# Condensed Matter Seminars History

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/97001220790

**Meeting ID:** 970 0122 0790

**Password:** 498831

**Meeting ID:**

**Password:**Thursday, April 29, 2021

3:15 PM

Online, Room via Zoom

## "Towards Room Temperature Superconductivity in Hydride-Based Materials Under Pressure"

**Dr. Eva Zurek , SUNY Buffalo**

[Host: Bellave Shivaram]

The pressure variable opens the door towards the synthesis of materials with unique properties, e.g. superconductivity, hydrogen storage media, high-energy 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.

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/95253668001

**Meeting ID:** 952 5366 8001

**Password:** 656540

**Meeting ID:**952 5366 8001

**Password:**Thursday, April 22, 2021

3:30 PM

Online, Room via Zoom

## "The anomalous thermal relaxations in linear chemical reactions"

**Saikat Bera , University of Virginia - Department of Physics**

[Host: Marija Vucelja]

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 non-equilibrium 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.

**Special Condensed Matter Seminar**

**Join Zoom Meeting:****Meeting ID:** 260 917 9512

**Password:** No Password Required

**Meeting ID:**

**Password:**Tuesday, April 20, 2021

3:30 PM

Online, Room via Zoom

## "Gutzwiller Quantum Molecular Dynamics Simulation in Liquid"

**Chen Cheng , University of Virginia - Department of Physics**

[Host: Gia-Wei Chern]

The Gutzwiller approximation is a method for strongly-correlated systems, it is the simplest theory that successfully captures the correlated induced metal-insulator transition, i.e. mott transition. Density function theory (DFT) is a very efficient method to deal with many-electron 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 metal-insulator 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.

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/8377396898

**Meeting ID:** N/A

**Password:** No Password Required

**Meeting ID:**

**Password:**Thursday, April 15, 2021

3:30 PM

Online, Room via Zoom

## "Machine Learning Enable the Large Scale Kinetic Monte Carlo for Falicov-Kimball Model"

**Sheng Zhang , University of Virginia - Department of Physics**

[Host: Gia-Wei Chern]

The Falicov-Kimball (FK) model was initially introduced as a statistical model for metal-insulator 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 time-step of dynamical simulations of the FK model, is very time-consuming. Here we apply the modern machine learning (ML) technique to enable the first-ever large-scale kinetic Monte Carlo (kMC) simulations of FK model. Using our neural-network model on a system of unprecedented 10^{5} lattice sites, we uncover an intriguing hidden sub-lattice symmetry breaking in the phase separation dynamics of FK model.

**Join Zoom Meeting:**##### https://virginia.zoom.us/my/israel.klich

**Meeting ID:** N/A

**Password:** No Password Required

**Meeting ID:**

**Password:**Thursday, April 8, 2021

2:30 PM

via Zoom, Room Online

## "A time-dependent approach to inelastic scattering spectroscopies in and away from equilibrium: beyond perturbation theory"

**Adrian Feiguin , Northeastern University**

[Host: Israel Klich ]

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 time-dependent 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 X-ray scattering (RIXS). To circumvent these hurdles, we introduce a time-dependent 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 time-dependent Schrödinger equation. The spectrum is recovered by measuring the momentum and energy lost by the scattered particles, akin an actual energy-loss experiment. The method can be used to study transient dynamics and spectral signatures of correlation-driven non-equilibrium processes, as I illustrate with several examples and experimental proposals using the time-dependent 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.

**RESERVED**

**Join Zoom Meeting:**##### https://virginia.zoom.us/j/92170693950

**Meeting ID:** 921 7069 3950

**Password:** 942161

**Meeting ID:**

**Password:**Thursday, March 25, 2021

3:30 PM

Online, Room via Zoom

## "Quantum Wakes and Measurement Induced Chirality"

**Matthew Wampler , University of Virginia - Department of Physics**

[Host: Israel Klich]

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 half-filling 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 so-called 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.

**Available**

**Join Zoom Meeting:**

##### https://virginia.zoom.us/j/95308132506

**Meeting ID:** 953 0813 2506

**Password: ** 253352

**Meeting ID:**

**Password:**

Thursday, March 11, 2021

3:30 PM

Online, Room via Zoom

## "Ferrimagnetic materials for room temperature small skyrmions"

**Wei Zhou , University of Virginia - Department of Physics**

[Host: Joe Poon]

The magnetic skyrmions are topologically protected spin configuration, which stabilized by Dzyaloshinskii-Moriya interaction (DMI). Due to skyrmions’ ability to be small, stable, and controllable by electric current [1], they have considerable potential for high-density data storage applications. It is theoretically predicted that ferrimagnetic materials prefer holding small skyrmions at room temperature (RT) [2,3]. 10-15nm ferrimagnetic CoGd heterostructures and 10-15nm 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.

**Available**

**Condensed Matter**

Thursday, February 25, 2021

3:30 PM

Physics Building, Room TBA

**RESERVED**

**Available**

**Available**

**Available**

**Condensed Matter**

Thursday, November 19, 2020

3:30 PM

Physics Building, Room TBA

**RESERVED**

**Condensed Matter**

Thursday, November 12, 2020

3:30 PM

Physics Building, Room TBA

**Available**

**Join Zoom Meeting:**

https://virginia.zoom.us/my/israel.klich

Thursday, November 5, 2020

9:30 AM

Online, Room via Zoom

## "Probing the Universality of Topological Defect Formation in a Quantum Annealer"

**Adolfo del Campo , Ikerbasque & DIPC**

[Host: Israel Klich]

The number of topological defects created in a system driven through a quantum phase transition exhibits a power-law scaling with the driving time. This universal scaling law is the key prediction of the Kibble-Zurek mechanism (KZM), and testing it using a hardware-based 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

formation via the one-dimensional transverse-field Ising model on two

different D-Wave quantum annealing devices. We find that the quantum simulator results can indeed be explained by the KZM for open-system quantum dynamics with phase-flip errors, with certain quantitative deviations from the theory likely caused by factors such as random control errors and transient effects. In addition, we probe physics beyond the KZM by identifying signatures of universality in the distribution and cumulants of the number of kinks and their decay, and again find agreement with the quantum simulator results. This implies that the theoretical predictions of the generalized KZM theory, which assumes isolation from the environment, applies beyond its original scope to an open system.

We support this result by extensive numerical computations. To check whether an alternative, classical interpretation of these results is possible, we used the spin-vector Monte Carlo model, a candidate classical description of the D-Wave device. We find that the degree of agreement with the experimental data from the D-Wave annealing devices is better for the KZM, a quantum theory, than for the classical spin-vector Monte Carlo model, thus favoring a quantum description of the device. Our work provides an experimental test of quantum critical dynamics in an open quantum system, and paves the way to new directions in quantum simulation experiments.

Ref.:

Yuki Bando, Yuki Susa, Hiroki Oshiyama, Naokazu Shibata, Masayuki

Ohzeki, Fernando Javier Gómez-Ruiz, Daniel A. Lidar, Sei Suzuki, Adolfo

del Campo, and Hidetoshi Nishimori, Phys. Rev. Research 2, 033369 (2020)

**Reserved - Please see HEP seminar schedule**

**Special Seminar**

**Join Zoom Meeting:**

##### https://virginia.zoom.us/j/92287909487

**Meeting ID:** 922 8790 9487

**Password: **HEPseminar

Wednesday, November 4, 2020

10:00 AM

Online, Room via Zoom

## "Crystal structure, rotational dynamics and vibrational dynamics of a two-dimensional perovskite"

**Xiao Hu , University of Virginia - Department of Physics**

[Host: Seung-Hun Lee]

Metal halide perovskites (MHPs) have come to the forefront in the photovoltaic and light-emitting 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 electron-phonon 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 three-dimensional (3D) perovskites. Compared with 3D perovskites, however, 2D perovskites exhibit a one-order-of-magnitude 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.

**Available**

**Available**

**Available**

**RESERVED**

**Available**

**Condensed Matter**

Thursday, September 24, 2020

3:30 PM

Physics Building, Room TBA

**Available**

**Condensed Matter**

Thursday, September 17, 2020

3:30 PM

Physics Building, Room TBA

**Available**

**Condensed Matter**

Thursday, September 10, 2020

3:30 PM

Physics Building, Room TBA

**RESERVED**

**Condensed Matter**

Thursday, September 3, 2020

3:30 PM

Physics Building, Room TBA

**Available**

**Available**

**Join Zoom Meeting:**#### https://virginia.zoom.us/j/96102363843

#### Meeting ID:

#### 961-0236-3843

#### Password: 32572

Thursday, April 16, 2020

3:30 PM

, Room via Zoom

## "Magnetic skyrmions and their applications"

**Hamed Vakili , University of Virginia - Department of Physics**

[Host: Avik Ghosh]

Skyrmions are topologically protected magnetic quasi-particles. 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, Anti-skyrmion). 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 non-destructive read-out.

**Available**

**Join Zoom Meeting:**#### Meeting ID:

#### https://virginia.zoom.us/j/775042517

#### Meeting ID:** **775-042-517

Tuesday, April 14, 2020

2:00 PM

, Room via Zoom

## "Atomic and Electronic Correlations in the Change Density Wave phase of Dichalcogenides"

**Sharon Philip , University of Virginia - Department of Physics**

[Host: Despina Louca]

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 in-plane re-orientation of Ta atoms to Star-of-David formation. But this alone doesn’t explain the notable electronic behavior of 1T-TaS_{2} and 1T-TaSe_{2} and why they differ from one another despite having the same trigonal symmetry. At very low temperatures, 1T-TaS_{2} undergoes a metal - insulator transition and is proposed to harbor a quantum spin liquid behavior whereas 1T-TaSe_{2} remains metallic. Investigating the local structure of pristine 1T-TaS_{2} and 1T-TaSe_{2 }in the CDW regime could tell us the differences in local atomic correlations in these compounds.

**Available**

**Webinar**### Join Zoom Meeting

### Join Zoom Meeting

https://virginia.zoom.us/j/5140654378 Meeting ID: 514 065 4378

Thursday, March 26, 2020

11:00 AM

Online, Room via Zoom (Zoom link and meeting ID provided above)

## "Investigation of the structural phase transitions in Weyl semimetal (Mo, W)Te2"

**Yu Tao , University of Virginia - Department of Physics**

[Host: Despina Louca]

Mo_{1-x}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_{1-x}W_{x}Te_{2} undergoes a structural phase transition from a high-temperature monoclinic 1T' phase, to a non-centrosymmetric orthorhombic T_{d} phase at low temperatures through a first-order structural phase transition. Both 1T' and T_{d }phases are comprised of weakly-bound layers, and differ mainly by shifts of the layers along the c-axis. Despite much research, the structural properties of Mo_{1-x}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_{1-x}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_{1-x}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.

**Available**

**RESERVED**

**Condensed Matter**

Thursday, February 27, 2020

3:30 PM

Physics Building, Room 204

## "Design principles of biological and chemical intelligence"

**Zhiyue Lu , University of North Carolina at Chapel Hill**

[Host: Marija Vucelja]

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 single-cell circadian clock (i.e. the Kai-ABC 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.

**Special Seminar**

Thursday, February 20, 2020

12:45 PM

Physics Building, Room 313

't Hooft anomalies provide a unique handle to study the nonperturbative dynamics of strongly-coupled 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**

Thursday, February 20, 2020

3:30 PM

Physics Building, Room 204

**RESERVED**

**Special Seminar**

Thursday, February 13, 2020

12:45 PM

Physics Building, Room 313

## "Exact results for 5-dimensional superconformal field theories"

**Christoph Uhlemann , University of Michigan**

[Host: Peter Arnold]

5-dimensional superconformal field theories play an intriguing role in the general understanding of quantum field theory. They provide strongly-coupled UV fixed points for many perturbatively non-renormalizable 5-dimensional gauge theories, and upon compactification they provide new insights into many lower-dimensional theories. This makes them both useful and interesting in their own right. In this talk I will discuss recent progress in the understanding of 5-dimensional superconformal field theories through AdS/CFT dualities and non-perturbative field theory methods, leading to interesting exact results and many cross checks.

**Condensed Matter**

Thursday, February 13, 2020

3:30 PM

Physics Building, Room 204

**Available**

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.

**RESERVED**

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 on-shell superspace Lagrangians directly from the symmetries of the theory. However, I will also demonstrate how the non-propagating 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 low-energy considerations, and has potential implications for supergravity, the Scattering Amplitudes program, and more.

**Reserved - Please see Atomic Seminar Schedule**

## "Duality between Space-like and Time-like Processes in Gauge Theory"

**Duff Neill , Los Alamos National Lab**

[Host: Peter Arnold]

Since the original papers of Drell-Levy-Yan, 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 in-elastic 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 space-time reciprocity relation (arXiv hep-th/0612247). I shall give an introduction to this space-time 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, up-to 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 time-like BFKL equation that resums the soft region of the fragmentation function of QCD, which maps to the space-like 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.

**Available**

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 quark-gluon plasma. It is an effective field theory formulated in terms of an infinite-order 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 level-crossings 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 quark-gluon plasma. In the second part of my talk, I will discuss a recently discovered phenomenon called “pole-skipping” that relates hydrodynamics to the underlying microscopic quantum many-body chaos. This new and special property of quantum correlation functions allows for a precise analytic connection between resummed, all-order hydrodynamics and the properties of quantum chaos (the Lyapunov exponent and the butterfly velocity).

**Available**

**Condensed Matter**

Thursday, December 5, 2019

11:00 AM

Physics Building, Room 313

## "Monopole Superconductivity and Density-Wave Order in Weyl Semi-metals"

**Professor Yi Li , Johns Hopkins University**

[Host: Dima Pesin]

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 density-wave orders: When the ordered pairs acquire non-trivial two-particle Berry phases, their pairing phases cannot be globally well-defined in the momentum space. Therefore, the conventional description of superconducting pairing symmetries in terms of spherical harmonics (e.g. s-, p-, d-waves) ceases to apply. Instead, they are characterized by topologically protected nodal gap functions represented by monopole harmonic functions. This so-called “monopole harmonic order” is expected to be realized and detected in Weyl semimetal materials.

**Condensed Matter**

Thursday, November 21, 2019

3:30 PM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, November 14, 2019

3:30 PM

Physics Building, Room 313

We obtain a new relation between the distributions μ_t at different times t ≥ 0 of the continuous-time TASEP (Totally Asymmetric Simple Exclusion Process) started from the step initial configuration. Namely, we present a continuous-time Markov process with local interactions and particle-dependent 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 discrete-space 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**

Thursday, November 7, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, October 31, 2019

11:00 AM

Physics Building, Room 313

**Reserved - Please see Atomic Seminar Schedule**

**Condensed Matter**

Thursday, October 24, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, October 17, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, October 10, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Available**

**Condensed Matter**

Thursday, September 26, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, September 19, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, September 12, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, September 5, 2019

11:00 AM

Physics Building, Room 313

**Available**

TBA

## "Two-dimensional magnetism and spintronics"

**Adam Wei Tsen , University of Waterloo**

[Host: Seunghun Lee]

The recent discoveries of ferromagnetism in single atomic layers have opened a new avenue for two-dimensional (2D) materials research. Not only do they raise fundamental questions regarding the requirements for long-range magnetic order in low-dimensional 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.

