmholtz Zentrum Berlin [Host: Seunghun Lee]
ABSTRACT:
CaV _{ 2 } O _{ 4 } is a quasione dimensional spin1 Heisenberg antiferromagnet. The magnetism arises from the partially filled t _{ 2g } levels of the V ^{ 3+ } ions, which in addition give an orbital degree of freedom to the system. In contrast to the isovalent vanadium spinel compounds, the low dimensionality in CaV _{ 2 } O _{ 4 } already arises from the crystal structure. It contains weakly coupled doublechains of edgesharing VO _{ 6 } octahedra, where the particular octahedral staggering creates a zigzaglike arrangement of the vanadium ions. This in return gives rise to strong magnetic direct exchange interactions between nearest and next nearest neighbor vanadium ions and to geometrical frustration. However, the strength of the exchange interactions is strongly influenced by the particular occupation of the t _{ 2g } orbitals. Depending on the type and degree of octahedral distortion, the system can be interpreted as a frustrated Haldane chain or a spin1 ladder.
We have used single crystal neutron diffraction and neutron spectroscopy to determine the spin structure as well as the complex excitation spectrum of CaV _{ 2 } O _{ 4 } . The results are analyzed theoretically and from this the leading exchange paths are deduced and discussed in terms of orbital ordering.

Condensed Matter Seminar Thursday, January 28, 2010 4:00 PM Physics Building, Room 204 
"Magnetic phase diagram of a spinchain system Ca2+xY2xCu5O10d: oxygen holedoping effects"Keeseong Park , Stony Brook University/ Brookhaven National Laboratory [Host: Despina Louca]
ABSTRACT:
Oxygen holedoping effects on a spinchain system, Ca2+xY2xCu5O10d
(CaYCuO) are reported. CaYCuO is a good specimen to study the magnetic
properties of the CuO2 chain at the ground state because it has no complex
structure other than the chain and it has hole dopability up to the formal
copper valence number of +2.4. Specifically, we can dope holes into the CuO2
chain by substituting Ca2+ for Y3+ or by utilizing oxygen deficiency. After
a systematic study of the two methods to dope holes, we found that oxygen
doping makes a more critical change in magnetic ordering in the chain than the
replacement of Ca2+. Oxygen deficiency effects of the chain on the magnetic
properties were explained using a mean field theory. A new relation for the
effective hole doping was found as p = x(2/3)d.

Condensed Matter Seminar Thursday, December 10, 2009 10:30 AM Physics Building, Room 313 
"Spin exchange interactions and magnetic properties"Mike Whangbo , NC State University [Host: Seunghun Lee]
ABSTRACT:
This talk will briefly review how the spin exchange interactions of magnetic solids are described quantitatively and qualitatively. When the choice of a spin lattice is made by inspecting the geometrical pattern of the magnetic ion arrangement or by seeking the novelty of the physics the chosen model generates, interesting but erroneous interpretations often result. The importance of choosing a spin lattice on the basis of electronic structure considerations is emphasized. The magnetic solids to be discussed in this talk include:
Cs _{ 2 } CuCl _{ 4 },
Na _{ 3 } Cu _{ 2 } SbO _{ 6 },
Bi _{ 4 } Cu _{ 3 } V _{ 2 } O _{ 14 } ,
Cu _{ 3 } (CO _{ 3 } ) _{ 2 } (OH) _{ 2 },
AgCrO _{ 2 } ,
Ca _{ 3 } CoMnO _{ 6 },
MnWO _{ 4 } ,
[Cu(HF _{ 2 } )(pyz) _{ 2 } ]BF _{ 4 } .

Condensed Matter Seminar Thursday, December 3, 2009 4:00 PM Physics Building, Room 204 
"NonFermi liquid fixed point for an imbalanced gas of fermions in 1+ ε dimensions"Andrew James , University of Virginia [Host: Austen Lamacraft]
ABSTRACT:
We consider a two species gas of fermions with population imbalance.
Using the renormalisation group in d=1+ ε dimensions, we show that for spinless fermions and ε > 0 a fixed point appears at finite attractive coupling where the quasiparticle residue Ζ vanishes, and identify this fixed point with the transition to LarkinOvchinnikovFuldeFerrell order (inhomogeneous superconductivity). When the two species of fermions also carry spin degrees of freedom we find a repulsive fixed point, indicating a transition to spin density wave order.

Condensed Matter Seminar Monday, November 23, 2009 3:30 PM Physics Building, Room 204 
"Counting statistics of electron transport in nanostructures"Christian Flindt , Harvard [Host: Israel Klich]
ABSTRACT:
Fluctuations of the electrical current running through a nanoscale conductor reveal information beyond what is contained in the conductance alone. In this talk I will give an overview of my recent works on counting statistics, the stochastic theory of charge transport in nanostructures. As an example, I will show how current fluctuations can be a useful tool to detect mechanical bistabilities and frequency shifts in nanoelectromechanical systems. While most works have focused on systems whose dynamics is Markovian, I will discuss the influence of memory effects on the counting statistics, illustrated with a model of transport through a double quantum dot in a dissipative environment. Finally, I show that highorder currentcurrent correlation functions (cumulants) in general oscillate as functions of basically any system parameter. A prediction that has been confirmed by recent experiments, as I will describe.
Recent papers: C. Flindt et al., PRL 100, 150601 (2008), PNAS 106, 10116 (2009)

Condensed Matter Seminar Thursday, November 19, 2009 4:00 PM Physics Building, Room 204 
"New Physics in Multicomponent Ultracold Atomic Gases"Ryan Barnett , Joint Quantum Institute [Host: Austen Lamacraft]
ABSTRACT:
BoseEinstein Condensation in multicomponent systems often exhibits physics which goes beyond traditional condensed matter paradigms due to the extra degrees of freedom. Examples of multicomponent condensates include condensed mixtures of atoms or atoms with internal spin degrees of freedom. In this talk I will discuss examples of such multicomponent condensates focused on during my recent research. The first example I will consider is a rotating condensate composed of two types of atoms with different masses. I will discuss the structure of the vortex configurations for such mixtures, and argue the existence of a counterintuitive phase where the two superfluids and external drive all rotate at different rates. During the remainder of the talk I will focus on spinor condensates. In optical traps the macroscopic spin configuration is determined by the spinexchange interaction.
The resulting meanfield ground states have a variety of pointgroup symmetries. I will discuss a geometrical representation of the meanfield states of these systems, and will also describe how this method is useful for understanding the coherent spinor dynamics, collective excitations, and topological defects. I will next explain how the phenomenon of orderbydisorder from quantum magnetism is naturally exhibited in spintwo condensates. Finally, I will discuss the rich structure of the vortex lattices which can occur when spinor condensates are rotated.

Condensed Matter Seminar Thursday, November 12, 2009 4:00 PM Physics Building, Room 204 
ABSTRACT:
Combining scanning probe microscopy with electrical
transport measurements is a powerful approach to probe lowdimensional
systems. The local information provided by scanning probe microscopy
is invaluable for studying effects such as electronelectron
interactions and scattering. Using this approach, we have probed the
local electronic properties of mono and bilayer graphene with atomic
resolution. We studied the effect of ripples, charged impurities and
defects on the local density of states. We find that longrange
scattering from ripples and impurities shifts the Dirac point leading
to electron and hole puddles. Shortrange scattering from lattice
defects mixes the two sublattices of graphene and tends to be strongly
suppressed away from the Fermi energy. In addition, in bilayer
graphene we observe an opening of a band gap due to the application of
a transverse electric field.

Condensed Matter Seminar Thursday, October 29, 2009 4:00 PM Physics Building, Room 204 
"Refrigeration using superconducting tunnel junctions"Sukumar Rajauria , NIST, Maryland [Host: Bellave Shivaram]
ABSTRACT:
In the recent years, nanorefrigeration using electron tunneling in hybrid Normal metal  Insulator  Superconductor (NIS) junctions has gained increasing attention. Its basic principle is the energy selective tunneling due to the presence of an energy gap in the superconductor density of states. With a subgap voltage bias, only the most energetic electrons can tunnel out of the normal metal, leaving behind the electrons with less energy.
We have measured with a high resolution the differential conductance of SINIS junctions, whose analysis gives us an access to the normal metal electronic temperature as a function of the voltage. A quantitative model is proposed, that includes the electronphonon coupling and the Kapitza resistance at the interface with the substrate. With this model, we have achieved a thorough description of the charge and heat currents. We have also shown that the normal metal phonon temperature drops significantly below the substrate temperature.
At very low temperature (T < 200mK) and low bias, the coherent Andreev current dominates the quasiparticle current. By analyzing quantitatively the heat balance in the SINIS junction, we demonstrate that the Andreev current does carry heat. This thermal contribution heats the normal metal electrons, overriding over a large voltage range the tunnelingbased cooling.
References
[1] S. Rajauria, P. S. Luo, T. Fournier, F. W. J. Hekking, H. Courtois, and B. Pannetier, Phy. Rev. Lett. 99, 047004 (2007).
[2] S. Rajauria, Ph. Gandit, T. Fournier, F. W. J. Hekking, B. Pannetier, and H. Courtois, Phy. Rev. Lett. 100, 047004 (2008).
[3] S. Rajauria, H. Courtois and B. Pannetier, submitted (2009).

Condensed Matter Seminar Thursday, October 22, 2009 4:00 PM Physics Building, Room 204 
"Classicalquantum mappings for geometrically frustrated systems"Stephen Powell , Joint Quantum Institute [Host: Andrew James]
ABSTRACT:
Several systems have recently been demonstrated to show "nonLGW"
quantum phase transitions, with different orderings on two sides of a continuous transition. We present two examples of classical statistical systems  spin ice in a [100] magnetic field and an ordering transition of closepacked dimers on a cubic lattice  that appear to show continuous (secondorder) transitions that lie outside the Landau paradigm.
In both cases, strong local constraints mean that neither of the neighboring phases can be understood as thermally disordered, excluding the standard route to a continuum critical theory. Instead, we derive critical theories for both transitions by mapping from threedimensional classical problems to twodimensional quantum problems. For the dimer model, this mapping provides a direct connection to previous work on nonLGW transitions of lattice bosons at fractional filling factors.

Condensed Matter Seminar Thursday, October 15, 2009 4:00 PM Physics Building, Room 204 
"Toricboson model: Toward a topological quantum memory at finite temperature"Alioscia Hamma , Perimeter Institute [Host: Israel Klich]
ABSTRACT:
We discuss the existence of stable topological quantum memory at finite
temperature. At stake here is the fundamental question of whether it is, in
principle, possible to store quantum information for macroscopic times
without the intervention from the external world, that is, without error
correction. We study the toric code in two dimensions with an additional
bosonic field that couples to the defects, in the presence of a generic
environment at finite temperature: the toricboson model. Although the
coupling constants for the bare model are not finite in the thermodynamic
limit, the model has a finite spectrum. We show that in the topological
phase, there is a finite temperature below which open strings are confined
and therefore the lifetime of the memory can be made arbitrarily
(polynomially) long in system size. The interaction with the bosonic field
yields a longrange attractive force between the end points of open strings
but leaves closed strings and topological order intact.

Condensed Matter Seminar Thursday, September 17, 2009 4:00 PM Physics Building, Room 204 
"Vertical nanocomposite heteroepitaxial thin films with manganites and ferroelectrics"Yonghang Pei , University of Virginia [Host: Bellave Shivaram] 
Condensed Matter Seminar Thursday, May 7, 2009 3:30 PM Physics Building, Room 204 
"The Effect of Thermal Fluctuations on the Problem of Euler Buckling Instability"Luke Langsjoen , University of Virginia [Host: Genya Kolomeisky]
ABSTRACT:
Renormalization techniques are used to study the effect of thermal fluctuations on the classical Euler buckling instability of a homogenous rod in 2 spatial dimensions. It is discovered that temperature has the effect of exponentially stiffening the rod, yielding a nonzero minimum critical force as a function of the length of the rod.

Condensed Matter Seminar Monday, April 20, 2009 3:30 PM Physics Building, Room 204 
"The thin film Giaever transformer  vortex drag in a superconductor thinfilm bilayer "Gil Refael , Caltech [Host: Israel Klich ]
ABSTRACT:
The normalfield induced superconductorinsulator transition in
amorphous thin films, and the possible intervening metallic state, can
be qualitatively explained within two paradigms: as vortex condensation,
or as a percolation transition between the competing normal and
superconducting phases. An experiment that may qualitatively distinguish
these two paradigms is a drag experiment on a bilayer system, consisting
of two parallel films, where a voltage response in one layer to current
in the other is measured. The drag due to vortices is expected to
dramatically exceed the Coulomb drag that may arise from the charge
carriers in the percolation scenario. In my talk, I will present recent
results estimating the drag response within the vortex and percolation
paradigms, in the limit of no tunneling between the layers.

Condensed Matter Seminar Thursday, April 16, 2009 4:00 PM Physics Building, Room 204 
"Mechanical Behavior of a Nibased Crystalline and a Zrbased Amorphous Materials Subjected to Surface Severe Plastic Deformation "Jiawan Tian , University of Tennessee [Host: Despina Louca]
ABSTRACT:
A surfacetreatment process, surfacesevereplastic deformation (S2PD), is developed and applied on both crystalline and amorphous materials to introduce plastic deformation in the nearsurface layer. A S2PDprocessed crystalline component is expected to have enhanced fatigue properties because the refined grains in the nearsurface layer and the coarse grains in the interior have good resistance to the crack initiation and propagation, respectively.
The microstructures and mechanical properties of the processed specimens were systematically investigated. It is shown that the S2PD process has the capability of simultaneously creating (a) a workhardened surface layer, (b) a nanocrystalline
(nc) surface layer, (c) a surface region with compressiveresidual stresses, and (d) a grainsize gradient with a nc surface and a coarsegrained interior for the polycrystalline superalloy. Improved yield strength and fatigue strength were found after the process.
For the amorphous material, thermal properties of the processed nearsurface layer were characterized by means of the differentialscanning calorimetry (DSC). Effects of the treatment on the microhardness were studied by the nanoindentation. After the treatment, the plasticflow deformation in the unconstrained sample edge was observed. In the subsurface layer, the impactinduced shearband operations generate the extrusion and intrusion marks on the side surface. XRD and highenergy synchrotron diffraction techniques were used to inspect the possible crystalline phase. A nanoindentation test shows that on the side surface, the hardness increases and, then, decreases with the distance from the processed surface. Fourpointbendingfatigue behavior has been studied and related to the modified surface structure and the compressiveresidual stress induced by the process.

Condensed Matter Seminar Wednesday, April 15, 2009 3:30 PM Physics Building, Room 204 
"Characterization of Doped Silicon and Endofullerenes by Raman Spectroscopy for Sensing and Transport Devices"Brian Burke , University of Virginia [Host: Bellave Shivaram ] 
Condensed Matter Seminar Thursday, April 9, 2009 4:00 PM Physics Building, Room 204 
"Simple Elements at high densities: EnRoute to metallic hydrogen and insulating lithium"Shanti Deemyad , Harvard University [Host: Despina Louca] 
Condensed Matter Seminar Tuesday, April 7, 2009 3:30 PM Physics Building, Room 204 
"Neutron and Xray scattering studies of frustrated, quantum, and multiferroic transition metal oxides"Junghwa Kim , University of Virginia [Host: Seunghun Lee] 
Condensed Matter Seminar Monday, April 6, 2009 3:30 PM Physics Building, Room 204 
"A study on the spinstate transition and complex magnetic coupling in Perovskite Cobaltite"Juan Yu , University of Virginia [Host: Bellave Shivaram] 
Condensed Matter Seminar Thursday, April 2, 2009 4:00 PM Physics Building, Room 204 
ABSTRACT:
The CalogeroSutherland family of models consist of point particles moving on a line or circle and interacting with a repulsive inversesquare potential. Both the classical and quantum versions are completely integrable, and are the subject of an extensive literature. I will describe their origin, their connection with the Hall effect and the BenjaminOno equation, and some of the subtleties that arise from the existence of two inner products.