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.

## "Tunneling-induced restoration of classical degeneracy in quantum kagome ice"

**Professor Ying-Jer Kao , National Taiwan University**

[Host: Gia-Wei Chern]

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 off-diagonal and diagonal quantum processes leading to the quasi-degeneracy of states and effectively, the classical degeneracy is restored.

## "Exciton polarons in two-dimensional organic-inorganic hybrid perovskites"

**Professor Carlos Silva , Georgia Tech**

[Host: Seunghun Lee]

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. In this seminar, we demonstrate that there are non-negligible 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 counter-cation. In these systems, organic cations not only serve as spacers between slabs consisting of corner-sharing metal-halide octahedra, but also determine lattice structure by inducing varying degree of distortion of the octahedra via the organic-inorganic interactions. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by 35±5 meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys 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 high-resolution 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 low-frequency (≲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, excitation-induced dephasing, etc.) reflect the polar solvation-like 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.

Through string theory, people found interesting relations in particle theory. For example, Kawai-Lewellen-Tye (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 Yang-Mills 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".

## "Thermoelectric transport properties of topological Bi-Sb cryogenic materials"

**Xixiao Hu , University of Virginia - Physics**

[Host: Joe Poon]

Bi-Sb alloys have shown promising thermoelectric (TE) properties at cryogenic temperature (<200 K). Over six decades, the figure of merit *zT *of n-type polycrystalline Bi-Sb has plateaued at ~0.4, while its p-type counterpart has remained even lower at ~0.1. We have studied the TE properties of melt-spun and spark plasma sintered (SPS) Bi-Sb alloys. We obtained a *zT *of 0.55 @100-150 K for n-type undoped Bi_{85}Sb_{15 }based on a low thermal conductivity 1.5 W/(m*K) measured with the hot-disk method. For p-type Bi-Sb, 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 low-temperature SPS processing. The transport properties (resistivity and Seebeck coefficient) of n-type undoped and p-type doped Bi_{85}Sb_{15} were analyzed using the two-band effective mass model within the Boltzmann transport theory. A band gap decreasing phenomenon was observed which poses challenges to the improvement of p-type Bi-Sb’s *zT*.

## "Topological phases with two-fold spatial antiunitary symmetries"

**Meng Hua , University of Virginia - Physics**

[Host: Jeffrey Teo]

An interesting theme in topological materials has been classification and prediction of symmetry protected topological(SPT) phases. Despite the Altland-Zirnbauer(AZ) classification under time-reversal symmetry, particle-hole 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 two-fold spatial antiunitary symmetries. We also generalize SPT phases to non-Hermitian system with two-fold spatial antiunitary symmetries and give an example of dissipative topological superconductors.

## "The three-body problem: periodic solutions, topological classification"

**Milovan Suvakov , Institute of Physics Belgrade**

[Host: Marija Vucelja ]

The three-body problem dates back to the 1680s. Isaac Newton had

already shown that his law of gravity could always predict the orbit

of two bodies held together by gravity, such as a star and a planet,

with complete accuracy. The periodic two-body orbit is always an

ellipse (circle). For two centuries, scientists tried different tacks

to find similar solution for three-body problem, until the German

mathematician Heinrich Bruns pointed out that the search for a general

solution for the three-body problem was futile, and that only specific

solutions that work only under particular conditions, were possible.

Only three families of such collisionless periodic orbits were known

until recently: 1) the Lagrange-Euler (1772); 2) the Broucke-Henon

(1975); and 3) Cris Moore's (1993) periodic orbit of three bodies

moving on a "figure-8" trajectory. Few years ago we reported the

discovery of 13 new families of periodic orbits. Meanwhile, hundreds

of new topologically different solutions have been reported by our and

other groups. We discuss the numerical methods used to find orbits and

to distinguish them from others. Additionally, we found that period T

of an orbit depends on its topology. This dependence is a simple

linear one, when expressed in terms of appropriate variables,

suggesting an exact mathematical law. This is the first known relation

between topological and kinematical properties of three-body systems.

https://scholar.google.com/citations?hl=en&user=dEJ0ThoAAAAJ&view_op=list_works&sortby=pubdate

**Reserved - Please see the Nuclear Seminar Schedule**

[Host: Simonetta Liutu]

## "Magnetic Skyrmions on a racetrack"

**Hamed Vakili , University of Virginia - Physics**

[Host: Avik Ghosh]

Skyrmions are topologically protected magnetic quasi-particles. 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, Anti-skyrmion). 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.

**Reserved - Please see Atomic Seminar Schedule**

## "Spin dynamics in two distinct types of classical spin liquids "

**Preetha Saha , University of Virginia - Physics**

[Host: Gia-Wei Chern]

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 Kitaev-type 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 spin-orbit magnets where a dominant off-diagonal exchange, the so-called Γ term, results in a macroscopic ground-state 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 Landau-Lifshitz 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 SrCr9Ga12-9pO19 [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)

**Available**

**Special Presentation**

Thursday, April 4, 2019

11:00 AM

Physics Building, Room 313

## "Introduction to Rivanna"

**UVa's Advanced Research Computing Services , University of Virginia**

[Host: Bryan Wright]

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 7,000-core cluster with more than a petabyte of storage. It includes a subset of GPU-equipped nodes. Please drop in if you have any interest in high-performance computing.

Slides from the talk can be found here:

http://galileo.phys.virginia.edu/compfac/faq/IntroductionToRivanna.pdf

## "Quantum gas microscopy of many-body dynamics in Fermi-Hubbard and Ising systems"

**Peter Schauss , University of Virginia - Physics**

[Host: Dmytro Pesin]

The ability to probe and manipulate cold atoms in optical lattices at the atomic level using quantum gas microscopes enables quantitative studies of quantum many-body dynamics. While there are many well-developed theoretical tools to study many-body 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.

First, I will present a study that probes the relaxation of density modulations in the doped Fermi-Hubbard model. This leads to a hydrodynamic description that allows us to determine the conductivity. We observe bad metallic behavior that we compare to predictions from finite-temperature Lanczos calculations and dynamical mean field theory.

Second, I introduce a new platform to study the 2D quantum Ising model. Via optical coupling of atoms in an optical lattice to a low-lying Rydberg state, we observe quench dynamics in the resulting Ising model and prepare states with antiferromagnetic correlations.

## "Metal-Insulator Transition in Phase Change Material Ge2Sb2Se5xTe5-5x"

**Zhenyang Xu , University of Virginia - Physics**

[Host: Despina Louca]

Ge2Sb2Te5 (GST-225) is a phase change material which has wide use in fabricating random access memories. With different quenching process, the GST-225 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 GST-225 an ideal material for RAM with fast speed. In our project, the Se-doped GST-225 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 metal-insulator transition happen for both furnace cooled samples and liquid N2 quenched samples at this limit. A tentative hypothesis is proposed to explain this metal-insulator transition.

**Special Condensed Matter Seminar**

Wednesday, March 13, 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]

The study of low-dimensional 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 scale-invariant tensor network have eluded us so far. Here we show that the features of the quantum-critical 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 two-dimensional 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.

**Available**

**Condensed Matter**

Thursday, February 28, 2019

3:30 AM

Physics Building, Room 204

**Reserved - Joint Nuclear and High Energy Seminar**

**Condensed Matter**

Thursday, February 21, 2019

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, February 14, 2019

11:00 AM

Physics Building, Room 313

## "Thermodynamic properties of disordered unconventional superconductors with competing interactions"

**Maxim Dzero , Kent State**

[Host: Israel Klich]

A topic of interplay between disorder and competing electronic phases in multiband superconductors have recently got renewed interest in the context of iron-based 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 two-band model with quasi-two-dimensional 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 quasi-classical approximation I will present the analysis of the quasi-classical 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**

Thursday, February 7, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Reserved - Please see the Nuclear Seminar Schedule**

[Host: Kent Paschke]

**Condensed Matter**

Thursday, January 31, 2019

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, January 24, 2019

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, January 17, 2019

11:00 AM

Physics Building, Room 313

**Reserved - Please see the Colloquia Schedule**

[Host: Bob Jones]

**Condensed Matter**

Wednesday, January 16, 2019

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, December 6, 2018

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, November 29, 2018

11:00 AM

Physics Building, Room 313

**RESERVED**

**Condensed Matter**

Thursday, November 15, 2018

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, November 8, 2018

11:00 AM

Physics Building, Room 313

## "Fractionalized excitations towards a non-Abelian phase in a Kitaev honeycomb magnet"

**Arnab Banerjee , Oak Ridge National Laboratory**

[Host: Bellave Shivaram]

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 alpha-RuCl3 which has honeycomb layers held together with weak van-der-Waals interactions. A strong spin-orbit coupling and an octahedral crystal field makes the Kitaev interactions arguably the leading order term in the Hamiltonian. Prominently, despite a long-range 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 long-range order vanishes and the continuum becomes gapped, supporting a state where a direct evidence of non-Abelian 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**

Thursday, November 1, 2018

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, October 25, 2018

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, October 18, 2018

11:00 AM

Physics Building, Room 313

## "Competing orders in a quantum spin model with long-range interactions"

**Erhai Zhao , George Mason University**

[Host: Bellave Shivaram]

Quantum spin liquids evade long-range 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 many-spin systems based on interacting dipoles. I will argue that long-range 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**

Thursday, October 11, 2018

11:00 AM

Physics Building, Room 313

## "Transport in Strongly Correlated 2D Electron Fluids"

**Alex Levchenko , University of Wisconsin-Madison**

[Host: Dmytro Pesin]

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.

**Reserved for Special Colloquium**

**Condensed Matter**

Thursday, September 27, 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]

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 present-day 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**

Thursday, September 20, 2018

11:00 AM

Physics Building, Room 313

## ""A materials-driven approach to the novel topological states of matter""

**Nirmal Ghimire , George Mason University**

[Host: Bellave Shivaram]

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 [1-3] 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 chiral-lattice 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**

Thursday, September 13, 2018

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, September 6, 2018

11:00 AM

Physics Building, Room 313

**Special Condensed Matter Seminar**

Friday, August 31, 2018

3:30 PM

Physics Building, Room 204

## "The mother of all states of the kagome quantum antiferromagnet"

**Hitesh J. Changlani , Florida State University**

[Host: Bellave Shivaram]

Strongly correlated systems provide a fertile ground for discovering exotic states of matter, such as those with topologically non-trivial 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 many-body 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 near-ideal 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 spin-1 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 well-studied spin-1/2 case [2]. I explain the existence of an exactly solvable point in the XXZ-Heisenberg 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 "three-coloring" 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].

[1] H. J. Changlani, A.M. Lauchli, Phys. Rev. B 91, 100407(R) (2015).

[2] K. Kumar, H. J. Changlani, B. K. Clark, E. Fradkin, Phys. Rev. B 94, 134410 (2016).

[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.

## "Gravitational Weyl Anomalies in 1+1-Dimensional Lifshitz Field Theories"

**Amr Ahmadain , UVA-Department of Physics**

[Host: Israel Klich ]

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 non-relativistic (non-Lorentz-invariant) symmetry group. More specifically, I will focus on the only anomaly found in 1+1-dimensional Lifshitz field theories that arise after coupling to Newton-Cartan 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+1-dimensional Weyl-invariant Chern-Simons Horava-Lifshitz 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.

## "Magnetic-Field Induced Weyl Semimetal from Wannier-Function-based Tight-Binding Model"

**Kyungwha Park , Virginia Tech**

[Host: Jeffrey Teo]

Weyl semimetals (WSMs) have a three-dimensional (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 magneto-transport signatures. We develop a tight-binding model based on Wannier functions directly from density functional theory (DFT) calculations for a topological DSM Na3Bi. We add spin-orbit coupling and Zeeman terms in the tight-binding 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 Fermi-arc surface states and other topological surface states as a function of chemical potential in the presence of the external magnetic field.

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 non-equilibrium stochastic growth, optimal paths in ill-condensed matter, as well as the extremal statistics of random matrices & increasing subsequences in random permutations.

1. J. Stat. Phys. 160, 794 (2015).

## "Frustrated Kondo chains and glassy magnetic phases on the pyrochlore lattice"

**Jing Luo , UVA-Department of Physics**

[Host: Gia-Wei Chern]

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 cross-linking network of Kondo or double-exchange 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$_4$.

## "Crystal Structures and Photoluminescence of a Two-Dimensional Perovskite"

**Depei Zhang , UVA - Department of Physics**

[Host: Seunghun Lee]

Arguably the biggest challenge of the high-efficiency 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})_{n-1}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 temperature-dependent crystal structures, along with the photovoltaic properties of the 2D 1-layer (n = 1) perovskite material. We have performed elastic and inelastic neutron scattering, Raman scattering, and photoluminescence measurements on a powder sample of the 1-layer 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.

## "Opportunities in Quantum Materials Research using Neutrons"

**Clarina Dela Cruz , Oak Ridge National Laboratory**

[Host: Despina Louca]

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 ultra-low 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.

**Available**

## "New Directions in Theoretical Studies of High Tc Superconductors"

**Adriana Moreo , University of Tennessee**

[Host: Despina Louca]

The discovery of high critical temperature superconductivity in iron-based 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:

i) The development of spin-fermion models [1] that allow studies in the difficult nematic regime with a finite

short-range antiferromagnetic correlation length above the ordering critical temperatures. This type of studies

also allow the inclussion of doping, quenched disorder, and the study of transport and real-frequency responses;

ii) The application of the Density Matrix Renormalization Group (DMRG) approach to multi-orbital Hubbard

models in chain and ladder structures [2] triggered by the discovery of superconductivity at high pressure in ladder

iron-based compounds such as BaFe_{2}S_{3} and BaFe_{2}Se_{3}. In this context,

the recently reported [2] pairing tendencies unveiled at intermediate Hubbard U will be discussed;

iii) Results for a newly developed multi-orbital spin-fermion model for the CuO_{2} planes in high Tc cuprates.[3]

[1] S.Liang {\it et al.}, Phys.Rev.Lett.{\bf 109}, 047001 (2012) and Phys. Rev. Lett. {\bf 111} 047004 (2013); Phys. Rev. B{\bf 92} 104512 (2015); C. Bishop {\it et al.}, Phys. Rev. Lett. {\bf 117} 117201 (2016); Phys. Rev. B{\bf 96} 035144 (2017). [2] N.D. Patel {\it et al.}, Phys. Rev. B{\bf 96}, 024520(2017). See also N.D. Patel {\it et al.}, Phys. Rev. B{\bf 94}, 075119(2016). [3] Mostafa Hussein et al., in preparation.