Condensed Matter Seminar Thursday, March 26, 2009 4:00 PM Physics Building, Room 204 
"Understanding Magnetism in Multiferroics"Micky Holcomb , University of California, Berkeley [Host: Stu Wolf]
ABSTRACT:
Magnetoelectric multiferroics are materials that exhibit multiple order parameters, such as ferroelectricity and ferromagnetism. The potential for coupling between such order parameters and their manipulation through external stimuli (electric or magnetic fields) allows the exploration of novel physics and device functionalities.
Among a large number of materials systems, the BiFeO3 system has emerged as an attractive model system, mainly because both the ferroelectric Curie temperature and the antiferromagnetic Neel temperature are well above room temperature.
In order to understand magnetoelectric coupling, the individual order parameters must first be understood. While the ferroelectric order can be probed using conventional capacitive measurements as well as by scanned probe techniques (such as piezoforce microscopy) probing the antiferromagnetic order requires the use of optical probes, such as SHG and xray photoemission spectromicroscopy.
Angle and temperature dependent absorption measurements using a stateoftheart highresolution photoemission microscope allowed imaging and direction determination of the order parameters in this multiferroic. These studies reveal the first observation of electrical control of antiferromagnetism and the extension to electrical control of ferromagnetism through exchange bias. Though this study, a generic method for separating order parameters in complex systems has been developed and applied.

Condensed Matter Seminar Monday, March 23, 2009 11:00 AM Physics Building, Room 313 
"SixPort Reflectometer: an Alternative Network Analyzer for THz Region"Guoguang Wu , University of Virginia [Host: Bellave Shivaram] 
Condensed Matter Seminar Thursday, March 19, 2009 4:00 PM Physics Building, Room 204 
"Geometrical Interpretation of the Nonuniversal Casimir Energy of An Infinite Cylindrical Wedge"Hussain Zaidi , University of Virginia [Host: Genya Kolomeisky]
ABSTRACT:
The majority of calculations in the literature on the Casimir energy of curved bodies focus on the universal part of the energy, implicitly assuming that the nonuniversal terms subtracted from the formal expression for the energy have no physical consequence. We explicitly calculate the nonuniversal terms for the particular case of an infinite cylindrical wedge and show that these terms are important quantities that arise out of the dependence of the surface tension and the bending/rigidity coefficients of a body on the energy cutoff. This lends support to a recent phenomenological argument in favor of a geometrical interpretation of the nonuniversal parts of the Casimir energy.

Condensed Matter Seminar Tuesday, March 17, 2009 3:30 PM Physics Building, Room 204 
"Orbital degrees of freedom on trianglebased lattice"Takuro Katsufuji , Waseda University [Host: Seunghun Lee ]
ABSTRACT:
Orbital degrees of freedom arising from the degeneracy of
d states give rise to intriguing behaviors in various
transitionmetal oxides. For example, spinel vanadates
(ZnV2O4, MnV2O4, FeV2O4, AlV2O4), where two d electrons
occupy the triply degenerate t2g orbitals, exhibit various
types of orbital ordering. In this talk, orbital ordering of
V ions on trianglebased lattice (SrVxGa12xO19, BaV10O15)
is discussed. In these compounds, V trimers in a spinsinglet
state are formed, but spins on a part of the V ions survive,
which are frustrated or ordered at low temperatures. Such a
spontaneous segregation into spinsinglet clusters and
magnetic ions could be a characteristic of the spinorbital
system on trianglebased lattice.

Condensed Matter Seminar Friday, March 13, 2009 11:00 AM Physics Building, Room 313 
"Singlemolecule biophysics with a protein nanopore"Liviu Movileanu , Syracuse University [Host: Keith Williams]
ABSTRACT:
Advances in rational protein design and singlemolecule technology allow for biochemical sampling at high temporal and spatial resolution and for the detection, manipulation, and exploration of individual molecules. We have developed a methodology for examining single biopolymer dynamics within a protein nanopore, a simple system that is highly pertinent to several more complex biological processes such as the translocation of nucleic acids and polypeptides through transmembrane pores. The ionic current through a single protein nanopore was determined by singlechannel electrical recordings in lipid bilayers. The results revealed unprecedented details of biopolymer behavior at singlemolecule resolution. These examples demonstrate an unusual control of single biomolecules and porebased nanostructures by using simple principles learned from physics and modern biology.

Condensed Matter Seminar Thursday, March 12, 2009 4:00 PM Physics Building, Room 204 
"Imaging Dirac Fermions in a Twodimensional Sheet of Carbon"Yuanbo Zhang , University of California, Berkeley [Host: Despina Louca]
ABSTRACT:
It has been four years since the first successful isolation of graphene, a single atomic sheet of carbon, and enthusiasm for this material is still growing. Part of this excitement arises from the fact that electrons in graphene behave as massless "relativistic" particles (Dirac
fermions) with an effective speed of light equal to c/300. Microscopic disorders in graphene, such as ripples in the carbon sheet or static "puddles" of charge, profoundly alter the behavior of these electrons. I will describe our recent experiments aimed at directly probing these perturbations and imaging their influence on Dirac fermions down to the atomic scale. Our measurements, performed using the techniques of scanning tunneling microscopy and spectroscopy, reveal unexpected electronic interference patterns that vary as a function of both electron energy and applied electric field (via a gate electrode). I will show that electron interference in graphene nanodevices arises from scattering off of static puddles of electrons, and that these puddles are caused by charged impurities underneath the graphene sheet.

Condensed Matter Seminar Thursday, February 19, 2009 3:30 PM Physics Building, Room 204 
"Quenching spin decoherence in diamond and singlemolecule magnets"Susumu Takahashi , University of California, Santa Barbara [Host: Despina Louca]
ABSTRACT:
Overcoming spin decoherence is critical to spintronics and spinbased quantum information processing devices. For spins in the solid state, an interaction with fluctuations of the surrounding spin bath is a major source of spin decoherence. One approach to reducing spin bath fluctuations is to bring the spin bath into a wellknown quantum state that exhibits little or no fluctuations. A prime example is the case of a fullypolarized spin bath. We present our recent demonstrations of significant suppression of spin decoherence measured with highfield electron paramagnetic resonance (EPR). One example is nitrogenvacancy (NV) centers in diamond [1]. Another is S=10 Fe8 singlemolecule magnets [2]. We will also present the development of UCSB freeelectron laser (FEL)based pulsed EPR spectrometer which aims for nanosecond time resolution.[1] S. Takahashi et al., Phys. Rev. Lett. 101, 047601 (2008).
[2] S. Takahashi et al., Phys. Rev. Lett. inpress; arXiv: 0810.1254.

Condensed Matter Seminar Thursday, February 12, 2009 3:30 PM Physics Building, Room 204 
"Dimensional Reduction at a Quantum Critical Point"Cristian Batista , Los Alamos National Laboratory [Host: Seunghun Lee]
ABSTRACT:
Competition between ground states near a quantum critical point is expected to lead to unconventional behavior in low dimensional systems. New phases of matter have been predicted, and explanations proposed for unsolved problems including nonFermi liquid behavior and high temperature superconductivity using twodimensional (2d) theories. In this talk, I will present a theory that describes the BoseEinstein condensate (BEC) quantum critical point (QCP) in layered systems with a frustrated interlayer coupling. I will demonstrate that the main effect of this geometric frustration is to reduce the dimensionality of the QCP (its critical exponents are the ones expected for a 2d system). In addition, I will present the first experimental evidence of dimensional reduction at a QCP observed in the Mott insulator BaCuSi _{ 2 } O _{ 6 } (Han Purple).

Condensed Matter Seminar Thursday, February 5, 2009 4:00 PM Physics Building, Room 204 
ABSTRACT:
A family of new Fe superconductors of SmFeAsO1xFx and others have been discovered in 2008 that contain the puckered FeAs planes instead of the hallmark CuO2 planes in the cuprate superconductors. Central to any superconductor is the nature of its superconducting gap, its value, its structure if any, its temperature dependence. We used Andreev reflection spectroscopy to investigate the gap of these new (1111) Fe superconductors and its temperature dependence. Although the gap values and transition temperatures are different for the Fe superconductors with different F doping, the values of 2 Δ /kBTC are all close to 3.53 that of a BCS swave superconductor. The gap is nearly isotropic with temperature dependence close to that of an swave superconductor. We did not find evidences of pseudogaps, but the spin density wave transition can induce spurious âpseudogapsâ in the system. These characteristics of the Fe superconductors are dramatically different from those of the cuprate superconductors.

Condensed Matter Seminar Wednesday, February 4, 2009 3:30 PM Physics Building, Room 204 
"Nanopores and nanofluidics for single DNA studies"Derek Stein , Brown University [Host: Keith Williams]
ABSTRACT:
Labonachip fluidic technology takes inspiration from electronic integrated circuits, from which its name, its fabrication methods, and its ?smaller, cheaper, faster? paradigm are derived. For siliconbased electronics, miniaturization eventually gave rise to qualitatively different behavior, as quantum mechanical phenomena grew increasingly important. As we shrink fluidic devices down to the nanoscale to probe samples as minute as a single molecule, what physical phenomena will dominate in this new regime, and how might we take advantage of them? This talk will focus on our studies of single DNA molecules using nanofluidic devices and solidstate nanopores.
We are studying how nanofluidic structures, whose critical dimensions are tens to hundreds of nanometers, can manipulate long DNA molecules by a variety of nanoscale phenomena, including electrokinetics, hydrodynamics, Coulomb interactions, and the statistical properties of polymers. Our work also focuses on solidstate nanopores, singlenanometerscale devices that can not only manipulate single molecules, but also detect them electronically. The basic principle behind this is that when DNA is electrophoretically driven through a nanopore, it blocks a measureable fraction of the ionic current that is transmitted through the pore. Thanks to its size, the nanopore also forces each base along the DNA to pass through in sequence, suggesting intriguing possibilities for genetic analysis.

Condensed Matter Seminar Thursday, January 29, 2009 4:00 PM Physics Building, Room 204 
"Combining ferroelectricity and magnetism: the lowenergy electrodynamics"Diyar Talbayev , LANL [Host: Despina Louca]
ABSTRACT:
Multiferroics, materials with coexisting magnetic and ferroelectric orders, hold the promise of implementing multifunctional devices, for example, an electricwrite magneticread memory element. The success of this venture depends on our fundamental understanding of magnetoelectric coupling â the interaction that mixes magnetism with ferroelectricity. Although the electrical switching of a magnetic domain has yet to be achieved, various degrees of electric control of magnetism and magnetic control of ferroelectric state have been demonstrated. Magnetic and lattice vibrations and their mixing by the magnetoelectric coupling play a central role in the properties of multiferroics. This lowenergy electrodynamics can help unravel the fundamental interactions between magnetic and lattice degrees of freedom. I will present a study of longwavelength magnetic excitations in several important classes of multiferroics and demonstrate the relationship between the magnetic excitations and the materialsâ magnetoelectric functionality.

Condensed Matter Seminar Monday, January 26, 2009 3:30 PM Physics Building, Room 204 
ABSTRACT:
A topological quantum computer is a hypothetical device in which intrinsic faulttolerance is embedded in the hardware of the quantum computer. It is envisioned that in these devices quantum information will be stored in certain "topologically ordered" states of matter, and quantum computation is carried out by braiding the worldlines of quasiparticle excitations that obey nonAbelian statistics, around one another, in specific patterns. Certain fractional quantum Hall states are among the prime candidates for realizing nonAbelian quasiparticles that can be used for topological quantum computation. I will review some of the properties of these states, and describe a method for finding braiding patterns which can be used to carry out a universal set of quantum gates on encoded qubits based on nonAbelian quasiparticles that can be realized as excitations of the ReadRezayi series of fractional quantum Hall states.

Condensed Matter Seminar Thursday, January 15, 2009 4:00 PM Physics Building, Room 204 
"A Study of the GroupIV Diluted Magnetic Semiconductor GeMn"Melissa Commisso , University of Virginia [Host: Bellave Shivaram] 
Condensed Matter Seminar Thursday, December 4, 2008 4:00 PM Physics Building, Room 204 
"Timereversal symmetry breaking and spontaneous anomalous Hall effect in Fermi fluids"Kai Sun , University of Illinois [Host: Paul Fendley]
ABSTRACT:
We study the spontaneous nonmagnetic timereversal symmetry breaking in a 2D Fermi liquid without breaking either the translational symmetry or the
U(1) charge symmetry. Using a Berry phase approach, we found that for a large class of models, including all one and twoband models, the timereversal symmetry breaking states can be classified into two classes, dubbed type I and II, depending on the accompanying spatial symmetry breaking patterns. The properties of each class are studied. In particular, we show that the states breaking both timereversal and chiral symmetries (type II) are described by spontaneously generated Berry phases and exhibit anomalous Hall effect in the absence of magnetic fields and magnetic impurities. We also show examples of the timereversal symmetry breaking phases in several different microscopically motivated models and calculate their associated Hall conductance within a meanfield approximation. In particularly, we found a simple lattice structure in which the timereversal symmetry breaking phases can be stabilized by infinitesimal interactions.

Condensed Matter Seminar Monday, November 3, 2008 3:30 PM Physics Building, Room 204 
"Study of Silicon Devices by Inelastic Tunneling Spectroscopy"James Kushmerick , NIST [Host: Keith Williams] 
Condensed Matter Seminar Thursday, October 23, 2008 4:00 PM Physics Building, Room 204 
"Neutron scattering and magnetization studies of the kagome lattice antiferromagnet"Kit Matan , ISSP, U of Tokyo [Host: Seunghun Lee]
ABSTRACT:
The collective behavior of interacting magnetic moments can be strongly inﬂuenced by the topology
of the underlying lattice. In geometrically frustrated spin systems, interesting spin dynamics and
chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. I
will talk about our studies of the spinwave excitations and spin chirality on a twodimensional
geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large
single crystal samples of KFe3(OH)6(SO4)2, an ideal kagomÃ© lattice antiferromagnet. The spinwave
excitations have been measured using highresolution inelastic neutron scattering. We directly
observe a ﬂat mode which corresponds to a lifted "zero energy mode," verifying a fundamental
prediction for the kagomÃ© lattice. A simple Heisenberg spin Hamiltonian provides an excellent ﬁt to
our spinwave data. The antisymmetric DzyloshinskiiMoriya interaction is the primary source of
anisotropy and explain the lowtemperature magnetization and spin structure.
In addition, combined thermodynamic and neutron scattering measurements reveal that the phase
transition to the ordered groundstate is unusual. At low temperatures, application of a magnetic ﬁeld
induces a transition between states with different nontrivial spintextures. The transition indicated
by the sudden increase in the magnetization arises as the spins on alternating layers, which are
previously oppositely canted due to the ferromagnetic interplane coupling, rotate 180Â° to align the
canting moment along the caxis. These observations are consistent with the ordering induced by
the DzyloshinskiiMoriya interaction. Elastic neutron scattering measurements in high ﬁeld verify the
180Â° spin rotation at the transition.

Condensed Matter Seminar Thursday, October 9, 2008 4:00 PM Physics Building, Room 313 
"MeasurementOnly Topological Quantum Computation"Parsa Bonderson , Microsoft / UCSB [Host: Paul Fendley]
ABSTRACT:
The topological approach to quantum computing derives intrinsic faulttolerance by encoding qubits in the nonlocal state spaces of nonAbelian anyons. The original prescription required topological charge measurement for qubit readout, and used braiding exchanges of anyons to execute computational gates. We present an anyonic analog of quantum state teleportation, and use it to show how a series of topological charge measurements may replace the physical transportation of computational anyons in the implementation of computational gates.

Condensed Matter Seminar Thursday, October 2, 2008 4:00 PM Physics Building, Room 204 
"Exploration of Novel Tunnel Barrier Materials for STTRAM "Wei Chen , University of Virginia [Host: Jongsoo Yoon]
ABSTRACT:
The basic structure of magnetic tunnel junctions (MTJs) consists of two ferromagnetic(FM) layers sandwiched by an ultra thin insulating barrier, and it shows high or low resistance depending on the relative direction of the the magnetization of two FM layers. Conventional Magnetic random access memory (MRAM) using MTJs as storage units are switched using external filed, so it has the scaling problem beyond 65nm node. A new switching mechanism called Spin Torque Transfer (STT) has been proposed and experimentally confirmed. In our work, new tunnel barrier materials are being explored to enhance the performance of this new STTRAM technology. One of the particular tunnel barrier materials VO2 has the metalinsulator transition close to room temperature, and we're trying to incorporate VO2 into MTJs stack as the smart barrier for the STT switching so that the MTJs could be switched in low resistance state and read at high resistance state.

Condensed Matter Seminar Thursday, April 24, 2008 4:00 PM Physics Building, Room 204 
"The Mysterious Metallic Phase in 2D Superconductors and the Resulting Phase Diagram"Yize (Stephanie) Li , University of Virginia [Host: Jongsoo Yoon]
ABSTRACT:
Conventional treatments of electronic transport predict that no metallic phase exists in twodimensional (2D) superconducting materials at zero temperature (T=0). This view has been challenged by the observation of magnetic field (B) induced metallic behavior in amorphous MoGe and Ta thin films. We have demonstrated that the metallic phase in Ta thin films has an intrinsic origin and associates with nonlinear voltagecurrent (IV) characteristics that are qualitatively different from those of superconducting and insulating phases. Based on transport measurement, we can map the phase diagram of Ta thin films in BTDisorder space and study the physics it reveals. We also investigated the nature of the Binduced insulating phase. We found that the peak structure of differential IV traces displayed a nonmonotonic change as a function of B, which might be a signature for the localized Cooper pairs.

Condensed Matter Seminar Thursday, April 10, 2008 4:00 PM Physics Building, Room 204 
"Effects of ShortRange Order on the Structure and Dynamics of Relaxor Ferroelectrics"Peter Gehring , NIST [Host: Seunghun Lee]
ABSTRACT:
Relaxors are disordered perovskite (ABO3) oxides, typically characterized by the presence of mixed valence Bsite cations, that have found widespread use in numerous device applications because they exhibit low hysteresis and recordsetting piezoelectric coefficients. Relaxors derive their name from an unusually frequencydependent dielectric susceptibility, but they also display a rich variety of unusual physical phenomena including simultaneously soft zonecenter and zoneboundary phonons, temperature dependent diffuse scattering, and an anomalous thermal expansion where a transition to a lowtemperature invarlike behavior is observed. Recent neutron elastic and inelastic scattering results on the leadbased relaxors PbMg1/3Nb2/3O3 (PMN),PbZn1/3Nb2/3O3 (PZN), and their solid solutions with PbTiO3 will be discussed that indicate the development of static, shortrange polar order at high temperatures is central to these phenomena. These results can be understood by analogy with randomfield models in which just two temperature scales are required to describe the essential features of relaxor compounds.

Condensed Matter Seminar Thursday, April 3, 2008 4:00 PM Physics Building, Room 204 
"Investigation of high temperature TE compounds"Jack Simonson , University of Virginia [Host: Jongsoo Yoon] 
Condensed Matter Seminar Thursday, March 27, 2008 4:00 PM Physics Building, Room 204 
"Fascinating Exotic Phenomena in Layered Ruthenates"Zhiqiang Mao , Tulane University [Host: Seunghun Lee]
ABSTRACT:
Perovskite ruthenates (Sr,Ca)_{ n+1 } Ru _{ n } O _{ 3n+1 } exhibit a rich variety of fascinating ordered ground states. Spintriplet superconductivity, metamagnetic quantum criticality, itinerant ferromagnetism, antiferromagnetic Mott insulating, and halfmetallic behavior were all found in close proximity to one another. These diverse ground states originate from the strong interplay of charge, spin, lattice, and orbital degrees of freedom. They offer a unique opportunity to tune the system and study the physics of novel quantum phases. In this talk, I will first give a brief overview on studies in this area, and then present our recent work on double layered ruthenates (Sr _{ 1x } Ca _{ x } ) _{ 3 } Ru _{ 2 } O _{ 7 } (0 ≤ x ≤ 1). We have established a magnetic phase diagram for this system using the high quality single crystals grown by the floatingzone technique; this phase diagram exhibits significant new phenomena. We find a very unique magnetic state in close proximity to a twodimensional ferromagnet with T _{ c } =0 K for 0.1 < x < 0.4. This state exhibits a surprisingly large Wilson ratio RW (e.g. R _{ W } ≈ 700 for x = 0.2); it freezes into a cluster glass (CG) at low temperatures. Furthermore, we observe evidence of nonFermi liquid behavior as the frozen temperature of the CG phase approaches zero near x = 0.1. The origin of such a state will be discussed.

Condensed Matter Seminar Thursday, March 20, 2008 4:00 PM Physics Building, Room 204 
"Random Telegraph Signal in Carbon Nanotube Device"Jack Chan , University of Virginia [Host: Jongsoo Yoon]
ABSTRACT:
Due to the low dimensionality of carbon nanotubes (CNTs), charging of a single defect site near a CNT may have a profound effect on modifying carrier transport mobility in a long CNT channel. Random Telegraph Signals (RTS) have been studied in carbon nanotube field effect transistors (CNTFETs). If the energy of the defect center is close to the Fermi level of the CNTFET, trapping and detrapping of carriers would modify the carrier transport in the channel, and give rise to RTS. RTS is observed as a switching between discrete current levels, representing a carrier being trapped and detrapped successively in the defect center. We speculate that RTS spectra could provide a characteristic signature of specific adsorbates or adducts on the nanotube channel. This capability is of interest not only for potential sensing technology but also provides a way to introduce controllable quantum interference resonances in the channel transport.

Condensed Matter Seminar Thursday, February 28, 2008 4:00 PM Physics Building, Room 204 
ABSTRACT:
Relaxors have been extensively studied for a variety of applications as ideal dielectric materials because they often exhibit extremely large dielectric and piezoelectric constants while the dielectric loss and temperature dependence are small. Although it is now widely believed that heterogeneity embedded and appearing in relaxors are relevant to various phenomena specific to relaxors, there is still no established microscopic theory for relaxors.
The prototypical relaxors Pb(Mg1/3Nb2/3)O3 (PMN) consists of Pb2+ on the A site of the ABO3 Perovskite structure and Mg2+ and Nb5+ on the B site. To keep the charge neutrality, Mg2+ and Nb5+ have to form a solid solution with a ratio of 1:2 so as to have an average valence of 4+. However, it is likely that a large difference between the ionic radius of Mg2+ and that of Nb5+ prefers the 1:1 solid solution resulting in an alternating arrangement of Mg2+ and Nb5+. The 1:2 state and the 1:1 state are mutually exclusive, thus the system falls in a state of frustration. The concept of frustration has been studied almost exclusively in magnetism, e.g., a geometrical frustration in an antiferromagnetic triangular lattice, though phenomena related to frustration are widely seen in nature. Since there is no unique ground state in a frustrated system, the system becomes unstable among various different states, which may cause large fluctuations leading to a extremely large susceptibility against an external field and to a novel exotic phase. We now consider that heterogeneous structures appearing in relaxors can originate from such a frustration.
In this presentation, a review is given on a series of neutron and xray scattering experiments on spatial structures and dynamics of polar nano regions in relaxors. We then would like to discuss how such experimental results can be understood in the frame work of frustration. We also discuss what we will be able to study the microscopic mechanism of relaxors by controlling the frustration through lattice distortion, charge imbalance and dimensionality.

Condensed Matter Seminar Thursday, February 14, 2008 4:00 PM Physics Building, Room 204 
"Order by distortion and chiral magnetism in CdCr2O4"GiaWei Chern , Johns Hopkins University [Host: Seunghun Lee] 
Condensed Matter Seminar Monday, February 11, 2008 3:30 PM Physics Building, Room 204 
"Neutron scattering study on static and dynamic spin structures in quasicrystalline magnets"Taku Sato , University of Tokyo [Host: Seunghun Lee]
ABSTRACT:
Quasicrystals have distinct spatial symmetry characterized by highlyordered but nonperiodic (quasiperiodic) atomic structure, which differs both from the periodic and random structures. Ordering and excitations of quasiperiodically arranged magnetic moments (spins) are yet fundamental open problems, despite the intensive efforts continuously made since the discovery of the quasicrystal. In this talk I will present recent development of understanding on the static and dynamic spin structures in quasicrystalline magnets, using the extensively studied ZnMgRE systems as typical examples.
In magnetization measurements, the ZnMgRE quasicrystals all show spinglasslike behavior, indicating random freezing of spins at low temperatures, however, welldefined shortrange order have been observed in the neutron scattering. The inelastic response of ZnMgRE falls into two classes: For RE = Tb and Dy, a broad inelastic peak has been observed with very weak Q dependence, suggesting an existence of the strongly localized collective excitation modes. On the other hand, for RE = Ho, temperatureindependent S(Q, h ω ) was observed in the neutronenergygain side (h ω >0) for an incredibly large temperature range (up to 200 K!). The anomalous spin fluctuations may be related to criticality of electron wave functions in the quasiperiodic lattice.

Condensed Matter Seminar Thursday, February 7, 2008 4:00 PM Physics Building, Room 204 
"Frustrated and Satisfied Ground States in Pyrochlore Magnets"Bruce Gaulin , McMaster University [Host: SeungHun Lee]
ABSTRACT:
Geometrical frustration arises quite generally when pairwise interactions in magnetic materials are incompatable with their local geometry. This often involves magnetic materials made up of assemblies of triangles or tetrahedra. The frustration is manifest by disordered low temperature states for the magnetic material  some of which are described by spin liquids, spin glasses, and spin ice. I will discuss (mostly) neutron scattering work using DCS at NIST on two magnetic pyrochlores Tb2Ti2O7 and Ho2Ti2O7, which can be thought of as Isinglike moments decorating a network of cornersharing tetrahedra. Tb2Ti2O7 displays a spin liquid, or cooperative paramagnetic ground state, but can be brought to order in an applied magnetic field. Ho2Ti2O7 displays a static disordered "spin ice"
state at low temperatures.

Condensed Matter Seminar Thursday, January 31, 2008 4:00 PM Physics Building, Room 204 
"Giant Negative Magnetization in a Class of Layered MolecularBased Magnets"Dr. Randy Fishman , Oak Ridge National Laboratory [Host: Jongsoo Yoon]
ABSTRACT:
Bimetallic oxalates are a class of layered molecularbased magnets with transition metals M(II) and M'(III) coupled by oxalate molecules in an open honeycomb structure. Energy, structure, and symmetry considerations are used to construct a reduced Hamiltonian, including exchange and spinorbit interactions, that explains the magnetic compensation and giant negative magnetization in some of the ferrimagnetic Fe(II)Fe(III) compounds. By shifting the Fe(II) ions with respect to the oxalate molecules, the organic cation between the magnetic layers alters the C_3 symmetric crystal field and the orbital angular momentum of the groundstate doublet at the Fe(II) sites. We provide new predictions for the spinwave gap, the effects of uniaxial strain, and the optical flipping of the negative magnetization in Fe(II)Fe(III) bimetallic oxalates.

Condensed Matter Seminar Thursday, December 6, 2007 4:00 PM Physics Building, Room 204 
"Understanding short and medium range order in materials using total neutron scattering"Dr. Thomas Proffen , Los Alamos National Laboratory [Host: Despina Louca]
ABSTRACT:
Determination of the atomic structure is mainly based on the measurement of Bragg intensities and yields the average structure of the infinite crystalline material. However, this approach ignores any defects or local structural deviations that manifest themselves as diffuse scattering. It also fails in case of disordered materials, badly crystalline such as many nanomaterials, or not crystalline at all, such as glasses. In some cases crystalline and amorphous phases coexist making the traditional crystallographic structure refinement difficult or incomplete. The total scattering pattern or the derived atomic pair distribution function (PDF), however, contains structural information over all length scales [1] and can be used to obtain a complete structural picture of complex materials.
One of the great advantaged of the PDF is the fact that one can limit the range on atomatom distance over which the structural model is refined. Focusing on small distances up to a few Angstroms will illuminate the local structure where as refinements over a wide range will yield the medium and long range structure. It is interesting to consider, that instruments such as the high resolution neutron powder diffractometer NPDF located at the Lujan Neutron Scattering Center at Los Alamos National Laboratory allows the measurement of PDFs up to distances in excess of 200Ã
or 20nm. As a result one can obtain a âcompleteâ structural fingerprint of nanoparticles that are frequently smaller in size as demonstrated in a recent study of gold nanoparticles [2].
[1] Th. Proffen, S.J.L. Billinge, T. Egami and D. Louca, Z. Krist. 218, 132143 (2003).
[2] K.L. Page, Th. Proffen, H. Terrones, M. Terrones, L. Lee, Y. Yang, S. Stemmer, R. Seshadri and A.K. Cheetham, Chem. Phys. Lett. 393, 385388 (2004).

Condensed Matter Seminar Thursday, November 29, 2007 4:00 PM Physics Building, Room 204 
"Mesoscopic physics in a quantum magnet chromium"YeongAh Soh , Dartmouth College [Host: Seunghun Lee]
ABSTRACT:
Cr is a simple metal and quintessential spin density wave antiferromagnet that has a quantum critical point. The Hall number collapses on entering the antiferromagnetic state because the Fermi surface is reduced as some of the carriers are localized to produce the magnetic order. Our studies of the Hall effect in Cr[1] were the first to measure the Hall effect in any metal near an antiferromagnetic quantum critical point and showed that it can be a very sensitive probe to study quantum phase transitions. We recently extended our studies to films to explore a twodimensional system. By growing extremely high quality Cr films we observe quantization of spin density waves and scattering from antiferromagnetic domain walls, effects which can be easily tuned by the film thickness.