**RESERVED**

**Available**

## "Local structure and the Jahn-Teller effect in TiSe2-xTex charge density waves"

**Aaron Wegner , UVA- Department of Physics**

[Host: Despina Louca]

Transition metal dichalcogenide (TMDC) materials exhibit a wide variety of interesting physical phenomena. This diverse family of materials forms a quasi-two dimensional layered hexagonal structure of X-M-X 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 graphene-like 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 Jahn-Teller 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**

Thursday, February 22, 2018

11:00 AM

Physics Building, Room 313

**RESERVED**

**Condensed Matter**

Thursday, February 15, 2018

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, February 8, 2018

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, February 1, 2018

11:00 AM

Physics Building, Room 313

## "Phase transitions and mode coupling in PbZrO3 and Zr-rich PbZr1-xTixO3 solid solutions"

**Sergey Vakhrushev , Ioffe Institute, St. Petersburg, Russia**

[Host: Despina Louca]

(PbZrO3)1-x-(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 Zr-rich (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 Zr-rich 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_{1-x}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 M-points 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 0.5-δ -δ) and q_{2} =(0.5-δ 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 M-point, 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 X-ray scattering measurements

# References

[1] A. K. Tagantsev et al., *Nat. Commun., *4, 2229* *(2013) [2]R. G. Burkovsky, et al. Phys. Rev. B 90, 144301 (2014) [3] R.G. Burkovsky, et al.. *J. Phys.: Condens. Matter, * 27, 335901 (2015) [4] R.G. Burkovsky, et al.., *Sci. Reports,* 7, 41512 (2017)

**Special Nuclear Seminar**

Thursday, January 25, 2018

3:30 PM

Physics Building, Room 204

## "Searching the origin of the knee of cosmic ray spectrum -LHAASO experiment"

**Cunfeng Feng , Shandong University**

[Host: Xiaochao Zheng]

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**

Thursday, January 18, 2018

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, November 30, 2017

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, November 16, 2017

11:00 AM

Physics Building, Room 313

## "Interface symmetry and non-helical states in topological insulator-semiconductor heterostructures"

**Ilya Vekhter , Louisiana State University**

[Host: Utpal Chatterjee]

Heterostructures combining topological and non-topological 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 topologically-trivial 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 well-studied helical Dirac surface states, SE-TI 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 in-plane magnetic field. I will also discuss how symmetry breaking at the interface controls proximity-induced superconductivity of the TI surface state.

**Condensed Matter**

Thursday, November 9, 2017

11:00 AM

Physics Building, Room 313

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 one-dimensional (1D) chain-like polymer called “carbyne” in 1960 and later zero-dimensional (0D) carbon fullerenes, 1D carbon nanotube, and two-dimensional (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 meta-stable 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

^{1-7}. These include functionalized C

_{60 }fullerenes for hydrogen storage

^{1, 2}, semi-hydrogenated graphene for metal-free ferromagnet

^{3}, metal-organic complexes with large electron affinity

^{4}, 3D metallic carbon made of hybridized

*sp*

^{2}and

*sp*

^{3}bonded atoms

^{5}, a Cairo-tilling inspired quasi-2D penta-graphene 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.

- Sun, Q., Jena, P., Wang, Q., and Marquez, M.: “First-principles study of hydrogen storage on Li
_{12}C_{60}”,*J. Am. Chem. Soc.***128**, 9741 (2006). - Berseth, P. A., Harter, A. G., Zidan, R., Blomqvist, A., Araujo, C. M., Scheicher, R. H., Ahuja, A., and Jena, P.: “Carbon Nanomaterials as Catalysts for Hydrogen Uptake and Release in NaAlH
_{4}”,*Nano Letters.***9**, 1501 (2009). - Zhou, J., Wang, Q., Sun, Q., Chen, X. S., Kawazoe, Y., and Jena, P.: “Ferromagnetism in semihydrogenated graphene”,
*Nano Letters***9**, 3867 (2009). - Giri, S., Child, B., Zhou, J., and Jena, P: “Unusual Stability of Multiply Charged Organo-metalic Complexes”,
*RSC Advances***5**, 44003 (2015). - Zhang, S., Wang, Q., Chen, X., and Jena, P.: “Stable Metallic 3D Metallic Phase of Carbon with Interlocking Hexagons”,
*Proc. Nat. Acad. Sci.***110**, 18809 (2013).

**Condensed Matter**

Thursday, November 2, 2017

11:00 AM

Physics Building, Room 313

## "Real-space condensation in mass transport models: statics, dynamics, and large deviations"

**Ori Hirschberg**

[Host: Israel Klich ]

The formation of traffic jams on highways, the clustering of particles in shaken granular gases, and the emergence of macroscopically-linked hubs in complex networks are all examples of real-space condensation. This real-space analogue of Bose-Einstein 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**

Thursday, October 26, 2017

11:00 AM

Physics Building, Room 313

## "Dissipation bounds on far-from-equilibrium fluctuations"

**Jordan Horowitz , MIT**

[Host: Marija Vucelja]

Near equilibrium, linear response theory has proven to be a powerful tool. At its core is the fluctuation-dissipation 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 nano-devices under energy restrictions.

**Condensed Matter**

Thursday, October 19, 2017

11:00 AM

Physics Building, Room 313

## "Skyrmion Lattices in Random and Ordered Potential Landscapes"

**Charles Reichhardt , Los Alamos National Lab**

[Host: Gia-Wei Chern]

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 non-dissipative 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 type-II 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 current-induced 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 skyrmion-skyrmion 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.

[1] S. Muhlbauer et al Science 323 915 (2009).

[2] X. Z. Yu et al. Nature 465, 901–904 (2010).

[3] X.Z. Yu et al Nature Materials, 10, 106 (2011).

[4] A. Fert, V. Cros, and J. Sampaio Nature Nanotechnology 8, 152 (2013).

**Condensed Matter**

Thursday, October 12, 2017

11:00 AM

Physics Building, Room 313

**RESERVED**

## "Topological mechanics and hidden symmetry in rigid origami"

**Bryan Chen , University of Pennsylvania**

[Host: Jeffrey Teo]

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 19th-century Maxwell-Cremona 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**

Thursday, September 28, 2017

11:00 AM

Physics Building, Room 313

## "Geometric theory of nonlocal transport in metals"

**Dmytro Pesin , University of Utah**

[Host: Gia-Wei Chern]

I will discuss the topological and geometric aspects of optical

and transport phenomena in metals with nontrivial band geometry, and outline

the full theory of linear-in-q contribution to the non-local conductivity in

a disordered metal. Physical applications of the theory include the natural

optical activity of metals and the dynamic chiral magnetic effect, as well

as the kinetic magnetoelectric effect/the current-induced magnetization in

metallic systems. The theory is similar in spirit to the one of the

anomalous Hall effect in metals, and can be used for the analysis of the

typical optical and transport measurements (e.g. Faraday rotation,

current-induced magnetization) in the THz frequency range.

**Condensed Matter**

Thursday, September 21, 2017

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, September 14, 2017

11:00 AM

Physics Building, Room 313

**RESERVED**

**Condensed Matter**

Thursday, September 7, 2017

11:00 AM

Physics Building, Room 313

## "Structure and dynamics of hydrogen in nanocrystalline palladium"

**Maiko Kofu , JPARC, Japan**

[Host: Despina Louca]

The behavior of hydrogen in metals has attracted much attention in fundamental and applied research areas. Palladium hydride (PdHx) is a typical metal-hydrogen 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 nanometer-scale. 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.

## "SUBATOMIC STATES IN CONDENSED MATTER AND MAGNETORESISTANCE OF SUPERCONDUCTORS"

**Boris I. Ivlev , Landau Institute and Instituto de Fisica, UASLP**

[Host: Genya Kolomeisky]

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*−*11*cm* 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 *T**c *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*.*18*T* and *n *= 0*, *1*, *2*, ...*) on the *R*(*H*) curve are in a good agreement with measurements.

## "Surfaces and Slabs of Fractional Topological Insulators"

**Alexander Sirota , UVA-Department of Physics**

[Host: Jeffrey Teo]

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 long-ranged 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.

**RESERVED**

## "Unusual magnetic order in some frustrated spin systems: "

**Bhanu Mahanti , Michigan State University**

[Host: Utpal Chatterjee]

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 many-body 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 one-dimensional Heisenberg spin chains and anisotropic two-dimensional 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.

## "Symmetry preserving gapping of Topological Weyl and Dirac semimetals"

**Syed Raza , UVA- Physics Department**

[Host: Jeffrey Teo]

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 symmetry-preserving gapping gives point-like and line-like anyonic excitations in the gapped bulk.

## "Understanding glassy systems using population annealing"

**Jon Machta , University of Massachusetts**

[Host: Marija Vucelja]

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 glass-forming 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.

## "Understanding and Controlling Organic Molecule and Metal Organic Framework Crystallization "

**Gaurav Giri , University of Virginia, Chemical Engineering**

[Host: Seunghun Lee]

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 metal-organic 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.

## "Energetics of a heat engine: a molecular dynamics simulation study"

**Mulugeta Bekele , Addis Ababa University, Ethiopia**

[Host: Bellave Shivaram]

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.

## "The Haldane phase in spin chains with SU(N) symmetry"

**Abhishek Roy , Institute for Theoretical Physics, University of Cologne**

[Host: Jeffrey Teo]

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.

**Available**

**Available**

**Condensed Matter**

Thursday, February 23, 2017

11:00 AM

Physics Building, Room 313

## "Dynamics of Kitaev-Heisenberg model on a Honeycomb lattice in the classical limit"

**Anjana Samarakoon , UVA-Department of Physics**

[Host: Seunghun Lee]

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 spin-1/2 Kitaev model using classical Monte-Carlo 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 pure-Kitaev 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 Majorana-like mode comparable to the Kitaev energy, which is spectrally narrowed compared to the quantum result, can be explained by magnon excitations on fluctuating one-dimensional 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 α-RuCl3 which is a prime candidate for experimental realization of Kitaev physics.

**Condensed Matter**

Thursday, February 23, 2017

3:30 PM

Physics Building, Room 313

**Reserved - Please see the Colloquia schedule**

**Condensed Matter**

Thursday, February 16, 2017

3:30 AM

Physics Building, Room 204

**Reserved - Please see the Colloquia schedule**

**Condensed Matter**

Thursday, February 9, 2017

11:00 AM

Physics Building, Room 313

**Reserved - Klich**

## "Between topological strings and topological phases"

**Jeffrey Teo , UVA - Department of Physics**

[Host: Eugene Kolomeisky]

Topological phases in two and three dimensions can be theoretically constructed by coupled-wire models whose fundamental constituents are electronic channels along strings. On the other hand the collective topological phases support further fractionalized emergent quasi-string excitations or defects such as flux vortices. In this talk I will describe topological superconductors and Dirac (or Weyl) semimetals using coupled-wire models, and discuss the fractional behavior of emergent topological strings.

**Condensed Matter**

Thursday, January 26, 2017

11:00 AM

Physics Building, Room 204

**RESERVED**

**Condensed Matter**

Thursday, January 19, 2017

11:00 AM

Physics Building, Room 204

## "Resonant inelastic X-ray scattering response of the Kitaev spin liquid"

**Gabor Halasz , KITP**

[Host: Marija Vucelja]

We propose that resonant inelastic X-ray scattering (RIXS) is an effective probe to detect spin-liquid 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 spin-conserving (SC) and the non-spin-conserving (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.

## "Formation probabilities, Post-measurement entanglement entropy and Casimir effect "

**Mohammed Ali Rajabpour , Fluminense Federal University**

[Host: Israel Klich ]

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 post-measurement 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.

Ref:

1: MAR, Europhysics Letters, 112, 66001 (2015), J. Stat. Mech. (2016) 123101 and K. Najafi, MAR, Phys. Rev. B 93, 125139 (2016).

2: F. C. Alcaraz, MAR, Phys. Rev. Lett. 111, 017201(2013), Phys. Rev. B, 90, 075132 (2014).

3: MAR, Phys. Rev. B 92, 075108 (2015) , J. Stat. Mech. (2016) 063109 and MAR,

K. Najafi, MAR, JHEP12(2016)124.

**Condensed Matter**

Thursday, December 1, 2016

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]

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 non-classical 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 (two-spheres-and-a-spring) 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**

Thursday, November 17, 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]

The progress of modern technology is increasingly driven by the development and evaluation of novel materials. Layered two-dimensional (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, semi-metallic, 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 temperature-dependent optical properties of MoTe2 and MoxW1-xTe2 using temperature-dependent and polarization-resolved 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 composition-dependent optical properties of MoxW1-xTe2 alloys. Our observations identify signatures of the hexagonal (H), monoclinc (1T’) and Td structural phases. Polarization-resolved 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 Raman-forbidden 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 MoxW1-xTe2 and provides new insights into the impact of disorder in transition metal dichalcogenides.

## "Conductance of a superconducting Coulomb blockade nanowire at finite temperature"

**Ching-Kai Chiu , University of Maryland**

[Host: Jeffrey Teo]

By applying a magnetic field, a superconducting proximity nanowire in the presence of spin-orbital 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 two-terminal 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**

Thursday, November 10, 2016

11:00 AM

Physics Building, Room 313

## "Interacting helical liquids in bilayer graphene"

**Chaoxing Liu , Penn State University**

[Host: Jeffrey Teo]

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 two-channel 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 charge-2e and spin-1, 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 two-channel 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 two-channel 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, Yi-Zhuang You, Andrea Young, Leon Balents, Chao-Xing Liu, Cenke Xu, arXiv:1602.03190v1, 2016

[2] Interacting topological phases in thin films of topological mirror Kondo insulators, Rui-Xing Zhang, Cenke Xu, Chao-Xing Liu, arXiv: 1607.06073, 2016

[3] Fingerprints of bosonic symmetry protected topological state in a quantum point contact, Rui-xing Zhang, Chao-xing Liu, arxiv: 1610.01236, 2016

**Condensed Matter**

Thursday, November 3, 2016

11:00 AM

Physics Building, Room 313

**RESERVED**

**Condensed Matter**

Thursday, October 27, 2016

11:00 AM

Physics Building, Room 313

## "Spin and orbital interactions in complex oxide heterostructures and transient states probed by x-rays"

**Mark Dean , Brookhaven National Lab**

[Host: Israel Klich ]

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 x-ray 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 LaNiO3-based heterostructures [2] and characterizing the spin behavior within the ultra-fast transient state of photo-doped 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, 601-605 (2016)

## "First-principles predictions of thermodynamically stable two-dimensional electrides"

**Mina Yoon , Center for Nanophase Materials Sciences, Oak Ridge National Laboratory**

[Host: Seunghun Lee]

Two-dimensional (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 alkaline-earth based 2D electrides by using a first-principles global structure optimization method, phonon spectrum analysis, and molecular dynamics simulation. The method was applied to binary compounds consisting of alkaline-earth 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**

Thursday, October 20, 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]

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**

Thursday, October 13, 2016

11:00 AM

Physics Building, Room 313

## "Anomalous cooling and heating – the Mpemba effect and its inverse"

**Oren Raz , University of Maryland**

[Host: Israel Klich ]

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 counter-intuitive behavior. Using the theoretical framework of non-equilibrium 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.

## "Multiferroics by design with frustrated molecular magnets"

**Yoshitomo Kamiya , RIKEN**

[Host: Gia-Wei Chern]

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 low-energy degrees of freedom consist of built-in 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**

Thursday, September 29, 2016

11:00 AM

Physics Building, Room 313

**Condensed Matter**

Thursday, September 22, 2016

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, September 15, 2016

11:00 AM

Physics Building, Room 313

## "Exploring metal-insulator transitions with optical microscopy and spectroscopy"

**Mumtaz Qazilbash , William and Mary**

[Host: Utpal Chatterjee]

The study of metal-insulator, 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 thermally-induced metal-insulator transitions accompanied by structural and/or magnetic instabilities: vanadium dioxide (VO2) and the manganite La0.67Sr0.33MnO3. Infrared micro-spectroscopy and micro-ellipsometry measurements on crystals of VO2 reveal that its insulating phases are Mott-Hubbard insulators, not Peierls insulators. Scanning near-field infrared microscopy (SNIM) allows us to directly image nano-scale metallic puddles that appear at the onset of the first-order metal-insulator 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 near-field infrared amplitude in a La0.67Sr0.33MnO3 film which we attribute to fluctuating conductivity in the vicinity of its second order metal-insulator transition.