Condensed Matter Seminar Thursday, November 8, 2007 4:00 PM Physics Building, Room 204 
"Classical and Quantum Frustrated Magnets "Yong Baek Kim , University of Toronto [Host: Seunghun Lee]
ABSTRACT:
We will discuss the properties of classical and quantum Heisenberg
models on the lattices with corner sharing triangles; namely
two dimensional Kagome and threedimensional HyperKagome lattices.
The roles of thermal and quantum fluctuations, and the resulting
ground states will be the focus of the discussion.
Connection to recent experiments on Volborthite,
Na4Ir3O8, and Znparatacamite will also be discussed.

Condensed Matter Seminar Thursday, November 1, 2007 4:00 PM Physics Building, Room 204 
ABSTRACT:
Electronic devices have traditionally relied on charge to encode, store, process and transmit information. A wellknown example is the celebrated Field Effect Transistor, which is the workhorse of all modern digital electronic chips. When a transistor channel is filled with charge, the transistor is âonâ and encodes the binary bit 1. When the channel is depleted of charge, the transistor is âoffâ and encodes the binary bit 0. Switching between bits is therefore accomplished by moving charge within the transistor which causes current flow and associated power dissipation. This is a fundamental shortcoming of all charge based devices. Since charge is a scalar, and has only a magnitude, logic bits 0 and 1 must be demarcated by a difference in the magnitude of the stored charge. As a result, switching between bits always involves changing the magnitude of charge and therefore causing a current flow and I2R dissipation. This cannot be avoided.
An electronâs spin, on the other hand, is a pseudo vector with a polarization. If spin polarization is made âbistableâ by placing the electron in a static magnetic field, then the two allowed polarizations (parallel and antiparallel to the field) can encode bits 0 and 1. Switching would require simply flipping the spin without moving charge in space and causing a current flow (I2R = 0). This can result in tremendous energy saving which is currently the most important goal in electronics.
The Single Spin Logic (SSL) idea1 is based on this paradigm. Spins of single electrons in quantum dots encode digital bits. Logic gates are configured by placing the dots in suitable layouts to engineer the spinspin interactions between them. Inputs are provided by aligning the spins in input dots along desired orientations using locally generated magnetic fields. The arrival of a new input string takes the system to an excited state. When the spins ultimately relax to the many body ground state, the spin polarizations in âoutputâ dots represent the result of a computation in response to the input bits. I will show how the universal NAND gate is configured in this way. With the NAND gate, any arbitrary circuit can be built.
Detailed quantum mechanical calculations show that switching in these circuits dissipate the minimum energy allowed by thermodynamics (the LandauerShannon limit), which is kTln(1/p) where p is the bit error probability. With p = 109, the energy dissipated is ~ 21 kT, whereas modern transistors dissipate 40,000 â 50,000 kT. The SSL is incomparably superior to spin based transistors (Spin Field Effect Transistors, Spin Junction Transistors, etc.) which do not compare well with traditional transistors. The reason is that they still rely on charge for encoding information and do not exploit the advantage of spin.
I will conclude by showing that SSL type constructs are best realized with organic nanostructures where the spin relaxation time can be extremely long. We have measured a spin relaxation time of 1 second at 100 K in a nanostructure of the conjugated organic tris(8hydroxyquinolinolato aluminum) popularly known as Alq3 2.

Condensed Matter Seminar Thursday, October 18, 2007 4:00 PM Physics Building, Room 204 
"Narrow Gap Semiconductors: spin splitting with no magnetic fields and more,â¦.."Giti Khodaparast , Virginia Tech [Host: Keith Williams]
ABSTRACT:
In light of the growing interest in spinrelated phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors (NGS). NGS offer several unique features such as small effective masses, large effective gfactors, high intrinsic mobilities, large spinorbit coupling effects and zerofield spin splitting. For example, In InSb quantum wells (QWs), spinresolved cyclotron resonance (CR) caused by the nonparabolicity of the conduction band and electron spin resonance have been observed. In order to increase our understanding of the dynamics of carriers and spins in NGS, we performed several timeresolved measurements such as magnetooptical Kerr effect. In this talk, magnetooptical studies on InSb QWs and ferromagnetic semiconductors such as InMnSb will be discussed. Our results are important for understanding the electronic and magnetic states in NGS.

Condensed Matter Seminar Thursday, October 11, 2007 4:00 PM Physics Building, Room 204 
"Superconductivity in graphenebased structures"Igor Mazin , Naval Research Laboratory [Host: SeungHun Lee]
ABSTRACT:
MgB2 and CaC6 are some of the most interesting new stars on the superconducting skies. The former, with Tc=39K is the most hightemperature conventional superconductor, and it is by far superior technologically to the cupratebase highTc materials. The latter has Tc >13K, nearly an order of magnitude higher than that of old intercalated graphites. Theory says that despite both materials being, essentially, doped graphenes, superconductivity comes from two different bands, one existing in MgB2 but not in CaC6 and the other in CaC6 but not in MgB2. In this talk I will explain what are this bands and why they are responsible for superconductivity in the respective compounds, and will discuss whether or not it is possible to invent a new material that would combine superconducting advantages of both.

Condensed Matter Seminar Thursday, October 4, 2007 4:00 PM Physics Building, Room 204 
"Recent Results on CMR and Multiferroic Manganese Oxides"Jeff Lynn , National Institute of Standards and Technology [Host: Seunghun Lee]
ABSTRACT:
A few results from recent neutron and xray scattering results will be presented on both CMR and multiferroic systems. For the multiferroics, we have been investigating the magnetic structure of hexagonal HoMnO _{ 3 } as a function of temperature and field [1], which is a commensurate antiferromagnetic (T _{ N } =72 K) ferroelectric (T _{ C } =875 K). Three different chiral symmetries describe the zero field magnetic phases, with strong dielectric anomalies associated with the phase transitions. The spin dynamics are well described by a Heisenberg model in two dimensions. Orthorhombic TbMnO _{ 3 } develops a longitudinally polarized spin density wave state below 41 K, with a change in magnetic structure at 28 K that permits the development of ferroelectricity, while the magnetic structure remains incommensurate [2]. The magnetism is particularly sensitive to Na doping [3]. For the RMn _{ 2 } O _{ 5 } system (R=Tb, Dy, Ho) strong anomalies in the specific heat, thermal expansion, and dielectric constant are a manifestation of the magnetic coupling to the ferroelectricity [4]. Strong magnetoelastic coupling is also found in the triangular antiferromagnetic multiferroic CuFeO _{ 2 } [5]. For the Kagome staircase system Co _{ 3 } V _{ 2 } O _{ 8 } the rich variety of magnetic phases and lockin transitions is a signature of competing interactions [6], and is quite different from ferroelectric Ni _{ 3 } V _{ 2 } O _{ 8 } . For the CMR systems, we will review recent results for the polaron dynamics in optimally doped LaBaMnO _{ 3 } and LaSrMnO _{ 3 } , and compare these results with LaCaMnO _{ 3 } [7] and the bilayer system [8]. The overall behavior observed in the CMR regime of the manganites is quite similar to that observed in the relaxor ferroelectrics as well as the spin and charge stripes found cuprate oxides, demonstrating a commonality of many of the underlying physical concepts of these perovskite oxides.

Condensed Matter Seminar Thursday, September 27, 2007 4:00 PM Physics Building, Room 204 
"Resonant Xray Scattering Study of QuadrupoleStrain Interactions in RareEarth Tetraborides "Sungdae Ji , UVA/NIST [Host: Seunghum Lee] 
Condensed Matter Seminar Thursday, September 6, 2007 4:00 PM Physics Building, Room 204 
"Coherence and optical spin rotations in quantum dots"Sophia Economou , Naval Research Lab [Host: Eddy Barnes/Paul Fendley]
ABSTRACT:
Optically controlled quantum dots, sometimes referred to as artificial atoms, have been the subject of intense research in recent years. This is due both to their potential role as qubits for quantum computing, and to novel emerging physics, as a result of the interplay of confinement and the semiconductor environment.
In the first part of the talk I will present such an effect, Spontaneously Generated Coherence (SGC), in which spontaneous emission of an excited level results in a coherent superposition of two lower levels. This phenomenon, predicted in the context of atomic physics, is so far unobserved in atoms. I will sketch our theory of SGC in quantum dots and present the experimental results of the first observation of this effect.
In the second part of my talk the focus will be on the design of arbitrary optical spin rotations, which are necessary operations for quantum computing. Simulations show that our approach yields high quality gates, up to two orders of magnitude faster that existing proposals.

Condensed Matter Seminar Thursday, April 26, 2007 4:00 PM Physics Building, Room 204 
"From fundamental understanding to predicting new nanomaterials for high capacity hydrogen storage and fuel cell technologies"Taner Yildirim , NIST [Host: Bellave Shivaram]
ABSTRACT:
Developing safe, costeffective, and practical means of storing hydrogen is crucial for the advancement of hydrogen and fuelcell technologies. The current stateoftheart is at an impasse in providing any materials that meet a storage capacity of 6wt% or more required for practical applications. The main obstacles in hydrogen storage are slow kinetics, poor reversibility and high dehydrogenation temperatures for the chemical hydrides, and very low desorption temperatures/energies for the physisorption materials such as metalorganic frameworks (MOF). Recently we have proposed a novel concept to overcome these obstacles. From accurate quantum mechanical calculations, we show that light transition metals (TM) such as a Tiatom affixed to several nanostructures such as nanotubes/C_{60} and small organic molecules (C_{2} H_{4} ) strongly bind up to five hydrogen molecules. The first H _{2} adsorption is dissociative with ~0.25 eV energy barrier while other adsorptions are molecular with significantly elongated HH bonds. The metalhydrogen interaction is found to be very unique, lying between chemi and physisorption, with a binding energy of 0.4 eV compatible with room temperature desorption and absorption. Simulations at high temperature indicate that such hybrid systems of transition metals affixed to nanostructures are quite stable and exhibit associative desorption upon heating, a requirement for reversible storage. These results not only advance our fundamental understanding of dissociative adsorption of hydrogen on transition metals in nanostructures but also suggest new routes to better storage and catalyst materials. Finally, time permitted, we will discuss the possibility of dimerization, polymerization, and incorporation of the predicted TMnanostuctures in nanoporous materials such as MOF to improve the lifecycle and kinetics of the predicted storage materials.

Condensed Matter Seminar Thursday, April 5, 2007 4:00 PM Physics Building, Room 204 
"Search for stripes in lightly hole doped antiferromagnetic YBCO"Andras Janossy , Budapest University of Technology and Economics [Host: Stu Wolf]
ABSTRACT:
Stripes, a periodic spin and charge modulation in superconducting and antiferromagnetic cuprates are a manifestation of an instability of the two dimensional hole system. A multifrequency electron spin resonance study of the antiferromagnetic domain structure in undoped and lightly Ca doped antiferromagnetic Y(Gd)Ba2Cu3O6 single crystals will be discussed. Gd substituting for Y serves as a nonperturbing localised ESR probe of the antiferromagnetic CuO2 layers. Hole doping, variation of temperature and magnetic fields change the antiferromagnetic structure. ESR and infrared transmission experiments in high magnetic fields show that the stripe structure is not an array of hole rich lines in YBCO.

Condensed Matter Seminar Thursday, March 29, 2007 4:00 PM Physics Building, Room 204 
ABSTRACT:
The history of the universe in a nutshell, from the Big Bang to now, and on to the future â John Mather will tell the story of how we got here, how the Universe began with a Big Bang, how it could have produced an Earth where sentient beings can live, and how those beings are discovering their history. Mather was Project Scientist for NASAâs Cosmic Background Explorer (COBE) satellite, which measured the spectrum (the color) of the heat radiation from the Big Bang, discovered hot and cold spots in that radiation, and hunted for the first objects that formed after the great explosion. He will explain Einsteinâs biggest mistake, show how Edwin Hubble discovered the expansion of the universe, how the COBE mission was built, and how the COBE data support the Big Bang theory. He will also show NASAâs plans for the next great telescope in space, the James Webb Space Telescope. It will look even farther back in time than the Hubble Space Telescope, and will look inside the dusty cocoons where stars and planets are being born today. Planned for launch in 2013, it may lead to another Nobel prize for some lucky observer.

Condensed Matter Seminar Thursday, March 22, 2007 7:30 PM Chemistry Building, Room 402 Hoxton Lecture 
"Neutron Scattering Investigation of Excitations Spectrum in Doped and Undoped SpinLadders"Bella Lake , HahnMeitner Institut [Host: Seunghun Lee]
ABSTRACT:
The âtelephone number compoundâ family, (La,Sr,Ca)_{ 14 }Cu _{ 24 } O _{ 41 }, is composed of copper oxide planes which form alternating layers of ladders and chains. In the absence of Sr these materials are intrinsically hole doped with a valence of +2.25 per Cu.In contrast, for La _{ 4 } Sr _{ 10 } Cu _{ 24 } O _{ 41 } the hole doping is much reduced and magnon heat conductivity measurements show that the ladders are free of holes [1]. A third member of the family, Sr _{ 2.5 } Ca _{ 11.5 } Cu _{ 24 } O _{ 41 } , is holedoped with ~2 holes per seven rungs of the ladder (the exact doping is debated). This compound is of particular interest because it shows a number of phenomena in common with highTc cuprates, including linear temperature dependence of resistivity above 130K, charge ordering below 60K and superconductivity under applied pressure [2]. Our neutron scattering measurements on the undoped ladder La _{ 4 } Sr _{ 10 }Cu _{ 24 } O _{ 41 } reveal gapped onemagnon and multimagnon excitations. The results have been modelled to find accurate values of the exchange constants [3]. The doped ladder Sr _{ 2.5 } Ca _{ 11.5 } Cu _{ 24 } O _{ 41 } has also been measured. The holes give rise to a number of changes in the excitation spectrum which will be described and discussed.

Condensed Matter Seminar Monday, March 12, 2007 3:30 PM Physics Building, Room 204 
"A unifying perspective on charge fractionalization"Alexander Seidel , NHMFL/Florida State [Host: Paul Fendley]
ABSTRACT:
Condensed matter physicists are generally faced with the task of
solving problems involving some 1024 particles that interact strongly.
Amazingly, in many cases this task of seemingly hopeless complexity is
amenable to the following simple strategy: Try to find a way to
(almost) switch of the interactions in a manner that preserves all the
fundamental properties of the system. If this is possible, one says
that the system is "adiabatically connected" to a noninteracting
system.
In the past 20 years, however, much focus has been on problems where
the traditional approach does not seem feasible. In particular, a new
paradigm has surfaced which applies to certain novel incompressible
quantum liquids that are said to have "topological order". This new
paradigm encompasses the fractional quantum Hall liquids, as well as
some theoretically proposed scenarios for novel magnetism in materials
similar to the parent compounds of high transition temperature
superconductors. The phenomenology of topologically ordered states is
very exotic, including fractionally charged excitations and fractional
braiding statistics. This fact seems to preclude the possibility that
these states have simple noninteracting limits. In spite of this, it
will be shown in this talk that such a trivial limit does exist for
fractional quantum Hall systems. In fact, by studying quantum Hall
states on cylinders with varying circumference, these states can be
adiabatically evolved into simple onedimensional chargedensitywave
systems. This point of view provides simple, intuitive pictures for
some of the exotic properties of fractional quantum Hall systems. In
particular, the principles of charge fractionalization in two spatial
dimensions and in one spatial dimension are completely unified by this
approach. The potential usefulness of this adiabatic continuity for
some unresolved problems will also be discussed.