**Condensed Matter**

Thursday, September 8, 2016

11:00 AM

Physics Building, Room 313

## "Topological Phase Transitions in Line-nodal Superconductors"

**Gil Young Cho , Advanced Institute of Science and Technology, Korea**

[Host: Jeffrey Teo]

Fathoming interplay between symmetry and topology of many-electron wave-functions has deepened understanding of quantum many body systems, especially after the discovery of topological insulators. Topology of electron wave-functions 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 line-nodal superconductors. The interplay may induce an exotic universality class in sharp contrast to that of the phenomenological Landau-Ginzburg theory. Hyper-scaling 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 temperature-tuning parameter phase-diagram.

**Condensed Matter**

Thursday, September 1, 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: Gia-Wei Chern]

Predicting material-specific physical properties is always a fascinating goal of physics research. The current state-of-the-art 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 Kohn-Sham 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.

A measurement of charge asymmetry in Bethe-Heitler (BH) pair production using the High Intensity Gamma-Ray Source at the Tri-Universities 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.

## "Aging and memory effects in the spin jam states of densely populated frustrated magnets"

**Anjana Samarakoon , UVA - Department of Physics**

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 zero-field 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.

## "Critical mechanical structures: topology and entropy"

**Xiaoming Mao , University of Michigan**

[Host: Marija Vucelja]

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.

## "Spin-Orbit Coupled Double Perovskites: Topology and Magnetism"

**Arun Paramekanti , University of Toronto**

[Host: Israel Klich ]

Double perovskites are a broad class of transition metal oxides which exhibit a wide range of phenomena - high temperature ferromagnetism, half-metallicity, and Mott insulators with geometrically frustrated magnetism. I will discuss how spin-orbit 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 half-metallic double perovskites as well as bulk iridium-based Mott insulators where we argue for dominant Kitaev exchange interactions.

**Nuclear Seminar**

Thursday, April 14, 2016

3:30 PM

Physics Building, Room 204

## "Experimental study of the 3He and neutron spin structure at low Q2 using a polarized 3He target"

**Nguyen Ton , UVA- Department of Physics**

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 *Q*2 by using a polarized 3He target as an effective polarized neutron target. The goals of the experiment are to make a bench-mark test of Chiral Perturbation Theory calculations and to check the Gerasimov-Drell-Hearn (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 *Q*2 points but with a defective equipment which complicate the data analysis. The second period covered higher *Q*2 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.

## "Spin-1/2 Heisenberg antiferromagnet in one dimension"

**Yasu Takano , University of Florida**

[Host: Seunghun Lee]

The one-dimensional spin-1/2 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 Tomonaga-Luttinger liquid, in which dynamic and static properties are inextricably linked. Low-energy 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.

**Reserved for special colloquium**

## "Simulating frustrated itinerant magnets at the mesoscale"

**Kipton Barros , LANL**

[Host: Gia-Wei Chern]

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 2-point 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).

## "Gauging and Orbifolding the topological phases in (2+1) and (3+1) dimensions"

**Xiao Chen , University of Illinois at Urbana-Champaign**

[Host: Jeffrey Teo]

Topological phases of matter are commonly equipped with global symmetries, such as electric-magnetic 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 bulk-boundary 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 twofold-symmetric D(Z_N ) quantum double model, SU(3)_1, and the S_3-symmetric SO(8)_1 state. Later on, we generalize this idea to (3+1) dimensional topological phases and study the bulk-boundary correspondence there

**Available**

## "Complex Magnetic Phase Diagrams of Models for Iron Based Superconductors"

**Elbio Dagotto , University of Tennessee/Oak Ridge National Laboratory**

[Host: Despina Louca]

This presentation will start with a brief introduction to iron-based

high temperature superconductors, emphasizing that these materials appear to

be located in an intermediate Hubbard coupling regime [1] which is difficult

to study with canonical analytical techniques. Moreover, competing

ferro- and antiferro-magnetic tendencies are present, leading to frustration.

For these reasons computational studies are important. After the introduction,

and time allowing, I will focus on three main areas of recent active research:

(i) the widely discussed spin nematic state analyzed from the perspective

of the spin fermion model [2];

(ii) the several zero temperature competing magnetic states that appear

in quasi one-dimensional two-leg ladder geometries [3];

(iii) the rich phase diagrams of multiorbital Hubbard models varying

couplings and temperatures from the perspective of a recently developed

technique that combines mean field and Monte Carlo aspects [4].

References:

[1] P. Dai, JP Hu, and E.D., Nat. Phys. 8, 709 (2012); E.D., RMP 85, 849 (2013).

[2] S. Liang et al., PRL 111, 047004 (2013); S. Liang et al., PRB 90, 184507 (2014).

[3] Q. Luo et al., PRB 84, 140506(R) (2011); Q. Luo et al., PRB 87, 024404 (2013); S.

Dong et al, PRL 113, 187204 (2014); J. Rincon et al., PRL 112, 106405 (2014).

[4] A. Mukherjee et al., PRB 90, 205133 (2014), and references therein;

A. Mukherjee et al., in preparation.

**Condensed Matter**

Thursday, February 25, 2016

1:00 PM

Physics Building, Room 313

## "Jamming and glassy behavior in dense biological tissues"

**Max Dapeng Bi , Rockefeller University**

[Host: Marija Vucelja]

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 density-driven glass transition in particle-based models for active matter, these cannot explain liquid-to-solid 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 single-cell properties such as cell-cell 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**

Thursday, February 18, 2016

1:00 PM

Physics Building, Room 313

## "Phase Diagram and Quantum Order by Disorder in the Kitaev K1-K2 Honeycomb Model"

**Natalia Perkins , University of Minnesota**

[Host: Gia-Wei Chern]

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 spin-liquid 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 second-neighbor Kitaev coupling is an important ingredient to such a microscopic description for the strong spin-orbit transition-metal oxide Na2IrO3.

We analyze the K1-K2 model from a variety of methodological perspectives. As a coherent picture emerges from the investigation, the K1-K2 model allows us to explain the onset of zigzag magnetic order that is also found experimentally. Furthermore, we find that the K1-K2 model is a suitable minimal description for resolving the substantially different nature of quantum and thermal fluctuations originating from such Kitaev couplings.

**Condensed Matter**

Thursday, February 11, 2016

11:00 AM

Physics Building, Room 313

**RESERVED**

**Condensed Matter**

Thursday, February 4, 2016

12:30 PM

Physics Building, Room 313

## "Resonant inelastic x-ray scattering as a probe of band structure"

**Marton Kanasz-Nagy , Harvard University**

[Host: Israel Klich ]

Resonant inelastic x-ray scattering (RIXS) has emerged as a powerful tool for studying high-temperature superconducting materials. In the talk, I will show how a theory based on non-interacting quasi-particles 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 angle-resolved photemisson spectroscopy, making RIXS a powerful band structure probe, potentially applicable to a wide range of materials.

**Condensed Matter**

Thursday, January 28, 2016

11:00 AM

Physics Building, Room 313

**Available**

**Condensed Matter**

Thursday, January 21, 2016

12:30 PM

Physics Building, Room 313

## "Extracting Hidden Hierarchies in Complex Spatial Networks"

**Carl Modes , Rockefeller University**

[Host: Marija Vucelja]

Natural and man-made 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 cycle-rich 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**

Thursday, December 3, 2015

11:00 AM

Physics Building, Room 313

## "Local probe investigation of interfaces in two-dimensional materials"

**Prof. Chenggang Tao , Virginia Tech**

[Host: Utpal Chatterjee]

Emerging two-dimensional (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 quasi-1D C_{60} nanostructures on graphene.

**Condensed Matter**

Thursday, November 19, 2015

3:30 PM

Physics Building, Room 204

**Reserved for GPSA**

**Condensed Matter**

Thursday, November 12, 2015

11:00 AM

Physics Building, Room 313

## "Magnetic orderings of XY-pyrochores"

**Prof. Haidong Zhou , University of Tennessee**

[Host: Seunghun Lee]

Due to the geometrically frustrated lattice and effective spin-1/2, 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**

Thursday, November 5, 2015

11:00 AM

Physics Building, Room 313

## "Topological superconductivity in Metal/Quantum-Spin-Ice heterostructure"

**Prof. Eun-Ah Kim , Cornell University**

[Host: Seunghun Lee]

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: Y2Sn2-xSbxO7 grown on the (111) surface of Pr2Zr2O7. The predicted order parameter symmetry is analogous to that of 2D superfluid He3-B phase and the realization of the proposal will amount to the first solid-state realization of such pairing state.

**Condensed Matter**

Thursday, October 29, 2015

11:00 AM

Physics Building, Room 313

## "Bulk Thermoelectric and Thermodynamic Probes of non-Abelian Anyons in Topological States of Matter"

**Kun Yang , FSU**

[Host: Israel Klich ]

Topological states of matter support quasiparticle excitations with fractional charge and possibly exotic statistics of the non-Abelian type, known as non-Abelian 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 non-Abelian quasiparticle statistics using bulk probes. We show that bulk thermopower is a promising way to detect their non-Abelian nature, and measure the quantum dimension (a key parameter that quantifies non-Abelian 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 insulator-superconductor hybrid systems. Some of the predicted behavior has been observed in recent experiments, which will also be discussed.

**RESERVED**

**Condensed Matter**

Thursday, October 15, 2015

11:00 AM

Physics Building, Room 313

## "Phononics in liquids and supercritical fluids"

**Dima Bolmatov , Brookhaven National Lab**

[Host: Genya Kolomeisky]

**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 condensed-matter 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 phonon-based 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 Delong-Petit and the ideal gas thermodynamic limits. The experimental evidence for the new thermodynamic boundary (the Frenkel line) on the pressure-temperature 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**

Thursday, October 8, 2015

11:00 AM

Physics Building, Room 313*

## "Quantum dissection of a covalent bond with the entanglement spectrum"

**Norman Tubman , UIUC**

[Host: Israel Klich]

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.

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 low-energy 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 one-dimensional 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**

Thursday, September 24, 2015

3:30 PM

Physics Building, Room 204

**RESERVED**

**Condensed Matter**

Thursday, September 17, 2015

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 10, 2015

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 3, 2015

3:30 PM

Physics Building, Room 204

## "Topological line nodes in Ca3P2 and other semi-metals"

**Ching-Kai Chiu , University of Maryland**

[Host: Jeffrey Teo]

In an ordinary three-dimensional metal the Fermi surface forms a two-dimensional closed sheet separating the filled from the empty states. Topological semimetals, on the other hand, can exhibit protected one-dimensional Fermi lines or zero-dimensional Fermi points, which arise due to an intricate interplay between symmetry and topology of the electronic wavefunctions. Here, we study how reflection symmetry, time-reversal symmetry, and inversion symmetry leads to the topological protection of line nodes in three-dimensional semi-metals. We derive the Z- and Z2-type 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 tight-binding model, derived from ab initio DFT calculations. We show that due to a bulk-boundary correspondence, Ca3P2 displays nearly dispersionless surface states, which take the shape of a drumhead. These topological surface states give rise to the charge polarization.

**Available**

**Condensed Matter**

Thursday, April 23, 2015

3:30 PM

Physics Building, Room 204

## "Magnetic Nematicity in the Hidden Ordered Material URu2Si2"

**Peter Riseborough , Temple University**

[Host: Bellave Shivaram]

The compound URu_{2}Si_{2} is a heavy-fermion 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 lambda-like anomaly in the specific heat and was originally thought to be a second-order 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 Fermi-surface. However, neutron scattering experiments have demonstrated that spin density wave order is absent below the 17.5 K transition and, likewise, x-ray 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 spin-rotational 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 four-fold symmetry of the magnetic susceptibility in the a-b plane is broken as the susceptibility develops a two-fold axis below the transition.

## "In-plane Charge Fluctuations in Bismuth Sulfide Superconductors"

**Anushika Athauda , University of Virginia**

[Host: Despina Louca]

## "Entanglement entropy from thermodynamic entropy"

**Mohammad Magrebi , JQI Maryland/NIST**

[Host: Israel Klich]

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 many-body systems, have remained elusive. In this talk, I will consider a simple setup where a dispersive medium becomes entangled with zero-point 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.

**Available**

## "Length Scale Dependent of Thermal Conductivity of Si-Ge Alloys"

**Long Chen , University of Virginia**

[Host: Joe Poon]

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 non-centrosymmetric 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**

Thursday, March 26, 2015

3:30 PM

Physics Building, Room 204

## "Micro-machined probes for the study of quantum fluids"

**Yoonseok Lee , University of Florida**

[Host: Seunghun Lee]

In the superfluid phases of liquid 3He with p-wave 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 quasi-particle sub-gap 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 B-phase, and the edge current and the chiral edge state in the A-phase, many of which are of topological origin. We have developed micro-machined probes or micro-electro-mechanical 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.

**Reserved for HEP Seminar**

## "Magneto-transport and domain wall scattering in the epitaxial L10 MnAl thin films"

**Linqiang Luo , University of Virginia**

[Host: Stu Wolf]

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 magneto-transport(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. Maze-like strip domains were observed by MFM after demagnetization. The difference of in-plane and out-of-plane 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.

**RESERVED**

**Condensed Matter**

Thursday, February 26, 2015

3:30 PM

Physics Building, Room 204

## "Problems in Human Motion Planning"

**Brian Skinner , Argonne National Laboratory**

[Host: Genya Kolomeisky]

Moving through a densely-populated environment can be surprisingly hard, owing to the problem of congestion. Learning to deal with congestion in crowds and in networks is a long-standing and urgently-studied problem, one that can be equally well described at the level of dense, correlated matter or at the level of game-theoretical 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 densely-packed repulsive particles, and I address the question: what is the form of the pedestrian-pedestrian 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" decision-making. Analysis of the model reveals a surprising connection between Nash equilibria from game theory and percolative phase transitions from statistical physics.

**Condensed Matter**

Thursday, February 19, 2015

3:30 PM

Physics Building, Room 204

**Reserved for Special Colloquium**

**Condensed Matter**

Thursday, February 12, 2015

3:30 PM

Physics Building, Room 204

Bending of double-stranded 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 single-molecule 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 worm-like chain model. Time permitting, I will also present a new method to compute the equilibrium distribution of forces exerted by a semi-flexible polymer loop.

**Condensed Matter**

Wednesday, February 11, 2015

3:30 PM

Physics Building, Room 204

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**

Thursday, February 5, 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]

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.

A metamagnet (MM) is characterized by the sharp rise of the low-temperature 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 heavy-fermion metals, such as UPt_{3}, and show how these systems are well described by a simple two-levels model, which appears to be applicable to all MMs, when suitably extended.

**Available**

**Condensed Matter**

Thursday, January 15, 2015

3:30 PM

Physics Building, Room 204

## "Uniaxial Pressure on Strongly Correlated Materials"

**Rena Zieve , University of California, Davis**

[Host: Bellave Shivaram]

We describe a helium-activated 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 pressure-induced phase transitions directly; and making smaller pressure adjustments than possible with typical hydrostatic pressure arrangements. Several families of unconventional superconductors, including high-Tc 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.

**Joint Nuclear/Condensed Matter Physics Seminar**

Thursday, December 4, 2014

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]

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 band-gap 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**

Thursday, November 20, 2014

3:30 PM

Physics Building, Room 204

## "Anyonics: Designing exotic circuitry with non-Abelian anyons"

**Kiril Shtengel , University of California, Riverside**

[Host: Israel Klich]

Non-Abelian 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 non-Abelian anyon, a Majorana zero energy mode bound to a vortex or a domain wall. One such candidate system, a so-called "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, s-wave superconductors and insulators with strong spin-orbit coupling. Similarly to a Majorana wire, the ends of our “quantum wire” would bind "parafermions", exotic non-Abelian 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".

## "Impact of electronic nematicity on the unconventional superconductivity of the iron pnictides"

**Rafael Fernandes , University of Minnesota**

[Host: Despina Louca]

Five years after their discovery, much of the interest in the iron pnictides remains in understanding not only their high-temperature 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 sign-changing s^{+-} superconducting state, although they may coexist under certain conditions. When the s^{+-} instability is in close competition with a d-wave 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 time-reversal or tetragonal symmetries.

**Condensed Matter**

Thursday, November 13, 2014

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, November 6, 2014

3:30 PM

Physics Building, Room 204

## "Negative Casimir Entropies in Nanoparticle Interactions"

**Kimball Milton , University of Oklahoma**

[Host: Israel Klich]

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.

**RESERVED**

**Available**

**Available**

## "Spin dynamics in a rare-earth based single molecule magnet"

**Maiko Kofu , Institute of Solid State Physics, University of Tokyo**

[Host: Despina Louca]

Single-molecule 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 easy-axis 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 rare-earth based SMMs attracts much attention. In this talk, I will show our recent results on Tb-based SMMs. We have investigated the energy scheme by inelastic neutron scattering (INS) and the relaxation phenomena by quasi-elastic neutron scattering (QENS) measurements. The mechanism of magnetization reversal through quantum tunneling in the Tb-based SMM will be discussed.

**RESERVED**

**RESERVED**

**Condensed Matter**

Thursday, September 25, 2014

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 18, 2014

3:30 PM

Physics Building, Room 204

## "Hybrid inorganic-organic nanomaterials as building blocks for designer solids and their applications in optoelectronic devices"

**Joshua Choi , University of Virginia**

[Host: Seunghun Lee]

Tunable optoelectronic properties of hybrid inorganic-organic 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 self-assembly 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 metal-organic 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**

Thursday, September 11, 2014

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 4, 2014

3:30 PM

Physics Building, Room 204

**RESERVED**

**Available**

**RESERVED**

**Reserved for Nuclear seminar**

**Condensed Matter**

Thursday, April 24, 2014

3:30 PM

Physics Building, Room 204

## "Amplification, entanglement and storage of microwave radiation using superconducting circuits"

**Jean-Damien Pillet , Columbia University**

[Host: Tom Gallagher]

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 non-degenerate three wave-mixing. 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.

## "Effect of surface disorder on transport in topological insulators"

**Kun Woo Kim , Caltech**

[Host: Israel Klich]

**Condensed Matter**

Thursday, April 17, 2014

3:30 PM

Physics Building, Room 204

^{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 spin-orbit interaction (intrinsic to large Z iridium cations) and their residual on-site Coulomb interaction U, conspires to stabilize a novel class of spin-orbit assisted Mott insulators with a proposed J

_{eff}=1/2 ground state wavefunction. The identification of this novel spin-orbit 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 spin-orbit coupled setting. Currently, however, there remains very little understanding of how spin-orbit Mott phases respond to carrier doping and, more specifically, how relevant U remains for the charge carriers of a spin-orbit Mott phase once the bandwidth is increased. Here I will present our group’s recent experimental work exploring carrier doping and the resulting electronic phase behavior in one such spin-orbit 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 system’s ground state. Our results reveal the stabilization of an electronically phase separated ground state in B-site doped Sr

_{3}Ir

_{2}O

_{7}, suggestive of an extended regime of localization of in-plane doped carriers within the spin-orbit Mott phase. This results in a percolative metal-to-insulator transition with a novel, global, antiferromagnetic order. The electronic response of B-site doping in Sr

_{3}Ir

_{2}O

_{7}will then be compared with recent results exploring A-site doping of electrons into the system and the resulting electronic phase diagrams discussed.

## "The internal structure of a vortex and its exited states, in a two dimensional superfluid"

**Oded Agam , Hebrew University**

[Host: Israel Klich]

**Condensed Matter**

Monday, February 24, 2014

3:30 PM

Physics Building, Room 204

## "Matrix Product States for the Fractional Quantum Hall Effect"

**Roger Mong , Caltech**

[Host: Paul Fendley]

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 density-matrix-renormalization 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**

Thursday, February 20, 2014

3:30 PM

Physics Building, Room 204

## "From Topological Insulators to Majorana Fermions"

**Fan Zhang , University of Pennsylvania**

[Host: Paul Fendley]

**Condensed Matter**

Monday, February 17, 2014

3:30 PM

Physics Building, Room 204

## "Exotic Zero Energy Modes at Topological Defects in Crystalline Superconductors"

**Jeffrey Teo , University of Illinois at Urbana-Champaign**

[Host: Paul Fendley]

**Condensed Matter**

Thursday, February 6, 2014

3:30 PM

Physics Building, Room 204

## "The Bulk-Edge Correspondence in Abelian Fractional Quantum Hall States"

**Michael Mulligan , Station Q/UCSB**

[Host: Paul Fendley]

## "Controlling Atomic Movement on the Nanoscale"

**Sinisa Coh , University of California, Berkeley**

[Host: Paul Fendley]

More details can be found in these publications:

Phys. Rev. Lett. 110, 185901 (2013)

http://link.aps.org/doi/10.1103/PhysRevLett.110.185901

Phys. Rev. B 88, 045424 (2013)

http://link.aps.org/doi/10.1103/PhysRevB88.045424

## "Casimir forces and torques. from quantum fluctuations to self assembly"

**Raul Esquivel-Sirvent , Universidad Nacional Autónoma de México**

[Host: Israel Klich]

## "Simulating condensed matter systems with tensor network states and discovery of algebraic decoherence"

**Thomas Barthel , Ludwig-Maximilians-Universität Munich**

[Host: Paul Fendley]

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 memory-less (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 many-body system. For the open spin-1/2 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 quasi-exact numerical simulation, I will explain the result on the basis of a perturbative treatment.

## "Equilibration and Thermalization in Quantum Many-Body Systems"

**Alioscia Hamma , Tsinghua University**

[Host: Paul Fendley]

**Condensed Matter**

Thursday, January 16, 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]

To understand DFT’s potential and limitations, we have extended the powerful density matrix renormalization group (DMRG) technique to solve one-dimensional 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 Kohn-Sham 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**

Thursday, December 12, 2013

3:30 PM

Physics Building, Room 313

**Condensed Matter**

Thursday, December 5, 2013

3:30 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, November 21, 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**

Thursday, November 7, 2013

3:30 PM

Physics Building, Room 204

## "Theory of Resonant X-Ray Scattering with Applications to high-Tc Cuprates"

**David Benjamin , Harvard University**

[Host: Israel Klich]

**Condensed Matter**

Thursday, October 31, 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]

## "Microcavity Exciton-Polariton Condensates Physics and Applications"

**Na Young Kim , Stanford University**

[Host: Seunghun Lee]

I will first discuss the characteristics of exciton-polariton condensates with emphasis on their intrinsic open-dissipative nature. I will present exciton-polariton-lattice systems, where we explore the non-zero momentum condensate order. We envision that the polariton-lattice systems would serve as a solid-state platform to investigate strongly correlated materials. Finally, I will show our recent progress on electrically pumped exciton-polariton coherent matter waves towards the development of novel coherent light sources operating at low threshold powers and at high temperatures.

## "The Interplay of Correlation, Electron-Phonon Coupling, Magnetism, and Superconductivity in High Tc Superconductors"

**Zhiping Yin , Rutgers University**

[Host: Joe Poon]

References:

[1] Z. P. Yin, A. Kutepov, and G. Kotliar, Phys. Rev. X 3, 021011 (2013).

[2] Z. P. Yin, K. Haule, and G. Kotliar, Nat. Mater. 10, 932-935 (2011).

[3] Z. P. Yin, K. Haule, and G. Kotliar, Nat. Phys. 7, 294-297 (2011).

[4] Z. P. Yin, K. Haule, and G. Kotliar, Phys. Rev. B 86, 195141 (2012).

[5] Z. P. Yin, K. Haule, and G. Kotliar, to be published (2013).

[6] Z. P. Yin and G. Kotliar, EPL 101, 27002 (2013).

[7] M. Retuerto et al. Chemistry of Materials, 10.1021/cm402423x (2013).

**Condensed Matter**

Thursday, September 26, 2013

3:30 PM

Physics Building, Room 204

## "Gauge dynamics of kagome antiferromagnets"

**Michael Lawler , Binghamton University**

[Host: Israel Klich]

**Condensed Matter**

Thursday, September 5, 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]

_{3}and related compounds.

## "Fock Parafermions and Self-Dual Representations of the Braid Group"

**Emilio Cobanera , Leiden University**

[Host: Israel Klich]

## "Metabolic Imaging: A Novel Diagnostic Strategy for Hypertensive Heart"

**Min Zhong , University of Virginia**

[Host: Bijoy Kundu]

## "Z_2 spin liquid in the Heisenberg antiferromagnet on kagome"

**Oleg Tchernyshyov , Johns Hopkins University**

[Host: Paul Fendley]

I will present a phenomenological Z_2 lattice gauge theory that describes low-energy 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 ground-state degeneracy, response to quenched disorder, and spinon states near the system's edge.

## "Quantum Critical Behavior in a Resonant Level Coupled to a Dissipative Environment"

**Gleb Finkelstein , Duke University**

[Host: Israel Klich]

**Quantum Information Seminar**

Wednesday, April 10, 2013

3:30 PM

Physics Building, Room 313

**Condensed Matter**

Thursday, April 4, 2013

3:30 PM

Physics Building, Room 204

## "Magnetic ground states and excitations in vanadates, BaV10O15 and CoV2O4"

**Sachith Dissanayake , University of Virginia**

[Host: Seunghun Lee]

## "Transport Phenomena in CaVO3 and SrVO3 thin films"

**Mandy Gu , University of Virginia**

[Host: Stu Wolf]

## "What is an essential factor for determining Tc in Iron Pnictides?

- In terms of crystal structure and spin fluctuations -"

**Chul-Ho Lee , National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan**

[Host: Seunghun Lee]

[1] C. H. Lee et al., J. Phys. Soc. Jpn. 77 (2008) 083704.

**Condensed Matter**

Thursday, February 14, 2013

3:30 PM

Physics Building, Room 204

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 long-lived hybrid plasmon-phonon 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 high-speed electronic devices and the possibility of strain engineering for exploratory novel graphene electronics.

**Condensed Matter**

Tuesday, November 13, 2012

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]

## "Spin-lattice coupling and anomalous magnetic excitations in the triangular lattice antiferromagnet Ag2CrO2"

**Masa Matsuda , Oak Ridge National Laboratory**

[Host: Seunghun Lee]

## "Non-abelian statistics of fractionalized Majorana Fermions"

**Netanel Lindner , Caltech**

[Host: Israel Klich]

## "Unconventional 'optics' with pseudospins in graphene: Metamaterials, Klein Tunneling and Subthermal switching"

**Avik Ghosh , University of Virginia**

[Host: Bellave Shivaram]

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 quasi-momentum 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 Landauer-Keldysh 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, metal-insulator transition switches, electron collimators and pseudospintronic analogs of electro-optic modulators.

**Condensed Matter**

Thursday, October 25, 2012

3:30 PM

Physics Building, Room 204

## "Modal analysis of Casimir interactions"

**Francesco Intravaia , Los Alamos National Laboratory**

[Host: Israel Klich]

## "Quantum criticality and superconducting gap evolution in Fe-based superconductors"

**Kyuil Cho , The Ames Laboratory and Iowa State University**

[Host: Seunghun Lee]

_{2}(As

_{1-x,}P

_{x})

_{2}by using tunnel diode resonator technique and identified the existence of the QCP beneath the superconducting dome [1]. In addition, the low-temperature analysis of the penetration depth measurements revealed that the superconducting gap structure shows a universal dome-like 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).

## "Unconventional temperature-enhanced magnetism in Fe1.1Te"

**Igor Zaliznyak , Brookhaven National Laboratory**

[Host: Seunghun Lee]

**RESERVED**

**A Joint Seminar Sponsored by Electrical and Computer Engineering, Mechanical and Aerospace Engineering, and Physics**

Friday, April 20, 2012

2:00 PM

, Room MEC 341

## "Novel Semiconductor and Epitaxial Nanocomposite Materials for Energy Conversion and Optoelectronic Applications"

**Joshua M. O. Zide , University of Delaware**

[Host: ECE, MAE, & Physics]

Specifically, I will discuss two material systems: (1) nanocomposites consisting of metallic nanoparticles (such as ErAs and TbAs) within III-V semiconductors (such as InGaAs and GaAs), and (2) dilute bismuthide semiconductors in which bismuth is incorporated into III-V 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.