Condensed Matter Seminar Monday, February 19, 2007 3:30 PM Physics Building, Room 204 
ABSTRACT:
Generic perturbations of a Fermi liquid create noise in the form of numerous
particlehole pairs. In the talk I will show that it is possible to excite
particles from a Fermi sea in a noisefree fashion by electromagnetic pulses
of a special form. The resulting manybody states are characterized by a
finite number of particles excited above the Fermi surface accompanied by no
disturbance below it. The excitations can be regarded as realization of
vertex operators. I will also discuss potential applications in creating
entangled electron hole pairs, and effects of interactions.

Condensed Matter Seminar Thursday, February 15, 2007 4:00 PM Physics Building, Room 204 
ABSTRACT:
The ordering of matter into different phases is a central
preoccupation of many areas of physics, from condensed matter to
cosmology. Hand in hand with the existence of different phases
goes the question of which dynamical processes are responsible for =
their
formation, which may be equally important in determining what is
observed in a given situation. Recent experimental advances in the
creation of degenerate atomic gases have begun to realize the
prospect of a rich variety of new phases in atomic matter, involving
the hyperfine degrees of freedom, mixtures of different species, or
spatial order on optical lattices. With each new phase comes the
issue of how that phase will appear under laboratory
conditions.=C2=A0
In this talk I'll discuss the theoretical treatment of phase ordering =
in=C2=A0
several recent experiments, and the possibility of observing new
dynamical phenomena in the future.

Condensed Matter Seminar Wednesday, February 7, 2007 3:30 PM Physics Building, Room 204 
"PATTERNS, STABILITY AND COLLAPSE FOR TWODIMENSIONAL BIOLOGICAL SWARMS"Maria D'Orsogna , UCLA [Host: Paul Fendley]
ABSTRACT:
One of the most fascinating biological phenomena is the selforganization
of individual members of a species moving in unison with one another,
forming elegant and coherent aggregation patterns.
Schools of fish, flocks of birds and swarms of insects
arise in response to external stimuli or by direct interaction, and are
able to fulfill tasks much more efficiently than single agents.
How do these patterns arise? What are their properties?
How are individual characteristics linked to collective behaviors?
In this talk we discuss various aspects of biological swarming
by investigating a nonlinear system of
self propelled agents that interact via pairwise attractive and repulsive
potentials. We are able to predict distinct
aggregation morphologies, such as flocks and vortices, and by
utilizing statistical mechanics tools,
to relate the interaction potential to the collapsing or
dispersing behavior of aggregates as the number of constituents increases.
We also discuss passage to the continuum and possible applications
of this work to the development of artificial swarming teams.

Condensed Matter Seminar Monday, February 5, 2007 3:30 PM Physics Building, Room 204 
" Mutual influence of vortices and quasiparticles in dwave superconductors "Predrag Nikolic , Harvard [Host: Paul Fendley]
ABSTRACT:
Vortices in clean dwave superconductors at low temperatures can
behave
as quantum particles. Their quantum dynamics is made possible by the
smallness of their cores, due to short coherence length in typical
cuprates, and by the presence of massless fermionic quasiparticles,
which give rise to certain universal effects. We calculate a small
finite renormalization of vortex mass by the nodal quasiparticles,
and
demonstrate the absence of BardeenStephen damping of vortex motion
in
the limits of zero temperature, no disorder and vanishing core size.
Being liberated from strong friction, light vortices can experience
significant quantum fluctuations that can explain several phenomena
observed in cuprates, including the Nernst effect and density waves.
We
also show that quantum fluctuations of localized vortices can
significantly affect quasiparticle spectra. The local electronic
density
of states (LDOS) near a quantum fluctuating vortex shows no
zeroenergy
peak, but has satellite features at energies set by the vortex
trapping
potential. These are proposed to be the origin of the subgap LDOS
peaks
observed in recent STM experiments near the vortex cores.

Condensed Matter Seminar Thursday, February 1, 2007 4:00 PM Physics Building, Room 204 
"Phase Transitions in Nanodimensional Materials"Syed Qadri , U. S. Naval Research Laboratory [Host: Stu Wolf]
ABSTRACT:
Research interest in the field of nanodimensional materials is growing rapidly due to their interesting sizedependent electronic, magnetic, optical and mechanical properties that have many potential industrial applications. Metals and semiconductors with dimensions in the nanometer realm exhibit novel optical, electrical, and chemical properties which are closely associated with their structural characteristics. In addition, metastable phases and sizedependent phase transitions are observed when the particle sizes are reduced to nanodimensions. Several examples from our previous work will be presented to illustrate the existence of these unusual structural characteristics. Xray diffraction is a nondestructive characterization tool that plays an important role in the synthesis and in understanding the physical properties of the nanoparticles.

Condensed Matter Seminar Thursday, November 30, 2006 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, November 23, 2006 4:00 PM Physics Building, Room 204 
"Charge ordering instead of JahnTeller distortion"Daniel Khomskii , Universitaet zu Koeln, Germany [Host: Seunghun Lee]
ABSTRACT:
Due to JahnTeller effect the systems with orbital degeneracy usually have an orbital ordering with corresponding lattice distortion. However this distortion weakens and finally disappears when the electrons become itinerant. We show that in the intermediate regime between localized and itinerant electrons there exist a new possibility: orbital degeneracy may be lifted not by JahnTeller distortion, but by charge ordering, or charge differentiation. We demonstrate, by theoretical calculations and experimentally, that this happens in particular in rare earths nickelates such as YNiO3 or LuNiO3. The same physics apparently operates also in several other systems, such as ferrates (e.g. CaFeO3) and some others. I will also discuss a possible role of small charge transfer gap and oxygen holes in this phenomenon.

Condensed Matter Seminar Thursday, November 16, 2006 4:00 PM Physics Building, Room 204 
"Unusual features in magnetism and magnetoresistance of double perovskite oxides"Prof. D.D. Sarma , Indian Institute of Science [Host: Bellave Shivaram]
ABSTRACT:
Sr2FeMoO6 is a magnetic metal with an unusually high Curie temperature (~ 420 K) and belongs to the double perovskite family of compounds. It shot in to fame a few years ago due to its remarkable magnetoresistive properties. [1] We discuss the origin of ferromagnetism in this and related compounds based on a mechanism driven by the kinetic energy, [24] establishing these as members of a new class of magnetic materials; this mechanism also explains [4,5] the occurrence of ferromagnetism in dilute magnetic semiconductors, such as Mndoped GaAs and in pyrochlore, Tl2Mn2O7. [6] We also show [7] that the magnetoresistance in Sr2FeMoO6 arises from a magnetically triggered nearlyresonant tunnelling condition in contrast to other mechanisms discussed so far in the context of CMR and GMR materials.

Condensed Matter Seminar Wednesday, November 15, 2006 3:30 PM Physics Building, Room 204 
"To tunnel or not to tunnel  or why clathrates are so fascinating"Veerle Keppens , University of Tennessee [Host: Joe Poon]
ABSTRACT:
Motivated by the search for improved thermoelectric materials, several compounds have attracted attention that combine the high electron mobilities found in crystals with a low thermal conductivity approaching values typical for glasses. The common structural feature of these âElectron Crystal Phonon Glassesâ (ECPG) is that they contain loosely bound atoms that reside in a large crystalline âcageâ. These ârattlersâ scatter phonons and greatly reduce the thermal conductivity of the material. One family of ECPGs is formed by the Geclathrates Sr _{ 8 } Ga _{ 16 } Ge _{ 30 } and Eu _{ 8 } Ga _{ 16 } Ge _{ 30 } . The presence of the rattler significantly softens the elastic behavior. Combined with results from lowtemperature ultrasonic attenuation, neutronscattering, thermal conductivity and microwave absorption measurements, it provides clear evidence for the existence of a new type of fourwell tunneling states.

Condensed Matter Seminar Thursday, November 2, 2006 4:00 PM Physics Building, Room 204 
"Quantum phase transitions and magnon stability in gapped spin chains and ladders"Andrey Zheludev , Oak Ridge National Laboratory [Host: SeungHun Lee]
ABSTRACT:
The lowest energy excitations in quantum disordered spin chains and ladders are a triplet of massive magnons. The stability of these quasiparticles depends critically on the symmetries of the actual spin Hamiltonian. When the spin system undergoes a quantum phase transition, for example one induced by a strong external magnetic field, the symmetry of the ground state changes, and so do the excitations. I will present a comparative inelastic neutron
scattering study of three distinct onedimensional disordered spin systems: the S=1 quasi1D bondalternating antiferromagnet NTENP, the uniform anisotropic S=1chain Haldanegap compound
NDMAP and the uniform isotropic ``composite'' Haldane spin chain IPACuCl3. For each material I will discuss the fieldinduced condensation of magnons, and analyze the spectra measured below,
at and above the transition point. The highfield phase will be characterized either as a BoseEinstein magnon condensate, with a
cartelistic gapless Goldstone mode, or as a gapped "quantum spin solid". The issue of magnon stability in each phase will be addressed in detail.

Condensed Matter Seminar Thursday, October 26, 2006 4:00 PM Physics Building, Room 204 
"Graphene: symmetries, phase transitions, Hall effect"Igor Herbut , Simon Fraser University [Host: Seunghun Lee]
ABSTRACT:
A single sheet or graphite, or graphene, in its natural state is a semimetal with two Fermi points in its Brillouin zone. The lowenergy excitations near those points are equivalent to massless relativistic fermions, with the Fermi velocity assuming the role of the speed of light. I will discuss the effects of Coulomb interaction between electrons in such a system at zero and finite magnetic field. The integer Hall effect in graphene will be explained as the combination of the Landau level quantization and spontaneous breaking of the emerging "chiral" symmetry in such a quasirelativistic system.

Condensed Matter Seminar Thursday, October 19, 2006 2:00 PM Physics Building, Room 203 
"Low Energy Vibrational Excitations in Metallic Glasses"Ricardo Schwarz , Los Alamos National Laboratory [Host: Vittorio Celli]
ABSTRACT:
The specific heat and elastic constants of metallic glasses show anomalies not seen in crystals. These anomalies result from lowenergy vibrational excitations in the glass. We have measured the lowtemperature heat capacity, the elastic constants, and the phonon density of states of both glassy and singlephase crystalline Pd_{40}Cu_{40}P_{20}. The specific heats of both alloys, plotted as C_{P}/T^{3} vs. T, show different humps (commonly known as "Boson Peaks"). The elastic constants of the crystal and glass have different Tdependence: the shear modulus of the glass varies as C'(T) = C'(0)[1  AT], whereas that of the crystal varies as C'(T) = C'(0)[1  BT^{2}  DT^{4}]. This suite of lowtemperature measurements enabled us to identify the lowenergy vibrational excitations responsible for these anomalies

Condensed Matter Seminar Wednesday, October 18, 2006 4:00 PM Physics Building, Room 204 
"Magnetic ordering in a halfpolarized magnetization plateau of the pyrochlore antiferromagnet"Doron Bergman , UC Santa Barbara [Host: Seunghun Lee]
ABSTRACT:
The classical pyrochlore antiferromagnet (AFM) is considered the
``most'' geometrically frustrated system. Classically, this leads to the
absence of any ordering transition at nonzero temperature, even in an
applied magnetic field. Recent experiments on the spinel chromites, ACr2o4(A=Cd,Hg) show the existance of a very robust magnetization plateau in a
strong magnetic field, and a simultaneous magnetic ordering.
We describe a model of spinlattice coupling that explains both the
plateau formation and the observed ordering on the plateau. The predictions are confirmed by recent neutron scattering and xray
scattering experiments (S. H. Lee et al.). The same model applied to
zero magnetic field predicts a reduced
but still large ground state degeneracy, including the states observed
in both the Cd and Hg materials. This is consistent with the dominance
of spinlattice interactions, with weak additional effects determining
the low field magnetic ordering.

Condensed Matter Seminar Thursday, October 12, 2006 4:00 PM Physics Building, Room 204 
ABSTRACT:
While electrostatics and magnetostatics are disparate phenomena in a vacuum, no symmetry forbids materials from responding magnetically to an electric field or vise versa. Materials with a strong magnetoelectric response are of interest for applications and challenge our understanding of magnetic dielectrics. I discuss specific examples of magnetoelectricity in metal oxides with triangular or kagomÃ© lattices of spins with competing antiferromagnetic interactions [13]. It is shown that inversion symmetry breaking magnetic order can act as an effective electric field through magnetoelastic distortions that relieve frustration. The results presented in this talk are based on magnetic neutron scattering experiments.

Condensed Matter Seminar Thursday, October 5, 2006 4:00 PM Physics Building, Room 204 
ABSTRACT:
Quantum criticality describes the strong fluctuations of a second order phase transition at zero temperature. There is growing evidence that it is
relevant to a large part of the phase diagram of a variety of strongly
correlated materials. In this talk, I will describe some of the basic issues that challenge the theoretical description of quantum criticality. I will go on to make the case for a new class of quantum critical point, with critical fluctuations that are genuinely quantummechanical.
In the case of heavy fermion metals, the theory of local quantum criticality characterizes such quantum fluctuations in terms of the collapse of a Kondo entanglement scale. Finally, I will summarize
some of the experimental evidences for this theory, particularly those
concerning the evolution of the Fermi surface.

Condensed Matter Seminar Thursday, September 28, 2006 4:00 PM Physics Building, Room 204 
"Applications of several MeV CW Superconducting Radio Frequency Accelerators"Ganapathi Myneni and Geoff Kraft , University of Virginia [Host: Bellave Shivaram] 
Condensed Matter Seminar Thursday, September 14, 2006 4:00 PM Physics Building, Room 204 
"Rapid Motion in the Plant Kingdom: Nature's Weapons of Mass Reproduction."Dwight Whitaker , Williams College [Host: Jongsoo Yoon]
ABSTRACT:
Plants and fungi have developed a number of remarkable methods to bring about rapid motion. The most rapid movements rely on stored elastic energy and take place in a timescale that is shorter than a single enzymatic reaction (~1 ms). Because these processes are entirely mechanical we can describe the motion with a straightforward biomechanical model based on classical mechanics. Highspeed video lets us test the predictions of our models, which enable us to isolate the key features required for each type of movement and to assess how effectively the system performs. This information, when combined with field observations, helps us to understand the adaptive significance of the motion and put it into evolutionary context with similar species. In this talk we will present a number common plants that exhibit a variety of uncommon methods to disperse seeds and spores.

Condensed Matter Seminar Thursday, August 31, 2006 4:00 PM Physics Building, Room 204 
"Desynchronization and Spatial Effects in Multistrain Diseases"Leah Chock , Naval Research Laboratories [Host: Jongsoo Yoon]
ABSTRACT:
Dengue fever, a multistrain disease, has four distinct coexisting
serotypes (strains). The serotypes interact by antibodydependent
enhancement (ADE), in which infection with a single serotype is
asymptomatic, but contact with a second serotype leads to serious illness
accompanied by greater infectivity. It has been observed from serotype
data that outbreaks of the four serotypes occur asynchronously (Nisalak et
al., Am. J. Trop. Med. Hyg. 68: 192). We present a compartmental model
for multiple serotypes with ADE, and consider autonomous, seasonally
driven, and stochastic versions of the model. For sufficiently small ADE,
we find that the number of infectives of each serotype synchronizes, with
outbreaks occurring in phase. However, when the ADE increases past a
threshold, the system becomes chaotic, and infectives of each serotype
desynchronize. Spatial effects are included in a multipatch model. We
observe desynchronization between spatially distinct regions.

Condensed Matter Seminar Thursday, April 27, 2006 4:00 PM Physics Building, Room 204 
"National Physics Day Show"Steve Thornton, Michael Timmins, Rob Watkins , UVA [Host: University of Virginia Physics Department]
ABSTRACT:
This is a familyoriented event. Come see an hour of exciting and intriguing demonstrations! For more information on this free public event, call 434 924 3781. Free public parking is available in nearby Scott Stadium lots.

Condensed Matter Seminar Thursday, April 20, 2006 6:00 PM Physics Building  McCormick Road, Room 203 12^{th} Annual National Physics Day Show 
"Quasiparticle Breakdown in a Quantum Spin Liquid"Igor Zaliznyak , Brookhaven National Laboratory [Host: Seunghun Lee]
ABSTRACT:
Quasiparticles are the elementary excitations carrying energy and momentum quanta in consensed matter, much like photons and elementary particles carry energy and momentum in the Universe surrounding us. Recent neutron scattering experiments demonstrate how quasiparticle description of energy spectra fails in magnetic crystals with nonmagnetic ground states that can
be identified as "quantum spin liquids" (QSL). The elementary excitations in such systems can be identified with massive (i.e. having nonzero rest
energy) quasiparticles, called magnons, which obey Bose statistics, forming a Bose liquid. In a Bose quantum liquid, however, the singleparticle dispersion can terminate at an energy where quasiparticle breakups into two excitations become allowed, i.e. where the singleparticle dispersion enters a continuum of twoparticle states. Such spectrum endpoint was originally
predicted by L. Landau for the superfluid helium4, where it was subsequently extensively studied both theoretically and experimentally. A
manifestation of this peculiar phenomenon in the case of a quantum spin liquid was found in the spin dynamics of the Haldanechain S=1 antiferromagnet CsNiCl3, where it was initially identified as a crossover from the single quasiparticle to a spincontinuum response. More recently, signatures of the quasiparticle spectrum termination were observed in the excitation spectrum of the twodimensional (2D) quantum spin liquid existing in the organometallic material piperazinium hexachlorodicuprate (PHCC), indicating a failure of the Bosequasiparticle description of the QSL state of the 2D S=1/2 Heisenberg antiferromagnet in an extended region of its phase space. These findings are of great current interest as they might have important implications for the type of hightemperature superconductivity found in cuprates.

Condensed Matter Seminar Thursday, April 13, 2006 4:00 PM Physics Building, Room 204 
"SUPERCONDUCTIVITY  An Emerging Technology for Power Systems"Don Gubser , Naval Research Laboratory [Host: Stu Wolf]
ABSTRACT:
As the world becomes more âelectrifiedâ, efficient distribution and use of electrical power becomes increasingly important. The use of superconducting materials significantly reduces electrical energy loss in the distribution and use of electrical power as well as producing significant reductions in size and weight of power components and machinery. Although superconductivity was first discovered in 1911 the requirement of an extreme âcryogenicâ environment (near absolute zero temperature) limited its utility. With the discovery in 1986 of a new class of âhigh temperature superconductors (HTS)â that operate at substantially higher temperatures (although still cryogenic), remarkable progress has been made in advancing a broader use for superconducting technology. Full scale demonstrations are now being built to develop engineering skills required for systems implementation of this new HTS technology and to better quantify system benefits. This talk will briefly review some of the fundamental attributes of superconductivity before turning to the main focus of the talk describing ongoing power demonstration projects (transmission lines, transformers, motors/generators, etc.). I will end with thoughts on what it will take to realize the full potential of these emerging superconducting technologies.

Condensed Matter Seminar Thursday, April 6, 2006 4:00 PM Physics Building, Room 204 
"Fate of the Josephson Effect in Thinfilm Superconductors"Gil Refael , Caltech [Host: Victor Galitski]
ABSTRACT:
The dc Josephson effect is a probe of the fundamental nature
of the superconducting state. In this talk, I will analyze the case of
two superconducting thin films connected by a point contact. Remarkably,
the Josephson effect is absent at nonzero temperature, even below the
KT transition of the films, and the resistance across the contact is
nonzero. Moreover, the point contact resistance is found to vary with temperature in a nearly activated fashion, with a universal energy
barrier determined only by the superfluid stiffness characterizing the
films, an angle characterizing the geometry, and whether or not the
Coulomb interaction between Cooper pairs is screened. As will be shown,
this should be testable even in finite systems at a proper range of
lengthscales and temperatures.

Condensed Matter Seminar Thursday, March 30, 2006 4:00 PM Physics Building, Room 204 
"Ultrafast Phase Transformation in Metals Induced by Laser Irradiation: a Molecular Dynamic Study"Zhibin Lin , UVA [Host: Jongsoo Yoon]
ABSTRACT:
Short (pico and femtosecond) laser irradiation has the ability to bring materials into a highly nonequilibrium state where the electron temperature is high while the lattice is still cold. By using Molecular Dynamic (MD) simulation, we can study the kinetics and mechanisms of laser melting and ablation phenomena at the atomic level. In this talk, I will talk about our simulation results on ultrafast melting induced by strong laser irradiation in metals and coherent acoustic phonon generation by laser excitation. In addition, I will relate to recent experimental observation in timeresolved diffraction experiments in probing the structure dynamic under extreme conditions.

Condensed Matter Seminar Thursday, March 23, 2006 4:00 PM Physics Building, Room 204 
"Field Theory Studies of a Unitary Fermi Gas"Matthew Wingate , University of Washington [Host: Hank Thacker]
ABSTRACT:
Fermion pairing leads to interesting phenomena in many physical systems, whether the constituents are atoms, nucleons, electrons, or quarks. The newfound ability to trap and cool fermionic atoms has provided a versatile avenue for experimental exploration of fermion pairing. I have been working on using field theory techniques to describe and calculate properties of these gases. In this talk, I focus on a dilute gas of 2component, nonrelativistic fermions whose scattering length is much larger than the average interparticle spacing. This is a setup which describes atomic gases tuned to a Feshbach resonance, as well as neutron matter. I discuss how Monte Carlo methods
constitute an ab initio method for theoretical study of this gas. As an
example, I show an exploratory calculation of the critical temperature
separating the normal and superfluid phases. Next, I outline how effective field theory enables one to study the low temperature properties of the gas beyond superfluid hydrodynamics. It turns out
that this system possesses a great deal of symmetry which tightly constrains the form of nexttoleading order behavior.

Condensed Matter Seminar Thursday, March 16, 2006 4:00 PM Physics Building, Room 204 
"Adsorption of Gases on Carbon Nanotubes"Aldo Migone , Southern Illinois University [Host: George Hess] 
Condensed Matter Seminar Thursday, March 2, 2006 4:00 PM Physics Building, Room 204 
"Full Counting Statistics From Interference Between Interacting Bose Liquids in One Dimension"Vladimir Gritsev , Harvard University [Host: Paul Fendley]
ABSTRACT:
In my talk first I will give a general introduction in physics of
onedimensional Bose condensates. Because of the absence of phase coherence,
the interference between one dimensional condensates gives rise to quantum
noise in the form of fluctuating fringe patterns. I describe the possible
experiments for measuring the probability distribution function of interference
fringe amplitudes. This probability distribution can be related to the boundary
sineGordon model which is also known in mesoscopic physics to describe an
impurity in a Luttinger liquid. The probability distribution function
demonstrates an interesting crossover from broad Poissonian distribution for
the case of impenetrable bosons to universal narrow asymmetric distributions at
weak interactions. Finally we argue that by measuring such distributions
experimentally, one can study the
properties of conformal field theories with negative central charge,
that appear in a variety of contexts ranging from stochastic growth
models to two dimensional quantum gravity.

Condensed Matter Seminar Thursday, February 23, 2006 4:00 PM Physics Building, Room 204 
"Emergence of Magnetism from a Nonmagnetic Mott Insulator"Daniel Phelan , UVA [Host: Despina Louca]
ABSTRACT:
The parent compound of La1xSrxCoO3 has a nonmagnetic, Mott insulating ground state. As temperature is increased, the parent undergoes a nonmagnetic to paramagnetic (PM) transition, and with holedoping, static ferromagnetic (FM) correlations develop. The mechanisms by which the PM and FM states emerge from the nonmagnetic state will be discussed with reference to our neutron scattering measurements. In the PM state, dynamic FM and antiferromagnetic (AFM) correlations likely result from the superexchange interactions in a dynamically orbitally ordered state. With doping, static isotropic FM droplets are formed due to double exchange, and the FM correlation length increases with hole concentration.

Condensed Matter Seminar Thursday, February 2, 2006 4:00 PM Physics Building, Room 204 
"The Low Temperature Phase Diagram of the RKKY Model: Competing Interactions and Magnetic Percolation"Donald Priour , University of Maryland [Host: Victor Galitski]
ABSTRACT:
We examine magnetic moments (e.g. Mn impuritiesin GaAs) coupled via the indirect exchange RKKY interaction. We obtain via Monte Carlo the T=0 phase diagram as a function of the Mn density n_{i} and the carrier concentration n_{c}. As evidenced by a diverging correlation length and the magnetic susceptibility, the boundary between the ferromagnetic (FM) and the paramagnetic (PM) phases represents a line
of zero temperature critical points with behavior very similar to a percolation transition. In particular, the ferromagnetic clusters increase in size and ultimately coalesce to span the system as the phase boundary is approached from the PM side. We have found this "ferromagnetic percolation" behavior in a variety of disordered
magnetic systems with competing interactions, including the EdwardsAnderson model. In the RKKY system, we find that in the dilute limit, bulk ferromagnetism vanishes for n_{c}/n_{i} > 0.1. We also examine the
impact of a strong local antiferromagnetic superexchange coupling between magnetic impurities, and we discuss the impact of a finite mean free path, which we include as a damping factor in the RKKY range function.

Condensed Matter Seminar Thursday, January 26, 2006 4:00 PM Physics Building, Room 204 
"Spindependent Properties of Siliconbased Epitaxial Structures"Frank Tsui , University of North Carolina, Chapel Hill [Host: Joe Poon] 
Condensed Matter Seminar Thursday, January 19, 2006 4:00 PM Physics Building, Room 204 
"Bosons on the Triangular Lattice: Mott Transition in the Presence of Geometrical Frustration"Anton Burkov , Harvard University [Host: Victor Galitski]
ABSTRACT:
Interest in the behavior of interacting bosons on twodimensional lattices
has recently been revived by the experimental realization of
superfluidinsulator transition of cold atoms in an optical lattice.
Such models are also interesting from a broader prospective since
they provide a simpler context to explore general aspects of
conductinginsulating transitions of electrons.
I will talk about our recent study of the interplay between Mott
localization and geometrical frustration in a system of interacting
bosons on the triangular lattice. Frustration prevents localization
into simple ordered states, that can be determined by minimizing
only the interaction energy. A variety of unconventional states
thus become possible, notably supersolids and valence bond ordered
insulators. I will describe a general phenomenological theory of these
states, based on a duality transformation from bosons to vortices
and compare this theory with numerical simulations.

Condensed Matter Seminar Thursday, December 1, 2005 4:00 PM Physics Building, Room 204 
"Quantum Phases of the Extended BoseHubbard Hamiltonian: Possibility of a Supersolid State of Cold Atoms in Optical Lattices"Vito Scarola , University of Maryland [Host: Victor Galitski]
ABSTRACT:
Cold atoms confined to optical lattices offer the unique opportunity for direct
simulation of strongly correlated models in a pristine environment. Recent
experimental progress in simulating the zerorange BoseHubbard model with
bosonic atoms in an optical lattice provides a direct experimental probe of the
superfluid to Mott phase transition. Another lattice model, the extended
BoseHubbard model, yields a rich but contested phase diagram which includes
the possibility of supersolid and density wave order. I will discuss a
specific proposal designed to extend the range of interaction in these
otherwise zerorange systems to effectively simulate an extended BoseHubbard
model in a cold atom optical lattice. I will also discuss potential issues
affecting experimental detection of supersolid and density wave order.

Condensed Matter Seminar Thursday, November 17, 2005 4:00 PM Physics Building, Room 204 
ABSTRACT:
After a brief introduction to the quantum Hall effects and the composite fermion model, I will discuss recent results in the second Landau level (LL). The data were obtained at very low temperatures (sample temperatures as low as 9 mK) in a very high mobility 2D electron system. We observe wellquantized FQHE states at the LL filling nu= 2+1/2, 2+1/3, and nu= 2+2/3 in coexistence with the reentrant integer quantum Hall states, and a new FQHE state at nu= 2+2/5. The origin of the 2+2/5 state is not clear and the numerical results strongly suggest that it is not a conventional FQHE state but a parafermionic state. There is also evidence for a second evendenominator FQHE state at nu= 2+3/8.
Most importantly, we discovered an unexpected quantization of the diagonal resistance, Rxx, at the edges of several quantum Hall states. Each quantized Rxx value is close to the difference between the two adjacent Hall plateaus in the offdiagonal resistance, Rxy. Surprisingly, we can trace this observation back to a small density gradient, about 1%/cm, in our sample. Under this scenario almost all Rxx features can be explained quantitatively by an electron density gradient. These findings have very important implications for any Rxx data taken on twodimensional electron systems, since Rxx seems now to be solely determined by Rxy, while its relationship to the diagonal resistivity, rxx, is unclear. These findings are further corroborated by data from a different sample at a temperature of 1.2K. There, Rxx shows a strictly linear dependence on the magnetic field, except for sharp spikes at Bfields where the IQHE develops. Interestingly, this linear magnetoresistance can also be explained by the density gradient model.

Condensed Matter Seminar Thursday, November 10, 2005 4:00 PM Physics Building, Room 204 
"Overcoming Degeneracy in Highly Frustrated Antiferromagnets"Chris Henley , Cornell University [Host: Paul Fendley] 
Condensed Matter Seminar Thursday, November 3, 2005 4:00 PM Physics Building, Room 204 
ABSTRACT:
We have observed superflow in both bulk solid He4 and
solid He4 confined in porous media by torsional oscillator technique.[1]
Below 0.25K nonclassical rotational inertia appears as an abrupt drop in the
resonant period of the torsional oscillator.
The temperature dependence of the supersolid fraction is different from
those found in liquid helium and weakly interacting alkali gases.
The nonclassical rotational inertia fraction is independent of the oscillation
speed below an extremely small critical value of 10 um/s, above which
strongly attenuated. The pressure dependence of the supersolid fraction in the
low temperature limit reveals an intriguing maxium around 55 bars, whereafter
the supersolid fraction monotonically decreases at least up to 136 bars and can
be linearly extrapolated to zero at 170 bars.
[1] E. Kim and M. H. W. Chan, Nature 427, 225 (2004); Science 305, 1941 (2004);
J. Low Temp. Phys. 138, 859 (2005).