## "Transport Properties of VO_{2} Films near the Metal-Semiconductor Transition"

**Salinporn Kittiwatanakul , University of Virginia**

[Host: Stu Wolf]

_{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 c-plane Al

_{2}O

_{3}substrates to study the effect of different O

_{2}flow rates (4.5-6.0 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}/c-Al

_{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 b-axis 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**

Thursday, April 12, 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]

## "Torsion and Viscosity in Condensed Matter Physics"

**Taylor Hughes , University of Illinois at Urbana-Champaign**

[Host: Israel Klich]

## "Magnetic excitation in the iron based superconductor, FeTe1-xSex: fluctuation near maximal quantum criticality"

**Jooseop Lee , University of Virginia**

[Host: Seunghun Lee]

**Condensed Matter**

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]

**RESERVED**

**How to Get into Undergraduate Research**

Thursday, December 1, 2011

5:45 PM

Physics Building, Room 204

## "How to Get into Undergraduate Research"

**Bob Jones, Despina Louca, & Blaine Norum , University of Virginia**

[Host: Elton Ho]

**Condensed Matter**

Thursday, November 17, 2011

3:30 PM

Physics Building, Room 204

## "The Linear and Nonlinear Magnetic Response in Strongly Correlated Metals"

**Bellave Shivaram , University of Virginia**

[Host: Genya Kolomeisky]

**Condensed Matter**

Thursday, November 10, 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]

**Reserved for Special Colloquium**

**Reserved for Special Colloquium**

**Available**

## "Resonant spin excitations in Iron Arsenide superconductors"

**Stephan Rosenkranz , Argonne National Lab**

[Host: Seunghun Lee]

**Condensed Matter**

Thursday, October 6, 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]

**Condensed Matter**

Thursday, September 29, 2011

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Monday, September 26, 2011

3:30 PM

Physics Building, Room 204

## "Fe-based superconductors at high magnetic fields"

**Alexander Gurevich , Old Dominion University**

[Host: Joe Poon]

**Condensed Matter**

Thursday, September 22, 2011

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 15, 2011

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 8, 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]

**Condensed Matter**

Thursday, September 1, 2011

3:30 PM

Physics Building, Room 204

**Available**

**Available**

**RESERVED**

## "Material Development for Magnetic tunnel junctions in Spin Transfer Torque Random Access Memory (STT-RAM)"

**Ms. Yishen Cui , University of Virginia**

[Host: Stu Wolf]

**Condensed Matter**

Thursday, April 21, 2011

3:30 PM

Physics Building, Room 204

## "Computational Study of ultra-short-pulse laser-metal interaction"

**Chengping Wu , UVa**

[Host: Genya Kolomeisky]

**Available**

**Condensed Matter**

Thursday, March 31, 2011

3:30 PM

Physics Building, Room 204

**Condensed Matter**

Tuesday, March 29, 2011

3:30 PM

Physics Building, Room 313

## "Neutron scattering study on the iron-based high-Tc superconductor systems"

**Wei Bao , Renmin University of China**

[Host: Seunghun Lee]

**RESERVED**

## "Nonconventional Odd Denominator Fractional Quantum Hall States"

**Gabor Csathy , Purdue University**

[Host: Jongsoo Yoon]

## "Quantum Phases of Dipolar Bosons"

**Carlos A. R. Sa de Melo , Georgia Institute of Technology**

[Host: Cass Sackett]

**Condensed Matter**

Thursday, February 24, 2011

3:30 PM

Physics Building, Room 204

## "Universal conductance in quantum multi-wire junctions"

**Armin Rahmani , Boston University**

[Host: Austen Lamacraft ]

**Condensed Matter**

Thursday, February 17, 2011

3:30 PM

Physics Building, Room 204

## "Quantum Information Processing with Prethreshold Superconducting Qubits"

**Michael R. Geller , University of Georgia**

[Host: Eugene Kolomeisky ]

**Condensed Matter**

Thursday, February 10, 2011

3:30 PM

Physics Building, Room 204

## "Photons as phonons: from cavity QED to quantum crystallization"

**Sarang Gopalakrishnan , Urbana-Champaign**

[Host: Austen Lamacraft]

**Available , Urbana**

## "Study of spin state transition in La _{2-x }Ca _{ x }CoO _{ 4 } by using neutron and soft x-ray scattering"

**Kazumasa Horigane , Tohoku University**

[Host: Despina Louca]

_{ 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 J-PARC [2]. Figure 1 shows the magnetic excitation of La

_{ 1.5 }Ca

_{ 0.5 }CoO

_{ 4 }below and above T

_{ N }(=50K). Spin-wave 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 x-ray 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)

**Available**

**RESERVED**

**Condensed Matter**

Thursday, November 18, 2010

4:00 PM

Physics Building, Room 204

## "Fractional statistics and beam splitters in quantum Hall systems"

**Smitha Vishveshwara , UIUC**

[Host: Israel Klich ]

**Condensed Matter**

Thursday, November 11, 2010

4:00 PM

Physics Building, Room 204

## "Interlayer exchange couplings in Ga1-xMnxAs/GaAs diluted ferromagnetic semiconductor multilayers"

**Jae-Ho Chung , Korea University**

[Host: Seung-Hun Lee]

To test this idea, we have fabricated GaMnAs-based 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 cm-3 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 Be-doped spacers. Their field cycling behavior clearly indicated that the observed antiferromagnetic coupling is spontaneous and robust. All of these samples showed GMR-like transitions in magnetotransport measurements. In contrast, none of the samples without Be-doped spacers showed such behavior. These results indicate that the interlayer exchange couplings in GaMnAs-based ferromagnetic multilayers can be controlled via engineering of the spacer properties.

**RESERVED**

**Available**

**Available**

**Available**

**RESERVED**

**Condensed Matter**

Thursday, September 30, 2010

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, September 23, 2010

3:30 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]

**Condensed Matter**

Thursday, September 16, 2010

3:30 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 9, 2010

3:30 PM

Physics Building, Room 204

**RESERVED**

**Condensed Matter**

Thursday, September 2, 2010

4:00 PM

Physics Building, Room 204

**Available**

## "Structural Investigations of Hydrides Studied by Neutron Scattering "

**Toyoto Sato , Tohoku University**

[Host: Despina Louca]

**Available**

## "Physical basis for the spatial organization of DNA"

**Bae-Yeun Ha , University of Waterloo**

[Host: Seunghun Lee ]

**Available**

**RESERVED**

**Available**

**Condensed Matter**

Thursday, April 1, 2010

4:00 PM

Physics Building, Room 204

## "Projection Hamiltonians for clustered quantum Hall wavefunctions"

**Thomas Jackson , Yale**

[Host: Israel Klich]

**Condensed Matter**

Thursday, March 25, 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]

**RESERVED**

**Condensed Matter**

Thursday, March 4, 2010

4:00 PM

Physics Building, Room 204

## "Electron itinerancy, orbital symmetry and itinerant spin fluctuations in Fe-based superconductors as revealed by soft x-ray spectroscopies."

**Norman Mannella , University of Tennessee - Knoxville**

[Host: Seung-Hun Lee]

_{ 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**

Thursday, February 25, 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 ]

**Condensed Matter**

Thursday, February 18, 2010

4:00 PM

Physics Building, Room 204

## "Effect of order parameter fluctuations on the spectral density in d-wave superconductors"

**Maxim Khodas , Brookhaven**

[Host: Israel Klich ]

**Available**

## "Orbital ordering in CaV _{ 2 } O _{ 4 } : A neutron scattering study"

**Oliver Pieper , Helmholtz- Zentrum Berlin**

[Host: Seunghun Lee]

_{ 2 }O

_{ 4 }is a quasi-one dimensional spin-1 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 double-chains of edge-sharing VO

_{ 6 }octahedra, where the particular octahedral staggering creates a zigzag-like 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 spin-1 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.

**RESERVED**

**Condensed Matter**

Thursday, December 10, 2009

10:30 AM

Physics Building, Room 313

## "Magnetic phase diagram of a spin-chain system Ca2+xY2-xCu5O10-d: oxygen hole-doping effects"

**Keeseong Park , Stony Brook University/ Brookhaven National Laboratory**

[Host: Despina Louca]

**Condensed Matter**

Thursday, December 3, 2009

4:00 PM

Physics Building, Room 204

## "Spin exchange interactions and magnetic properties"

**Mike Whangbo , NC State University**

[Host: Seunghun Lee]

_{ 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**

Monday, November 23, 2009

3:30 PM

Physics Building, Room 204

## "Non-Fermi liquid fixed point for an imbalanced gas of fermions in 1+ ε dimensions"

**Andrew James , University of Virginia**

[Host: Austen Lamacraft]

**Condensed Matter**

Thursday, November 19, 2009

4:00 PM

Physics Building, Room 204

## "Counting statistics of electron transport in nanostructures"

**Christian Flindt , Harvard**

[Host: Israel Klich]

**Condensed Matter**

Thursday, November 12, 2009

4:00 PM

Physics Building, Room 204

## "New Physics in Multicomponent Ultracold Atomic Gases"

**Ryan Barnett , Joint Quantum Institute**

[Host: Austen Lamacraft]

**RESERVED**

**Condensed Matter**

Thursday, October 22, 2009

4:00 PM

Physics Building, Room 204

## "Refrigeration using superconducting tunnel junctions"

**Sukumar Rajauria , NIST, Maryland**

[Host: Bellave Shivaram]

**Condensed Matter**

Thursday, October 15, 2009

4:00 PM

Physics Building, Room 204

## "Classical-quantum mappings for geometrically frustrated systems"

**Stephen Powell , Joint Quantum Institute**

[Host: Andrew James]

**Available**

**Condensed Matter**

Thursday, September 24, 2009

4:00 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 17, 2009

4:00 PM

Physics Building, Room 204

## "Toric-boson model: Toward a topological quantum memory at finite temperature"

**Alioscia Hamma , Perimeter Institute**

[Host: Israel Klich]

**Condensed Matter**

Thursday, September 10, 2009

4:00 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 3, 2009

4:00 PM

Physics Building, Room 204

**Available**

**RESERVED**

**Condensed Matter**

Thursday, May 7, 2009

3:30 PM

Physics Building, Room 204

## "Vertical nano-composite heteroepitaxial thin films with manganites and ferroelectrics"

**Yonghang Pei , University of Virginia**

[Host: Bellave Shivaram]

**RESERVED**

**Condensed Matter**

Monday, April 20, 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]

**Condensed Matter**

Thursday, April 16, 2009

4:00 PM

Physics Building, Room 204

## "The thin film Giaever transformer - vortex drag in a superconductor thin-film bilayer "

**Gil Refael , Caltech**

[Host: Israel Klich ]

**Condensed Matter**

Wednesday, April 15, 2009

3:30 PM

Physics Building, Room 204

## "Mechanical Behavior of a Ni-based Crystalline and a Zr-based Amorphous Materials Subjected to Surface Severe Plastic Deformation "

**Jiawan Tian , University of Tennessee**

[Host: Despina Louca]

**Condensed Matter**

Thursday, April 9, 2009

4:00 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**

Tuesday, April 7, 2009

3:30 PM

Physics Building, Room 204

## "Simple Elements at high densities: En-Route to metallic hydrogen and insulating lithium"

**Shanti Deemyad , Harvard University**

[Host: Despina Louca]

**Condensed Matter**

Monday, April 6, 2009

3:30 PM

Physics Building, Room 204

## "Neutron and X-ray scattering studies of frustrated, quantum, and multiferroic transition metal oxides"

**Junghwa Kim , University of Virginia**

[Host: Seunghun Lee]

## "A study on the spin-state transition and complex magnetic coupling in Perovskite Cobaltite"

**Juan Yu , University of Virginia**

[Host: Bellave Shivaram]

## "Understanding Magnetism in Multiferroics"

**Micky Holcomb , University of California, Berkeley**

[Host: Stu Wolf]

**Condensed Matter**

Thursday, March 19, 2009

4:00 PM

Physics Building, Room 204

## "Six-Port Reflectometer: an Alternative Network Analyzer for THz Region"

**Guoguang Wu , University of Virginia**

[Host: Bellave Shivaram]

**Condensed Matter**

Tuesday, March 17, 2009

3:30 PM

Physics Building, Room 204

## "Geometrical Interpretation of the Non-universal Casimir Energy of An Infinite Cylindrical Wedge"

**Hussain Zaidi , University of Virginia**

[Host: Genya Kolomeisky]

## "Orbital degrees of freedom on triangle-based lattice"

**Takuro Katsufuji , Waseda University**

[Host: Seunghun Lee ]

## "Single-molecule biophysics with a protein nanopore"

**Liviu Movileanu , Syracuse University**

[Host: Keith Williams]

**Condensed Matter**

Thursday, February 26, 2009

4:00 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, February 19, 2009

3:30 PM

Physics Building, Room 204

## "Imaging Dirac Fermions in a Two-dimensional Sheet of Carbon"

**Yuanbo Zhang , University of California, Berkeley**

[Host: Despina Louca]

**Condensed Matter**

Thursday, February 12, 2009

3:30 PM

Physics Building, Room 204

## "Quenching spin decoherence in diamond and single-molecule magnets"

**Susumu Takahashi , University of California, Santa Barbara**

[Host: Despina Louca]

**Condensed Matter**

Thursday, February 5, 2009

4:00 PM

Physics Building, Room 204

## "Dimensional Reduction at a Quantum Critical Point"

**Cristian Batista , Los Alamos National Laboratory**

[Host: Seunghun Lee]

_{ 2 }O

_{ 6 }(Han Purple).

**Condensed Matter**

Wednesday, February 4, 2009

3:30 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, January 29, 2009

4:00 PM

Physics Building, Room 204

## "Nanopores and nanofluidics for single DNA studies"

**Derek Stein , Brown University**

[Host: Keith Williams]

**Condensed Matter**

Monday, January 26, 2009

3:30 PM

Physics Building, Room 204

## "Combining ferroelectricity and magnetism: the low-energy electrodynamics"

**Diyar Talbayev , LANL**

[Host: Despina Louca]

**RESERVED**

**Condensed Matter**

Thursday, December 4, 2008

4:00 PM

Physics Building, Room 204

## "A Study of the Group-IV Diluted Magnetic Semiconductor GeMn"

**Melissa Commisso , University of Virginia**

[Host: Bellave Shivaram]

**Condensed Matter**

Thursday, November 20, 2008

4:00 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, November 13, 2008

4:00 PM

Physics Building, Room 204

**Available**

**RESERVED**

**Condensed Matter**

Monday, November 3, 2008

3:30 PM

Physics Building, Room 204

## "Time-reversal symmetry breaking and spontaneous anomalous Hall effect in Fermi fluids"

**Kai Sun , University of Illinois**

[Host: Paul Fendley]

**Reserved**

[Host: Dinko Pocanic]

## "Study of Silicon Devices by Inelastic Tunneling Spectroscopy"

**James Kushmerick , NIST**

[Host: Keith Williams]

**Available**

**Condensed Matter**

Thursday, October 9, 2008

4:00 PM

Physics Building, Room 313

## "Neutron scattering and magnetization studies of the kagome lattice antiferromagnet"

**Kit Matan , ISSP, U of Tokyo**

[Host: Seunghun Lee]

**Condensed Matter**

Thursday, October 2, 2008

4:00 PM

Physics Building, Room 204

## "Measurement-Only Topological Quantum Computation"

**Parsa Bonderson , Microsoft / UCSB**

[Host: Paul Fendley]

**Condensed Matter**

Thursday, September 25, 2008

4:00 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 18, 2008

4:00 PM

Physics Building, Room 204

**RESERVED**

**Condensed Matter**

Thursday, September 11, 2008

4:00 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 4, 2008

4:00 PM

Physics Building, Room 204

**Available**

**Available**

**Condensed Matter**

Thursday, April 24, 2008

4:00 PM

Physics Building, Room 204

## "Exploration of Novel Tunnel Barrier Materials for STT-RAM "

**Wei Chen , University of Virginia**

[Host: Jongsoo Yoon]

**Condensed Matter**

Thursday, April 10, 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]

**Condensed Matter**

Thursday, April 3, 2008

4:00 PM

Physics Building, Room 204

## "Effects of Short-Range Order on the Structure and Dynamics of Relaxor Ferroelectrics"

**Peter Gehring , NIST**

[Host: Seunghun Lee]

**Condensed Matter**

Thursday, March 27, 2008

4:00 PM

Physics Building, Room 204

## "Investigation of high temperature TE compounds"

**Jack Simonson , University of Virginia**

[Host: Jongsoo Yoon]

## "Fascinating Exotic Phenomena in Layered Ruthenates"

**Zhiqiang Mao , Tulane University**

[Host: Seunghun Lee]

_{ n+1 }Ru

_{ n }O

_{ 3n+1 }exhibit a rich variety of fascinating ordered ground states. Spin-triplet superconductivity, metamagnetic quantum criticality, itinerant ferromagnetism, antiferromagnetic Mott insulating, and half-metallic 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

_{ 1-x }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 floating-zone technique; this phase diagram exhibits significant new phenomena. We find a very unique magnetic state in close proximity to a two-dimensional 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 non-Fermi 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.

**RESERVED**

**Condensed Matter**

Thursday, February 28, 2008

4:00 PM

Physics Building, Room 204

## "Random Telegraph Signal in Carbon Nanotube Device"

**Jack Chan , University of Virginia**

[Host: Jongsoo Yoon]

**Condensed Matter**

Thursday, February 21, 2008

4:00 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, February 14, 2008

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Monday, February 11, 2008

3:30 PM

Physics Building, Room 204

## "Order by distortion and chiral magnetism in CdCr2O4"

**Gia-Wei Chern , Johns Hopkins University**

[Host: Seunghun Lee]

**Condensed Matter**

Thursday, February 7, 2008

4:00 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]

**Condensed Matter**

Thursday, January 31, 2008

4:00 PM

Physics Building, Room 204

## "Frustrated and Satisfied Ground States in Pyrochlore Magnets"

**Bruce Gaulin , McMaster University**

[Host: Seung-Hun Lee]

**RESERVED**

**Available**

**Condensed Matter**

Thursday, December 6, 2007

4:00 PM

Physics Building, Room 204

## "Giant Negative Magnetization in a Class of Layered Molecular-Based Magnets"

**Dr. Randy Fishman , Oak Ridge National Laboratory**

[Host: Jongsoo Yoon]

**Condensed Matter**

Thursday, November 29, 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]

**Condensed Matter**

Thursday, November 15, 2007

4:00 PM

Physics Building, Room 204

**Available**

## "Mesoscopic physics in a quantum magnet chromium"

**Yeong-Ah Soh , Dartmouth College**

[Host: Seunghun Lee]

**Condensed Matter**

Thursday, November 1, 2007

4:00 PM

Physics Building, Room 204

## "Classical and Quantum Frustrated Magnets "

**Yong Baek Kim , University of Toronto**

[Host: Seunghun Lee]

**RESERVED**

**Condensed Matter**

Thursday, October 18, 2007

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, October 11, 2007

4:00 PM

Physics Building, Room 204

## "Narrow Gap Semiconductors: spin splitting with no magnetic fields and more,….."

**Giti Khodaparast , Virginia Tech**

[Host: Keith Williams]

**Condensed Matter**

Thursday, October 4, 2007

4:00 PM

Physics Building, Room 204

## "Superconductivity in graphene-based structures"

**Igor Mazin , Naval Research Laboratory**

[Host: Seung-Hun Lee]

**Condensed Matter**

Thursday, September 27, 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]

_{ 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 lock-in 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 La-BaMnO

_{ 3 }and La-SrMnO

_{ 3 }, and compare these results with La-CaMnO

_{ 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**

Thursday, September 20, 2007

4:00 PM

Physics Building, Room 204

**Available**

**Condensed Matter**

Thursday, September 6, 2007

4:00 PM

Physics Building, Room 204

## "Resonant X-ray Scattering Study of Quadrupole-Strain Interactions in Rare-Earth Tetraborides "

**Sungdae Ji , UVA/NIST**

[Host: Seunghum Lee]

**Available**

**Condensed Matter**

Thursday, April 26, 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]

## "From fundamental understanding to predicting new nanomaterials for high capacity hydrogen storage and fuel cell technologies"

**Taner Yildirim , NIST**

[Host: Bellave Shivaram]

_{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 H-H bonds. The metal-hydrogen 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 nano-structures 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 TM-nanostuctures in nanoporous materials such as MOF to improve the life-cycle and kinetics of the predicted storage materials.

**Condensed Matter**

Thursday, March 29, 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]

## "Neutron Scattering Investigation of Excitations Spectrum in Doped and Undoped Spin-Ladders"

**Bella Lake , Hahn-Meitner Institut**

[Host: Seunghun Lee]

_{ 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 hole-doped 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 high-Tc 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 one-magnon and multi-magnon 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.

## "A unifying perspective on charge fractionalization"

**Alexander Seidel , NHMFL/Florida State**

[Host: Paul Fendley]

**Condensed Matter**

Thursday, February 15, 2007

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Wednesday, February 7, 2007

3:30 PM

Physics Building, Room 204

## "PATTERNS, STABILITY AND COLLAPSE FOR TWO-DIMENSIONAL BIOLOGICAL SWARMS"

**Maria D'Orsogna , UCLA**

[Host: Paul Fendley]

## " Mutual influence of vortices and quasiparticles in d-wave superconductors "

**Predrag Nikolic , Harvard**

[Host: Paul Fendley]

**Condensed Matter**

Thursday, November 30, 2006

4:00 PM

Physics Building, Room 204

## "Phase Transitions in Nano-dimensional Materials"

**Syed Qadri , U. S. Naval Research Laboratory**

[Host: Stu Wolf]

**Condensed Matter**

Thursday, November 23, 2006

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, November 16, 2006

4:00 PM

Physics Building, Room 204

## "Charge ordering instead of Jahn-Teller distortion"

**Daniel Khomskii , Universitaet zu Koeln, Germany**

[Host: Seunghun Lee]

**Condensed Matter**

Wednesday, November 15, 2006

3:30 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]

## "To tunnel or not to tunnel - or why clathrates are so fascinating"

**Veerle Keppens , University of Tennessee**

[Host: Joe Poon]

_{ 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 low-temperature ultrasonic attenuation, neutron-scattering, thermal conductivity and microwave absorption measurements, it provides clear evidence for the existence of a new type of four-well tunneling states.

## "Quantum phase transitions and magnon stability in gapped spin chains and ladders"

**Andrey Zheludev , Oak Ridge National Laboratory**

[Host: Seung-Hun Lee]

## "Graphene: symmetries, phase transitions, Hall effect"

**Igor Herbut , Simon Fraser University**

[Host: Seunghun Lee]

**Condensed Matter**

Wednesday, October 18, 2006

4:00 PM

Physics Building, Room 204

## "Low Energy Vibrational Excitations in Metallic Glasses"

**Ricardo Schwarz , Los Alamos National Laboratory**

[Host: Vittorio Celli]

_{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 T-dependence: 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 low-temperature measurements enabled us to identify the low-energy vibrational excitations responsible for these anomalies

## "Magnetic ordering in a half--polarized magnetization plateau of the pyrochlore antiferromagnet"

**Doron Bergman , UC Santa Barbara**

[Host: Seunghun Lee]

**Condensed Matter**

Thursday, September 28, 2006

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, September 14, 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]

## "Rapid Motion in the Plant Kingdom: Nature's Weapons of Mass Reproduction."

**Dwight Whitaker , Williams College**

[Host: Jongsoo Yoon]

## "Desynchronization and Spatial Effects in Multistrain Diseases"

**Leah Chock , Naval Research Laboratories**

[Host: Jongsoo Yoon]

**12th Annual National Physics Day Show**

Two shows: 6:00 pm and 7:15 pm.

Two shows: 6:00 pm and 7:15 pm.

Thursday, April 20, 2006

6:00 PM

Physics Building - McCormick Road, Room 203

## "National Physics Day Show"

**Steve Thornton, Michael Timmins, Rob Watkins , UVA**

[Host: University of Virginia Physics Department]

## "Quasiparticle Breakdown in a Quantum Spin Liquid"

**Igor Zaliznyak , Brookhaven National Laboratory**

[Host: Seunghun Lee]

## "SUPERCONDUCTIVITY - An Emerging Technology for Power Systems"

**Don Gubser , Naval Research Laboratory**

[Host: Stu Wolf]

## "Fate of the Josephson Effect in Thin-film Superconductors"

**Gil Refael , Caltech**

[Host: Victor Galitski]

## "Ultrafast Phase Transformation in Metals Induced by Laser Irradiation: a Molecular Dynamic Study"

**Zhibin Lin , UVA**

[Host: Jongsoo Yoon]

## "Field Theory Studies of a Unitary Fermi Gas"

**Matthew Wingate , University of Washington**

[Host: Hank Thacker]

******SPRING RECESS*******

## "Adsorption of Gases on Carbon Nanotubes"

**Aldo Migone , Southern Illinois University**

[Host: George Hess]

**Condensed Matter**

Thursday, February 23, 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]

## "Emergence of Magnetism from a Non-magnetic Mott Insulator"

**Daniel Phelan , UVA**

[Host: Despina Louca]

## "The Low Temperature Phase Diagram of the RKKY Model: Competing Interactions and Magnetic Percolation"

**Donald Priour , University of Maryland**

[Host: Victor Galitski]

## "Spin-dependent Properties of Silicon-based Epitaxial Structures"

**Frank Tsui , University of North Carolina, Chapel Hill**

[Host: Joe Poon]

## "Bosons on the Triangular Lattice: Mott Transition in the Presence of Geometrical Frustration"

**Anton Burkov , Harvard University**

[Host: Victor Galitski]

**Condensed Matter**

Thursday, November 24, 2005

4:00 PM

Physics Building, Room 204

******THANKSGIVING BREAK******

**Condensed Matter**

Thursday, November 17, 2005

4:00 PM

Physics Building, Room 204

## "Quantum Phases of the Extended Bose-Hubbard Hamiltonian: Possibility of a Supersolid State of Cold Atoms in Optical Lattices"

**Vito Scarola , University of Maryland**

[Host: Victor Galitski]

**Condensed Matter**

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**

Thursday, September 29, 2005

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, September 22, 2005

4:00 PM

Physics Building, Room 204

## "Current-Induced Spin Polarization of 2DEG in Perpendicular Magnetic Field"

**Maxim Vavilov , Yale University**

[Host: Jongsoo Yoon]

## "Novel, Collective Insulating Phase in 2D Superconductors in High Magnetic Fields"

**Sambandamurhy Ganapathy , Magnet Lab, Tallahassee and Princeton University**

[Host: Jongsoo Yoon]

## "Novel Optical Studies of Ion-erosion, Growth, and Diffusion on Metal Surfaces**"

**Petros Thomas , University of California at Davis**

[Host: George Hess]

## "Unusual Magnetic State in MnSi under Hydrostatic Pressure"

**Dmitry Reznik , Institute of Solid State Physics, Forschungszentrum Karlsruhe**

[Host: Seunghun Lee]

## "Topological Defects and Fractionally Quantized Vortices in Nanomagnets"

**Oleg Tchernyshyov , John Hopkins**

[Host: Paul Fendley]

## "Control of Electric Polarization by Using Magnetic Field in Magnetic Oxides with Long-Wavelength Magnetic Structures"

**Tsuyoshi Kimura , Los Alamos National Laboratory**

[Host: Seung-Hun Lee]

**Joint CM/Atomic Seminar**

Thursday, April 7, 2005

4:00 PM

Physics Building, Room 204

## "Antiferromagnetic Order as a Competing Ground State in Electron-doped High -Tc Superconductors"

**Pengcheng Dai , University of Tennessee/Oak Ridge National Laboratory**

[Host: Seung-Hun Lee]

## "How to Relax Frustrated Systems: Spin and Orbital Physics in V and Cr Spinels"

**Yuki Motome , RIKEN**

[Host: Seung-Hun Lee ]

## "Dipole-Dipole Excitation and Ionization in an Initially Frozen Rydberg Gas"

**Wenhui Li , UVA**

[Host: Tom Gallagher]

^{3}dependence on interatomic spacing R. The dipole-dipole 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 dipole-dipole forces lead to motion, which must be considered in proposed applications.

**Condensed Matter**

Thursday, February 24, 2005

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, February 17, 2005

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, February 10, 2005

4:00 PM

Physics Building, Room 204

## "Vortex Dynamics and Fluctuations Near the Magnetic Field Tuned Superconductor-Insulator Transition"

**Victor Galitski , Santa Barbara**

[Host: Paul Fendley]

## "Superfluid-Insulator Transition in a Moving System of Interacting Bosons"

**Anatoli Polkovnikov , Harvard University**

[Host: Paul Fendley]

## "Non-equilibrium Transport Through a Phonon Coupled Molecule"

**Aditi Mitra , Columbia University**

[Host: Paul Fendley]

## "Continuous Tuning of 2D-Superconductivity in Strontium Titanate Field Effect Transistors"

**Feng Pan , University of Colorado**

[Host: Jongsoo Yoon]

**Condensed Matter**

Thursday, November 18, 2004

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, November 11, 2004

4:00 PM

Physics Building, Room 204

## "Magnetic, Transport, and Thermodynamic Studies of Weakly Magnetic Systems of Transition Metal Oxides"

**Zhixian Zhou , FSU**

[Host: Jongsoo Yoon]

**Joint Materials Science and Condensed Matter Physics**

Thursday, September 30, 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]

## "Center for Spins in Nanotechnology -CeSpIN -a proposal for a new interdisciplinary center at UVa"

**Stuart Wolf , DARPA**

[Host: William A. Jesser]

## "Future Nanoelectronics: Materials Science and Physics"

**John R. Tucker , University of Illinois**

[Host: William A. Jesser]

## "Spintronics: Fundamentals and Applications"

**Igor Zutic , University of Maryland**

[Host: Olivier Pfister]

**Condensed Matter**

Thursday, February 26, 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]

**Special Condensed Matter Seminar.**

Monday, February 23, 2004

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, February 19, 2004

4:00 PM

Physics Building, Room 204

## "Fluctuations in electronic, spintronic and photonic systems"

**Eugene Mishchenko , Harvard University**

[Host: Paul Fendley]

**Condensed Matter**

Thursday, February 12, 2004

4:00 PM

Physics Building, Room 204

## "Multi-walled Carbon Nanotube-based electromechanical oscillators"

**Stergios Papadakis , UNC**

[Host: Yongsoon Yoon]

**Special Condensed Matter Seminar. Please note special time and day**

Wednesday, February 11, 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]

**Special Condensed Matter Seminar. Please note special day and time.**

Tuesday, February 10, 2004

4:00 PM

Physics Building, Room 204

## "Non-Abelian Anyons and Topological Order in Solids"

**Kirill Shtengel , Microsoft**

[Host: Paul Fendley]

## "Nernst effect: from simple to correlated metals"

**Vadim Oganesyan , Princeton University**

[Host: Paul Fendley]

**Condensed Matter**

Wednesday, February 4, 2004

3:30 PM

Physics Building, Room 204

## "Anisotropic Quantum Correlations in Nanostructures"

**Igor Altfeder , Harvard University**

[Host: Joe Poon]

## "Playing with Superconductivity and Magnetism in Nanoscale"

**Jiyeong Gu , Argonne National Laboratory**

[Host: Jongsoo Yoon]

## "Radiation-Induced Magnetoresistance Oscillations in a 2D Electron Gas"

**Adam Durst , Yale University**

[Host: Paul Fendley]

## "Magnetic Structures and Magnetovolume Anomalies in R2Fe17 intermetallic compounds"

**Oleksandr Prokhnenko , Institute of Physics, Academy of Sciences of the Czech Republic**

[Host: Despina Louca]