Condensed Matter Seminar Thursday, October 27, 2005 4:00 PM Physics Building, Room 204 
ABSTRACT:
According to the scaling theory of localization (1979), simple metallic
phases should not exist in two dimensional electronic systems. Experiments
showing the divergence of the low temperature resistance in ultrathin films
of metallic elements and 2d electron gases in semiconductor heterostructures
tended to support this prediction for about 15 years. More recently,
however, evidence of metallic transport has begun to emerge, popping up in
thin normally superconducting films and in very low density, high mobility
2D electron gas systems. The physics behind these metallic behaviors is
not known although it is generally agreed that explanations must go beyond
the scaling theory paradigm and include electronelectron interactions.
In an effort to uncover an understandable metallic phase in two dimensions,
we are studying ultrathin films composed of superconducting (S) and normal
metal (N) elements. Interactions are essential to their superconducting
state and such "SN" systems have been predicted to undergo a quantum
superconductor to metal transition (SMT) as N is increased. I will describe
how our transport and tunneling experiments on SN (Pb/Ag) bilayer films
exhibit deviations from standard superconductor proximity effect theories
that are consistent with an impending SMT. For example, the quasiparticle
density of states of superconducting bilayers acquires a hybrid
superconductormetal appearance. This characteristic suggests that
coexisting but separate superconducting and metal quasiparticle populations
develop in the approach to the metallic phase.

Condensed Matter Seminar Thursday, October 20, 2005 4:00 PM Physics Building, Room 204 
ABSTRACT:
In high magnetic field and at low temperatures, liquid He3 condenses into a unique superfluid phase (A1) in which the condensate is spinpolarized. Owing to the broken relative symmetry in gauge and spin spaces, A1 phase hydrodynamics has unusual properties. To demonstrate the unusual hydrodynamics, two experiments on (1) magnetic fountain effect and (2) spinentropy wave (SEW) propagation will be discussed. (1)Owing to the ability to respond to gradients in pressure, temperature as well as magnetic field, A1 phase should develop a steady pressure gradient across two chambers when a steady magnetic field gradient is applied across a superleak connecting the two chambers. Such magnetic fountain effect has been observed but with shorter relaxation time than expected. The observed relaxation is likely related to the important but not yet wellunderstood spin relaxation phenomenon occurring at the wall boundaries. (2)The superfluidnormal fluid counterflow propagating mode in A1 phase is accompanied by oscillations in both entropy and spin densities. The large âspin stiffnessâ makes the velocity of this SEW is much greater that it would be for âentropy stiffnessâ alone. The measured SEW velocity gives the superfluid density directly. Anisotropy in the superfluid density may be studied with SEW. The liquidcrystallike property of anisotropy âtextureâ may be probed and its kinetic anisotropy âtexture phase transitionâ has been observed.

Condensed Matter Seminar Thursday, October 13, 2005 4:00 PM Physics Building, Room 204 
ABSTRACT:
We demonstrate spatial separation of carriers with opposite spin
orientations in a nonmagnetic semiconductor. An ability to
manipulate spin of charge carries in a controllable fashion is
central to the rapidly developing field of spintronics, as well as
for the development of spinbased devices for quantum information
processing. However, creation of spinpolarized currents is proven
to be a formidable challenge and, previously, required either
injection from magnetic materials or application of strong Zeeman
magnetic field. We show that in a nonmagnetic semiconductor with
spinorbit interactions spins can be spatially separated in a
âspin spectrometerâ, utilizing difference in momenta and, thus,
cyclotron radii, for two spin polarizations. For holes in GaAs
almost 100% bipolar spin filtering has been achieved in magnetic
focusing geometry with spatial separation of polarized beams by
0.2 microns. We confirmed spin polarization of the injected
currents by applying strong Zeeman field and using point contacts
as spin filters. Spinorbit interaction constant has been measured
directly in these experiments. The new technique of spin
injection/detection opens a possibility to investigate density and
electric field dependence of spinorbit interactions, spin
dynamics at a few tenths of picoseconds without RF fields, and
shed light on such outstanding problems as â0.7 anomalyâ in
quantum point contacts by measuring spin polarization of charge
carriers.

Condensed Matter Seminar Thursday, September 29, 2005 4:00 PM Physics Building, Room 204 
"CurrentInduced Spin Polarization of 2DEG in Perpendicular Magnetic Field"Maxim Vavilov , Yale University [Host: Jongsoo Yoon]
ABSTRACT:
An electric field applied to a twodimensional electron gas with
spinorbit coupling generates spin magnetization. Based on a quantum
kinetic equation, we study the dependence of the magnitude and direction
of the spin magnetization on the strength of a perpendicular magnetic
field. We show that as magnetic field increases, the magnitude of the
magnetization decreases. However, in sufficiently strong magnetic
fields, the magnetization acquires a giant oscillating component; the
amplitude of this component may exceed the magnetization at zero
magnetic field. The large amplitude of magnetization oscillations is
related to the enhanced electronhole asymmetry in quantizing magnetic
fields.

Condensed Matter Seminar Thursday, September 22, 2005 4:00 PM Physics Building, Room 204 
"Novel, Collective Insulating Phase in 2D Superconductors in High Magnetic Fields"Sambandamurhy Ganapathy , Magnet Lab, Tallahassee and Princeton University [Host: Jongsoo Yoon]
ABSTRACT:
Superconductorinsulator transition (SIT) in 2D films has been an
active area in condensed matter physics research. The reason is the
continuing theoretical focus to understand zero temperature quantum
phase transitions (QPT) and the relative experimental ease with which
QPT can be tuned successfully in 2D films.
We present results from our study of the magnetic field (B)tuned SIT
in amorphous indium oxide films. We present three experimental
evidences that suggest that a novel, collective insulating state
emerges in the Binduced insulating phase. We argue that our results
lend support to the central assumption of the QPT approach.

Condensed Matter Seminar Thursday, August 25, 2005 4:00 PM Physics Building, Room 204 
"Novel Optical Studies of Ionerosion, Growth, and Diffusion on Metal Surfaces**"Petros Thomas , University of California at Davis [Host: George Hess]
ABSTRACT:
The novel optical techniques of obliqueincidence reflectivity
difference (OIRD) and linear optical diffraction (LOD) are applied to
several distinct surface physics problems. Studies of Ar and Neion
sputtering and thermal annealing of Nb(110) and Cu(111) using OIRD will
be discussed. The evolution of the average slope of the surface morphology
of a moderately stepped Ni(111) surface under ion erosion will be
presented. OIRD studies of the adsorption and desorption of atomic
hydrogen and deuterium on Nb(110) and Cu(111) will also be reported."
**If time allows, the following topic will also be discussed:
"The adsorption, growth and diffusion studies of Xe on Nb(110)".

Condensed Matter Seminar Thursday, August 18, 2005 3:30 PM Physics Building, Room 204 
"Unusual Magnetic State in MnSi under Hydrostatic Pressure"Dmitry Reznik , Institute of Solid State Physics, Forschungszentrum Karlsruhe [Host: Seunghun Lee] 
Condensed Matter Seminar Thursday, July 7, 2005 11:00 AM Physics Building, Room 204 
"Topological Defects and Fractionally Quantized Vortices in Nanomagnets"Oleg Tchernyshyov , John Hopkins [Host: Paul Fendley] 
Condensed Matter Seminar Thursday, May 26, 2005 11:00 AM Physics Building, Room 313 Informal CM Seminar 
"Control of Electric Polarization by Using Magnetic Field in Magnetic Oxides with LongWavelength Magnetic Structures"Tsuyoshi Kimura , Los Alamos National Laboratory [Host: SeungHun Lee]
ABSTRACT:
Recent observations of gigantic magnetoelectric and magnetocapacitive effects in rareearth manganites, TbMnO3 and DyMnO3 [1,2], provide a
novel approach to the mutual control of magnetization and electric polarization in magnetic ferroelectrics. We can control the magnitude and/or direction of the electric polarization vector by the application
of magnetic field in these manganites. In comparing the results from the both manganites, we noticed that a characteristic common to the both
materials is that they possess modulated magnetic structures with long wavelengths (as compared to the chemical unit cell) which arise from
competing magnetic interactions. Ferroelectricity in these materials appears to originate from the competing magnetic interactions which cause lattice modulations through magnetoelastic coupling. We extend our study to other modulated magnets with competing magnetic interactions.
In this talk, I show magnetic control of electric polarization in several modulated magnets [3], which may provide new route to design magnetoelectrics.
[1] T. Kimura, T.Goto, H. Shintani, K. Ishizaka, T. Arima, and Y.
Tokura, Nature 426, 55 (2003).
[2] T. Goto, T. Kimura, G. Lawes, A. P. Ramirez, and Y. Tokura, Phys.
Rev. Lett. 92, 257201 (2004).
[3] T. Kimura, G. Lawes, and A. P. Ramirez, Phys. Rev. Lett. (in press).

Condensed Matter Seminar Thursday, May 5, 2005 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, April 28, 2005 4:00 PM Physics Building, Room 204 
"Optimization of Complex MNiSn for HighTemperature Thermoelectric Power Generation"Slade Culp , UVA 
Condensed Matter Seminar Thursday, April 14, 2005 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, April 7, 2005 4:00 PM Physics Building, Room 204 Joint CM/Atomic Seminar 
"Antiferromagnetic Order as a Competing Ground State in Electrondoped High Tc Superconductors"Pengcheng Dai , University of Tennessee/Oak Ridge National Laboratory [Host: SeungHun Lee]
ABSTRACT:
In this talk, I will summarize recent progress on using neutron scattering to study electrondoped highTc superconductors. As a function of increasing doping, the asgrown antiferromagnetic (AF) insulator Pr0.88LaCe0.12CuO4 (PLCCO) can be transformed into a Tc=25 K superconductor without AF order by annealing at different temperatures. We find that underdoped PLCCO's are electronically phase separated with a three dimensional AF phase, a quasitwodimensional (2D) spin density wave, and a superconducting phase. An applied external field that suppresses superconductivity also enhances the quasi2D spin density wave order, thus suggesting AF order is a competing phase for superconductivity in these materials.

Condensed Matter Seminar Thursday, March 31, 2005 4:00 PM Physics Building, Room 204 
"How to Relax Frustrated Systems: Spin and Orbital Physics in V and Cr Spinels"Yuki Motome , RIKEN [Host: SeungHun Lee ]
ABSTRACT:
Geometrical frustration gives rise to a large number of degenerate ground states and suppresses a simpleminded longrange ordering in general. Here we discuss theoretically how the degeneracy is lifted in threedimensionally frustrated pyrochlore systems whose lattice structure consists of cornersharing tetrahedra. The pyrochlore systems are most typically found in the socalled B spinel oxides. Focusing on the spinels
with B=V and Cr, we explore the lowtemperature physics by the meanfield argument and Monte Carlo simulation for effective spinorbitallattice coupled models.
For V spinels, we clarify the strong interplay between spin and orbital degrees of freedom.
Orbital ordering reduces the magnetic frustration partially and realizes weakly coupled
onedimensional chains in the threedimensional pyrochlore lattice. For Cr spinels,
we show that the system exhibits novel phase transitions under the external magnetic
field due to the spinPeierls type mechanism. An emergence of a halfmagnetization
plateau is discussed in detail.

Condensed Matter Seminar Thursday, March 17, 2005 4:00 PM Physics Building, Room 204 
"DipoleDipole Excitation and Ionization in an Initially Frozen Rydberg Gas"Wenhui Li , UVA [Host: Tom Gallagher]
ABSTRACT:
In cold dense Rydberg atom samples, the dipoledipole interaction strength is effectively resonant at the typical interatomic spacing in the sample, and the interaction has a 1/R^{3} dependence on interatomic spacing R. The dipoledipole attraction leads to ionizing collisions of initially stationary atoms, which produces hot atoms and ions and initiates the evolution of initially cold samples of neutral Rydberg atoms into plasmas. More generally, the strong dipoledipole forces lead to motion, which must be considered in proposed applications.

Condensed Matter Seminar Thursday, March 3, 2005 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, February 24, 2005 4:00 PM Physics Building, Room 204 
ABSTRACT:
In quantum physics in 2+1 dimensions, particles need not behave as a boson or fermion. With nonabelian statistics, the wave function
depends not only on the particles being exchanged, but also on the order in which they are
exchanged. Such behavior probably occurs in the fractional quantum Hall effect. I discuss ways of building models with this behavior, and note the connection to quantum computers.

Condensed Matter Seminar Thursday, February 17, 2005 4:00 PM Physics Building, Room 204 
"Vortex Dynamics and Fluctuations Near the Magnetic Field Tuned SuperconductorInsulator Transition"Victor Galitski , Santa Barbara [Host: Paul Fendley]
ABSTRACT:
Superconductivity in two dimensions provides a unique area in which a fascinating variety of novel and fundamental phenomena occur. In this
talk, I will review recent theoretical and experimental results on disordered films, which undergo a magnetic field tuned superconductinginsulator transition at low temperatures. I will focus on the unusual phases and fluctuation phenomena evident in the experimental studies of the fieldtuned transition. First, I will explain how rare disorder fluctuations can enhance global
superconductivity and increase the critical magnetic field at which samples become superconducting. Next, I will briefly summarize the recently developed theory of quantum superconducting fluctuations, which explains transport properties above the transition. At the end of my talk, I will focus on the lowtemperature metallic phase observed in certain materials. This metallic state is truly mysterious
and can not be explained by any conventional theory (involving bosonic vortices as basic excitations). I will argue that under certain
circumstances the statistics of the vortices can change from bosonic to fermionic. Such a statistical transmutation may explain the nature
of the metallic state. I will discuss possible experimental signatures of the resulting vortex Fermi liquid.

Condensed Matter Seminar Thursday, February 10, 2005 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, February 3, 2005 4:00 PM Physics Building, Room 204 
"SuperfluidInsulator Transition in a Moving System of Interacting Bosons"Anatoli Polkovnikov , Harvard University [Host: Paul Fendley]
ABSTRACT:
Cold atomic systems with their high tunability and nearly perfect isolation from environment became very attractive for studying strongly correlated systems. Particularly exciting is the possibility to address problems of quantum dynamics far from
equilibrium, which are beyond the reach of conventional condensed matter systems. Most of theoretical studies of dynamics in these systems
employ essentially classical (GrossPitaevskii) equations of motion. At the same time in quilibrium a lot is known about various quantum
regimes, where the classical description does not work. In this talk I will discuss a problem where both quantum and dynamic effects are important. In particular, I will describe a moving system of interacting bosons in a periodic optical lattice
potential and generalize the conventional superfluidMott insulator transition to this
highly nonequilibrium situation. I will discuss implications of our results to recent and future
experiments.

Condensed Matter Seminar Thursday, January 27, 2005 4:00 PM Physics Building, Room 204 
"Nonequilibrium Transport Through a Phonon Coupled Molecule"Aditi Mitra , Columbia University [Host: Paul Fendley]
ABSTRACT:
The theory of nonequilibrium manybody systems is only partially understood, and developing the theory further is of relevance now as nanostructures (such as single molecule and NEMS devices) are routinely operated in the nonequilibrium regime. In this talk I will present a comprehensive theory for nonlinear transport through a single molecule device where the tunneling electrons are coupled to a vibrational mode of the molecule. I will present results for the current and the noise through
the device and show their sensitivity to phonon equilibration rates and hence to the steady state phonon distribution function. I will also outline a method to tackle the regime of low temperatures and strong electronphonon coupling where nonequilibrium conditions do not allow for
a variational treatment. Instead the key issue is the formulation of rate equations for the reduced density matrix that can be solved in the
semiclassical limit. The steadystate solutions lead to a multipeaked density matrix, implying an absence of bistability in the current but
structure in the noise. I will also demonstrate how departures from equilibrium produce decoherence that prevents the formation of
characteristically quantum features such as the polaron peak in the spectral function, and present generalizations of this method to other strongly correlated systems such as the nonequilibrium Kondo model.

Condensed Matter Seminar Thursday, January 20, 2005 4:00 PM Physics Building, Room 204 
"Continuous Tuning of 2DSuperconductivity in Strontium Titanate Field Effect Transistors"Feng Pan , University of Colorado [Host: Jongsoo Yoon]
ABSTRACT:
Understanding the doping mechanisms in metaloxide superconductors is an important part of probing the superconductor theory. Modulation of the carrier concentration using electric fields to tune superconductivity is a good method for this approach for its ability to minimize changes in both chemical impurities and structure. This talk explores the electric field modulation of normal state properties and superconductivity in thinfilm fieldeffect transistors based on lanthanumdoped strontium titanate channels and undoped strontium titanate gate insulation. Electric field tuning of normalstate sheet resistance and channel critical current are observed in enhancement (positive gate potential) and depletion (negative gate potential) modes, consistent with an electric field induced variation of channel depletion width. Detailed analysis of the nonlinear channel currentvoltage characteristics is consistent with electric field modulation of a 2dimensional KosterlitzThouless transition and shows the scaling of T_KT (KosterlitzThouless transition temperature) with conduction channel thickness and normal conductance at different gate fields. Also, small signal modulation experiments were performed on these devices. Modulation of critical current, sheet resistance, and T_KT is found for frequencies out to at least 1 kHz, which indicates possible exciting superconductor electronics.

Condensed Matter Seminar Thursday, December 9, 2004 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, November 18, 2004 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Tuesday, November 16, 2004 3:30 PM Physics Building, Room 204 
"Magnetic, Transport, and Thermodynamic Studies of Weakly Magnetic Systems of Transition Metal Oxides"Zhixian Zhou , FSU [Host: Jongsoo Yoon]
ABSTRACT:
LnBaCo2O5.5 (Ln=Gd, Eu) and Sr3Ru2O7 are examples of weakly magnetic systems of 3d
and 4d transition metal oxides, respectively. The former is nonmetallic and exhibits magnetic
properties of two sublattice magnetic systems with an inplane ferromagnetic interaction and a
relatively weak temperature dependent interplane magnetic coupling. The latter is a paramagnetic
metal with strongly correlated electrons near a magnetic instability.
The magnetization, resistivity and magnetoresistance (MR) of single crystals of
GdBaCo2O5.5 and EuBaCo2O5.5 are measured over a wide range of dc magnetic fields (up to 30 T)
and temperature. The data suggest an equal ratio of low spin (S=0) and intermediate spin (S=1)
Co3+ ions below TMI, with no indication of additional spin state transitions. The low field
magnetization shows a transition to a highly anisotropic ferromagnetic phase, followed by another
magnetic transition to an antiferromagnetic phase at a slightly lower temperature. Significant
anisotropy between the ab plane and c axis was observed in magnetic and magnetotransport
properties for both compounds.
The specific heat and electrical resistivity of Sr3Ru2O7 single crystals are measured in
several magnetic fields applied along the caxis for temperatures below 2 K and at fields up to 17
T. Near the critical metamagnetic field at B1
*~7.8 T, the electronic specific heat divided by
temperature increases logarithmically as the temperature decreases, over a large range of T, before
saturating below a certain T* (which is sample dependent). This crossover from a nonFermi
Liquid to a Fermi Liquid state is also observed in the resistivity data near the critical metamagnetic
field for I  c and B  c. At the lowest temperatures, a Schottkylike upturn with decreasing
temperature is observed.

Condensed Matter Seminar Thursday, November 11, 2004 4:00 PM Physics Building, Room 204 
"The Development and Analysis of Texture in Thin Metallic Films from Electrodeposition and Sputtering"Erik Svedberg , Seagate [Host: Bellave Shivaram] 
Condensed Matter Seminar Thursday, September 30, 2004 4:00 PM Physics Building, Room 204 Joint Materials Science and 
"Center for Spins in Nanotechnology CeSpIN a proposal for a new interdisciplinary center at UVa"Stuart Wolf , DARPA [Host: William A. Jesser]
ABSTRACT:
In this talk I will discuss the details of a proposed new center at UVa that will explore how electron spin can be utilized in electronics that will transcend Moore's law. There are two parallel paths that this center will explore. The first is the development of spin devices that will be nanoscale extensions of the spin transport electronics that will be introduced in the very near term as nonvolatile random access memory and magnetic sensors. The second path is the exploration of spin coherent effects that will directly impact the development of a quantum information processor. There are challenges in both paths but the key to a new paradigm of electronics will be the development of a single technology that can provide both conventional logic and memory as well as quantum logic and memory and spins seem to fit the bill.

Condensed Matter Seminar Thursday, July 29, 2004 4:00 PM Physics Building, Room 204 
"Future Nanoelectronics: Materials Science and Physics"John R. Tucker , University of Illinois [Host: William A. Jesser]
ABSTRACT:
A new nanoelectronics must be developed over the next two decades to sustain present rates of progress. The general outlines for what that technology should do are beginning to come into focus, but as yet there is no consensus on how to proceed toward these goals. In this talk, I will summarize research on nanometer MOS transistors and atomscale silicon devices by my colleagues and myself over the last ten years. Our longterm strategy envisions increasingly higher levels of synergy between materials science and quantum physics in developing this new technology.

Condensed Matter Seminar Thursday, July 22, 2004 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Wednesday, May 19, 2004 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, April 22, 2004 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, April 15, 2004 4:00 PM Physics Building, Room 204 

Condensed Matter Seminar Thursday, April 8, 2004 4:00 PM Physics Building, Room 204 
"Spintronics: Fundamentals and Applications"Igor Zutic , University of Maryland [Host: Olivier Pfister]
ABSTRACT:
Spintronics is an interdisciplinary field in which the central idea
is the manipulation of spin degrees of freedom in solid state systems.
The motivation to examine spintronics ranges from fundamental
studies, where the changes of the spin degrees of freedom can be a
sensitive probe for basic physical phenomena, to applications that
are neither feasible nor effective with conventional electronics.
This talk will focus on two examples: (1) spinpolarized transport in
hybrid structures containing superconductors and (2) a proposal
for magnetic pn junctions. Our prediction that a superconducting response
can be used to probe a novel class of ferromagnetic semiconductors has
recently led to the first direct measurement of the spin polarization
in these materials. In the second example, we develop a theory of
inhomogeneously doped semiconductors. We predict the spinvoltaic
effect, a spinanalogue of the photovoltaic effect. We show that
the direction of the charge current (which can even flow at no applied
bias) can be switched by the reversal of an equilibrium magnetization
or of a polarization of the injected spin. The same spinvoltaic effect
can be used to develop a novel class of tunable magnetic transistors.

Condensed Matter Seminar Thursday, March 4, 2004 4:00 PM Physics Building, Room 204 
"Nanoscopic Transport in the Ultrathin Metal Films and Molecular Organic Crystals"Vladimir Butko , Los Alamos [Host: Jongsoo Yoon]
ABSTRACT:
A description of the properties of low dimensional electron gases remains
one of the most important and challenging problems in condensed matter
physics. I will present electron transport and tunneling measurements on
ultrathin, metal films. I will show that in the strong disorder limit a
gap emerges in the density of states that is solely attributable to
fundamental many body electronelectron interaction effects, i.e. the
Coulomb gap. Interestingly, a quantum metal state can be realized, in
otherwise highly insulating films, by suppressing the Coulomb gap via a
magnetic field. In the second part of my talk I will describe ongoing
research into the properties of the two dimensional electron gas of Field
Effect Transistors (FETs) fabricated on high quality organic molecular
single crystals. Practical applications of the molecular organic materials,
such as flexible, largearea electronic devices will also be discussed.

Condensed Matter Seminar Thursday, February 26, 2004 4:00 PM Physics Building, Room 204 
ABSTRACT:
Electronelectron interactions in lowdimensional systems have
attracted a great deal of attention in recent years. We show that
arrays of large, strongly coupled quantum dots present an analytically
tractable, yet nontrivial model of such systems.
A single dot strongly coupled to leads exhibits almost no Coulomb
blockade (save for corrections that are exponentially small in the
dotlead conductance). In the array geometry with large intergrain
conductance g>>1, however, the interactions drive the system into
an insulating state with a charge gap proportional to exp(g).
The latter reflects the energy cost to create a largesize,
unitcharge soliton  the only charged excitation the system
supports. In 2d, such solitons bring about a
BerezinskiiKosterlitzThouless crossover at a certain (gdependent)
critical temperature.
Upon changing the charge imbalance (e.g. by a gate voltage), the array
undergoes a phase transition into the pinned Wigner crystal state. The
model, thus, allows one to follow the system from a good metal (at
high temperature) all the way to the Wigner crystal insulator (at low
temperature) within a single framework.

Condensed Matter Seminar Monday, February 23, 2004 4:00 PM Physics Building, Room 204 Special Condensed Matter Seminar. 
"Fluctuations in electronic, spintronic and photonic systems"Eugene Mishchenko , Harvard University [Host: Paul Fendley]
ABSTRACT:
I will present both the experimental results and the theory
for the nonequilibrium fluctuations of electric current in mesoscopic
systems (diffusive wires and chaotic quantum dots). Noninteracting
electrons exhibit Poissonian statistics while correlations in mesoscopic
systems lead to the universality of current fluctuations. I will
consider the effects of spinpolarized transport on the noise and
magnetoresistance of spintronic systems. Finally, the electronoptical
analogies will be discussed in relation to the fluctuations of radiation
from chaotic absorbing ('grey body') and amplifying ('random laser')
optical media.

Condensed Matter Seminar Thursday, February 19, 2004 4:00 PM Physics Building, Room 204 
"Multiwalled Carbon Nanotubebased electromechanical oscillators"Stergios Papadakis , UNC [Host: Yongsoon Yoon]
ABSTRACT:
I will describe our fabrication and characterization of
torsional oscillators which use multiwalled carbon nanotubes (MWNTs)
as the torsion springs. We have, through direct force measurements with
a scanning probe microscope, measured the torsional properties of
individual MWNTs. We discovered a surprising increase in the torsional
stiffness of the MWNTs with repeated deflections. We have also
actuated the devices electrostatically, demonstrating
a selfcontained electromechanical device.
We use optical interferometry to measure the onresonance
behavior of the electrostaticallydriven devices.

Condensed Matter Seminar Thursday, February 12, 2004 4:00 PM Physics Building, Room 204 
"Quantum Computation with Josephson Devices"Dr. Yang Yu , The Research Laboratory of Electronics at MIT [Host: Joseph Poon]
ABSTRACT:
Quantum computers offer exponential speedup over classical computers in solving certain tasks. Superconducting Josephson devices are promising candidates in realizing qubits for quantum computer due to the ease of scaling up. The main challenge of superconducting qubits is decoherence, arising from the coupling between the superconducting qubits and the environment. Here we directly measure the intrawell energy relaxation time au_d between macroscopic quantum levels in the double well potential of a Nb superconducting qubit. The qubit's decoherence time, estimated from au_d, is longer than 20 micro s, indicating a strong potential for quantum computing employing Nbbased superconducting qubits.

Condensed Matter Seminar Wednesday, February 11, 2004 4:00 PM Physics Building, Room 204 Special Condensed Matter Seminar. Please note special time and day 
"NonAbelian Anyons and Topological Order in Solids"Kirill Shtengel , Microsoft [Host: Paul Fendley]
ABSTRACT:
A concept of topological order originally introduced by Wen in the context of Fractional Quantum Hall Effect has drawn much attention from people working in the fields of HTSC, frustrated magnetism and quantum computation. Topological order is manifested by a nontrivial degeneracy of the ground state on 2D surfaces of nonzero genus (such as a torus) and nontrivial mutual statistics of excitations. E.g., a Z_2 topological order in a magnetic system should lead to spincharge separation  one of the interesting (yet unlikely) possible mechanisms for HTSC. From the point of view of quantum computation, one of the biggest challenges is making it faulttolerant. We hope to use topological properties to encode quantum information in a way that is highly resistant to decoherence. So far one (and only) type of systems where topological order is known to exist are systems with Fractional Quantum Hall Effect. After reviewing the current state of search for topological phases in condensed matter, I will discuss models with nonAbelian topological order. In particular, I will present a version of extended Hubbard model whose lowenergy physics can lead to a phase with such nonAbelian order. General arguments for stability of these exotic phases as well as some new ideas about physical implementation of such systems will be presented. If found experimentally, these systems will provide a basis for building a truly faulttolerant quantum computer.

Condensed Matter Seminar Tuesday, February 10, 2004 4:00 PM Physics Building, Room 204 Special Condensed Matter Seminar. Please note special day and time. 
"Nernst effect: from simple to correlated metals"Vadim Oganesyan , Princeton University [Host: Paul Fendley]
ABSTRACT:
Since the discovery of large Nernst effect in the pseudogap of
holedoped cuprates this transport coefficient is becoming a new tool
with which to probe correlated electronic behavior. I will first
survey the recent experimental data on materials as varied as
superconductors, charge and spin density waves, heavy Fermi liquids
and ferromagnets. In some cases I will suggest possible common origins
of such behavior and support them with calculations.

Condensed Matter Seminar Thursday, February 5, 2004 4:00 PM Physics Building, Room 204 
"Anisotropic Quantum Correlations in Nanostructures"Igor Altfeder , Harvard University [Host: Joe Poon]
ABSTRACT:
Recent advances in scanning tunneling microscopy (STM) have shown that a variety of exotic related phenomena, involving spins and anisotropic interactions, can be realized in atomically flat (MBE grown) thin metal films due to a nontrivial interplay of structural and electronic selforganization. The two of these phenomena will be discussed in my talk: anisotropic Kondolike "confinement" in metallic quantum wells, and unidirectional charge ordering (CDW stripes) on metal surfaces. In all studied cases, we observe the formation of wirelike objects, where the longrange electronic coherence is spontaneously confined to a single degree of freedom.

Condensed Matter Seminar Wednesday, February 4, 2004 3:30 PM Physics Building, Room 204 
"Playing with Superconductivity and Magnetism in Nanoscale"Jiyeong Gu , Argonne National Laboratory [Host: Jongsoo Yoon]
ABSTRACT:
Recent technological advances made it possible to create hybrid
nanostructures with high quality ferromagnet/superconductor (F/S)
interfaces. The F/S systems have been in focus of intensive experimental and
theoretical studies because of both exciting fundamental problems and
potential device applications. The physical properties of both F and S films
will be strongly modified near the interface due to mutual proximity effect,
for example, superconducting correlations penetrate into F and the spin
polarization can extend into S. Main questions to be addressed in this talk
will be the type and length scale of the induced superconducting
correlations and spin relaxation length at the interface. Also, first
experimental observation of superconducting switching effect depending on
the mutual magnetization directions of Flayers in F/S/F trilayer system
will be discussed.