## "Opportunities for new Physics in O-D/1-D Hybrids"

**Dr. Keith Williams , Delft Universities**

[Host: Joseph Poon]

**Joint Condensed Matter and Chemical Physics Seminar**

Thursday, December 18, 2003

4:00 PM

Physics Building, Room 204

## "Scanning Tunneling Spectroscopy of Nanostructures: Mirages in Quantum Corrals"

**J. W. Gadzuk , NIST**

[Host: Joseph Poon and Ian Harrison]

## "Statistical mechanics methods for genome-wide modeling of translation"

**Dr. Leah Shaw , Cornell University**

[Host: E. Kolomeisky]

## "Novel Radiation-induced zero-resistance states in high mobility two-dimensional electron systems"

**Dr. Ramesh Mani , Harvard University**

[Host: Paul Fendley]

## "Atomic Scale Structure of Giant Magnetoresistance and Spin Tunnel Junction Multilayers"

**Dr. Xiaowang Zhou , UVA- Engineering**

[Host: J. Ruvalds]

**Condensed Matter**

Thursday, September 25, 2003

4:00 PM

Physics Building, Room 204

## "Playing with dimensionality - magnetism and transport in hydrides"

**Dr. Bjorvin Hjovarson , Uppsala, Sweden**

[Host: B. Shivaram]

**Condensed Matter**

Thursday, September 18, 2003

4:00 PM

Physics Building, Room 204

**SPECIAL NUCLEAR/PARTICLE PHYSICS SEMINAR**

Thursday, September 11, 2003

4:00 PM

Physics Building, Room 204

## "Nuclear Modification of Jet Fragmentation"

**Dr. Xin-Nian Wang , Lawrence Berkeley Lab**

[Host: Blaine Norum]

## "The physics of magnetoresistive random access memory (MRAM) based on magnetic tunnel junctions"

**Nick Rizzo , Motorola Labs**

[Host: Bellave Shivaram]

**Special Condensed Matter Seminar**

Friday, July 18, 2003

3:30 PM

Physics Building, Room 313

## "X-ray Microscopy using synchrotron radiation -- a tool of nanotechnology"

**Dr. Hwa Shik Youn , Beamline Division Head, Pohang Accelerator Laboratory, Pohang, Korea**

[Host: George Hess]

## "Hydrogen in vacuum systems and SRF cavities"

**Dr. G. Myneni , Thomas Jefferson National Accelerator Lab**

[Host: Belave Shivaram]

## "Regulated Self-assembly of Silicon-Germanium Quantum Dots"

**Tom Vandervelde , UVA**

[Host: Jongsoo Yoon]

## "Measurement of Thermal Properties at Mesoscopic Scales"

**Pillip Kim , Columbia University**

[Host: Jongsoo Yoon]

## "Anomalous magnetoresistance effect in ultrathin manganite films"

**Qi Li , Penn State Univ.**

[Host: J. Yoon]

## "Emergent excitations and novel phase transitions in geometrically frustrated magnets"

**Seunghun Lee , National Institute of Standards and Technology**

[Host: Despina Louca]

## "ELECTROACTIVE POLYMERIC AND ORGANIC MATERIALS FOR THIN-FILM-TRANSISTOR APPLICATIONS"

**Andrew J. Lovinger , Bell Laboratories, Lucent Technologies**

[Host: Bellave S. Shivaram]

**Condensed Matter**

Thursday, February 27, 2003

3:30 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, February 20, 2003

3:30 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, February 13, 2003

2:30 PM

Physics Building, Room 313

## "The Effect of Quenched Impurities on First-Order Transitions"

**John Cardy , IAS and Oxford University**

[Host: Paul Fendley]

## "Valence bond liquids and solids in geometrically frustrated magnets"

**Oleg Tchernyshyov , Johns Hopkins**

[Host: Paul Fendley]

**Condensed Matter**

Thursday, November 14, 2002

3:30 PM

Physics Building, Room 204

## "Ground-state properties of one-dimensional matter and quantum dissociation of a Luttinger liquid "

**Eugene Kolomeisky , University of Virginia**

[Host: John Ruvalds]

## "Destructive regime and superconductor-normal metal transition in ultrathin cylinders"

**Prof. Ying Lu , Penn State**

[Host: Y. Yoon]

## "Atomically Engineered Active Sites and Environments for Supported Metal Catalysts"

**Prof. Matthew Neurock , UVA - Chemical Engineering**

[Host: J. Poon]

## "Superconductor Divices"

**Prof. A. Lichtenberg , UVA-Superconducting Materials, Device and Circuit Research for THz Receivers**

[Host: Bascom Deaver]

## "Photomission Spectroscopy of High Temperature Superconductors"

**Prof. D. Dessau , Univ. of Colorado**

[Host: J. Ruvalds]

## "The Materials Research Science and Engineering Center at UVA: From Materials to Quantum Cellular Automata"

**Prof. Robert Hull , UVA Materials Sciece and Engineering, MRSEC Director**

[Host: J. Ruvalds]

**Condensed Matter**

Thursday, September 12, 2002

4:00 PM

Physics Building, Room 204

**Condensed Matter**

Thursday, September 5, 2002

4:00 PM

Physics Building, Room 204

## "Quantum fluctuations of charge and phase transitions of alarge Coulomb-blockaded quantum dot"

**Xiaoya Qi , University of Virginia**

[Host: D. Louca]

## "Probing Superconductors with Infrared - Electron-boson coupling in high temperature superconductors"

**Jiufeng Tu , Brookhaven National laboratory**

[Host: D. Louca]

_{2}with a T

_{c}of 39K and electric field doped C

_{60}with a T

_{c}as high as 117K. Infrared spectroscopy has emerged as one of the most powerful experimental tools for the study of correlated electron systems and for high-T

_{c}superconductors in particular. This talk will be focused on the infrared studies of two representative high-T

_{c}superconductors: MgB

_{2}(T

_{c}= 39.6 K) and optimally doped Bi

_{2}Sr

_{2}CaCu

_{2}O

_{8}

_{+}

_{¦Ä}(T

_{c}= 91.5 K). Effects of electron-boson coupling are observed in optical conductivities for both systems and their significance with respect to superconductivity will be discussed. In general, having a small free carrier plasma frequency (< 3 eV) seems to be an universal characteristic shared by almost all high-temperature superconductors with a T

_{c}> 30 K, which means that the issue of reduced screening should be treated carefully in all of these systems.

**Joint Condensed Matter-Chemical Physics Seminar**

Thursday, February 7, 2002

4:00 PM

Physics Building, Room 204

## "Combinatorial approach to materials discovery "

**Professor I. Takeuchi , University of Maryland**

[Host: Joseph Poon and Ian Harrison]

**Joint Condensed-Matter/High Energy Seminar**

Wednesday, February 6, 2002

3:00 PM

Physics Building, Room 204

## "On the Bethe Ansatz Solution of the Two-Channel Anderson Impurity Model"

**Carlos Bolech-Gret , Rutgers**

[Host: P. Fendley]

**Condensed Matter**

Wednesday, January 30, 2002

3:00 PM

Physics Building, Room 204

## "Non-conventional metals: odd-frequency density waves and d-density wave"

**Eugene Pivovarov , Caltech**

[Host: P. Fendley]

**Note: Special Condensed Matter Seminar**

Monday, January 28, 2002

4:00 PM

Physics Building, Room 204

## "Advection of Particles by Burger's Turbulence"

**Barbara Drossel , Darmstadt(Germany)**

[Host: Alan McKane]

## "Photonic Bandgap Crystal, Flattop Narrow-Band Filter, Modulators, and Switches"

**Phuc Tran , China Lake/Phillip Morris**

[Host: V. Celli]

## "Carbon Nanotubes: a playground for Luttinger liquid physics"

**Smitha Vishveshwara , UCSB**

[Host: E. Kolomeisky]

## "Electron-nuclear double resonance-mediated dynamic nuclear polarization"

**Arthur Brill , University of Virginia**

[Host: D. Louca]

**31st Annual Llewellyn G. Hoxton Lecture**

Thursday, April 26, 2001

7:30 PM

Chemistry Building, Room Chemistry Auditorium

## "Electronics in the Internet Age"

**Justin Rattner , Intel Fellow and Director of Intel's Microprocessor Research Lab**

## "Molecular impurities in He clusters: microsolvation shells and superfluidity"

**F. A. Gianturco , University of Rome**

[Host: V. Celli]

^{3}to about 10

^{5}He atoms, picks up in flight one or more atoms or molecules (neutral or ionized) which can then be studied downstream with laser spectroscopy. The corresponding microscopic understanding of the possible structures of the weakly-interacting

^{4}He atoms around or "outside" the molecular impurities requires the treatment of quantum effects on nuclear motions and the selection of the main, dominant structures within each cluster as a function of its size. In the present work we present new results for the quantum structures and the vibrational and rotational shifts for an OCS molecule in

^{4}He clusters using the Diffusion Monte Carlo (DMC) method. A comparison with the recent results [S. Grebenev et al., J. Chem. Phys. 112, 4485 (2000)] from the experimental group in Göttingen will also be reported.

## "X-ray Diffraction and Spectroscopic Investigations of Nanophased Iron"

**Rama Balasubramanian , James Madison University**

[Host: D. Louca]

**Joint Physics & Chemistry Colloquium - Alan T. Gwathmey Seminar Series**

Thursday, April 5, 2001

4:00 PM

Chemistry Building, Room 304

## "Recent Advances in Single Molecular Manipulation in Biophysics"

**Prof. Carlos Bustamante , University California Berkeley**

[Host: Ian Harrison]

## "Growth and Optical Properties of III-Nitride Wide Bandgap Semiconductors"

**Professor Jingyu Lin , Kansas State University**

[Host: E. Kolomeisky and J. Poon]

**Special Condensed Matter**

Monday, March 26, 2001

4:00 PM

Physics Building, Room 204

## "Carbon nanotube quantum wires and quantum dots"

**David Cobden , Univ. of Warwick**

[Host: Despina Louca]

## "GaAs Varactor Based Frequency Tripler Technology"

**Gerhard Schoenthal , University of Virginia**

[Host: Eugene Kolomeisky]

**Condensed Matter**

Thursday, February 22, 2001

4:00 PM

Physics Building, Room 204

## "The Density of States in the Quantum Critical Regime of the Metal-Insulator Transition"

**Dr. Winfried Teizer , University of California at San Diego**

[Host: J. Poon]

*quantum critical regime*, Coulomb interactions modify the density of states at the Fermi level and ultimately open a soft Coulomb gap since the decreasing electron mobility inhibits efficient screening. I will present tunneling spectroscopy data on amorphous Gd

_{x}Si

_{1}

_{-}

_{x}

_{,}a material that can be continuously and reversibly tuned through the metal-insulator transition. On the metallic side, we find a signature of strong Coulomb interactions in the density of states, as theoretically expected. As the metal-insulator transition is approached from the metallic side, the tunneling spectrum shows a precursor of a soft Coulomb gap prior to reaching the insulating regime. I will extract the quantitative relationship of the density of states at the Fermi energy and the transport conductivity in the quantum critical regime.

## "Resonant Inelastic X-ray Scattering from Insulating Cuprates"

**Peter Abbamonte , University of Groningen**

[Host: D Louca]

## "Quantum Wigner crystal and apparent metal-insulator transition of dilute 2D holes in GaAs at B=0"

**Dr. Jongsoo Yoon , University of California at Berkeley**

[Host: B. Shivaram]

**Condensed Matter**

Thursday, November 30, 2000

4:00 PM

Physics Building, Room 204

## "Thermoelectric Properties in Sb doped TiNiSn Half-Heusler Alloys"

**Yu Xia , University of Virginia**

[Host: Joseph Poon]

**Condensed Matter**

Thursday, November 16, 2000

4:00 PM

Physics Building, Room 204

## "Nucleation When Diffusion Becomes Important: A Tale of Two Fluxes"

**Professor Ken Kelton , Department of Physics - Washington University**

[Host: Joseph Poon]

**Condensed Matter**

Thursday, September 28, 2000

4:00 PM

Physics Building, Room 204

## "Turbulent Shear Flow: Analytic Result for a Universal Amplitude"

**Amit Chattapadhyay , UVA-Chemistry**

[Host: Eugene Kolomeisky]

**Condensed Matter**

Thursday, September 21, 2000

4:00 PM

Physics Building, Room 204

## "The hidden world of the diffusion equation"

**Timothy Newman/**

**P0STP0NED**, University of Virginia, Physics and Biology[Host: Eugene Kolomeisky]

## "Thermoacoustic Refrigeration"

**Steven Garrett , United Technologies Corporation Professor of Acoustics - Penn State University**

[Host: Bellave S. Shivaram]

**SPECIAL HIGH ENERGY PHYSICS SEMINAR**

Tuesday, May 30, 2000

3:00 PM

Physics Building, Room 313

## "The Measurement of Lambda Polarization at Nomad Experiment"

**Kamal Benslama , University of Regina**

[Host: Craig Dukes]

## "Soft Matter in a Tight Spot: The Structural Transition and the Molecular Origin"

**Prof. Shenting Cui , Dept. of Chemical Engineering at Univ. of Tenn. and Chemical Technology Div. at Oak Ridge Lab.**

[Host: R. E. Johnson]

**Special AMO/Chemical Physics Seminar**

Thursday, March 30, 2000

4:00 PM

Physics Building, Room 204

## "Characterization and Control of Ultracold Collisions"

**Steve Gensemer , University of Connecticut**

[Host: Robert Jones]

## "Progress with SQUID Gradiometers and their Application to Biomagnetism and Non-destructive Testing"

**Professor Gordon Donaldson , University of Strathclyde**

[Host: Bascom Deaver]

## "Holography and the Cosmological Constant Problem"

**George Minic , University of Southern California - (USC)**

[Host: Ziad Maassarani]

**Joint Condensed Matter and Chemical Physics Seminar**

Thursday, February 24, 2000

4:00 PM

Physics Building, Room 204

## "Materials Research at Jefferson Lab"

**Dr. Edward Gilman , Jefferson Laboratory and Norfolk State University**

[Host: Joseph Poon]

## "Crossover Critical Phenomena in Fluids"

**Anna Wyczalkowska , University of Maryland**

[Host: Despina Louca]

## "Directed Polymers and Oppressive Population Control"

**Timothy Newman , University of Virginia - Physics**

[Host: E. Kolomeisky]

**Condensed Matter**

Thursday, November 18, 1999

4:00 PM

Physics Building, Room 204

**SPECIAL CONDENSED MATTER SEMINAR**

Tuesday, November 16, 1999

4:00 PM

Physics Building, Room 204

## "NSOM Studies of SiN Membranes and Photonic Bandgap Structures"

**Anthony Campillo , University of Virginia - Physics**

**Joint Condensed Matter and Chemical Physics Seminar**

Wednesday, February 24, 1999

4:00 PM

Physics Building, Room 204

## "Materials Research at Norfolk State University"

**Dr. Edward Gilman , Norfolk State and Jefferson Laboratory**

[Host: Joseph Poon]

To add a speaker, send an email to gc6u@Virginia.EDU Include the seminar type (e.g. Condensed Matter Seminars), date, name of the speaker, title of talk, and an abstract (if available). [Please send a copy of the email to phys-speakers@Virginia.EDU.]