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 Physics at Virginia

"Voxelated Bioprinting: Digital Assembly of Viscoelastic Bio-ink Particles"


Liheng Cai , University of Virginia
[Host: Bellave Shivaram]
ABSTRACT:

Analogues of pixels to two-dimensional (2D) pictures, voxels –– in the form of small cubes or spheres –– are the basic units of three-dimensional (3D) objects. Digital assembly of bio-ink voxels may provide an approach to engineering heterogeneous yet tightly organized 3D tissue mimics. However, this approach requires precisely manipulating highly viscoelastic bio-ink voxels in 3D space, which represents a grand challenge in both soft matter science and biomanufacturing. In this talk, I will introduce a voxelated bioprinting technology that enables the Digital Assembly of Spherical bio-ink Particles (DASP). First, I will discuss the criteria for the on-demand generation, disposition, and assembly of viscoelastic bio-ink droplets in an aqueous environment without the help of large interfacial tension. Second, I will describe how to use DASP to create 3D structures consisting of interconnected yet distinguishable bio-ink particles. Finally, I will share our recent progress in applying DASP to encapsulate islets into multiscale porous scaffolds to treat type 1 diabetes. I will also discuss immediate applications and emerging challenges associated with voxelated bioprinting.

Condensed Matter Seminar
Thursday, April 11, 2024
3:30 PM
, Room TBA
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"TBA"


Ephraiem Sarabamoun , University of Virginia
[Host: Josh Choi]
ABSTRACT:

TBA

Condensed Matter Seminar
Thursday, March 28, 2024
3:30 PM
Gibson Hall, Room 211
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"TBA"


Chau Dao , UCLA
[Host: Gia-Wei Chern]
ABSTRACT:

TBA

Condensed Matter Seminar
Thursday, March 21, 2024
3:30 PM
Gibson Hall, Room 211
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ABSTRACT:

Despite identical R3 crystal structures, honeycomb layered MTiO3 ilmenites exhibit diverse magnetic orders and transition temperatures (TN): G-type antiferromagnetic for MnTiO3 (TN=68 K) and A-type antiferromagnetic for CoTiO3(TN=38 K) and NiTiO3 (TN=22 K). This work focuses on this intriguing interplay between local structure, electronic properties, and magnetic configurations. CoTiO3 has two magnon peaks around 5-14 meV with a distinct gapless Dirac node nestled between them are observed and the magnon modes are renormalized to lower energies. For CoTiO3, magnetic excitations attributed to spin-orbit exciton multiplet transitions show the same temperature dependance as magnon with the intensity dissipating quickly above TN.  The energy levels arising from crystal field and spin-orbit coupling are gradually thermally populated through T and reaching maximum at 100 K. However, the NiTiO3 system shows a single low energy magnon peak around 2-4 meV which is renormalized into lower energies, but it does not show Dirac magnon properties. The calculated exchange interactions using SpinW confirm the weaker inter-plane interaction in CoTiO3 than NiTiO3. Across three system, both transition metal M+2 ion and Ti+4 ions are in distorted octahedra environment, and the first four nearest neighbors are Ti-O < M-O < Ti-O < M-O with the given bond length order. Across three systems Ti-O and short M-O bond length variation is minimum. However, M-O bond length (MnTiO3=2.28 Å, CoTiO3=2.17 Å and NiTiO3=2.12 Å) variation is significant which follows the same variation as reported dielectric constants (MnTiO3=20.4, CoTiO3=19.5 and NiTiO3=17.8 ) and TN and confirms the interplay between these parameters. Within the measured 100 K to 500 K, temperature dependance of local structure is insignificant and for the reported relative dielectric values, the variation is almost constant. This suggests that the interplay between local geometry and magnetic interactions governs the diverse behaviors observed in these honeycomb materials.

Condensed Matter Seminar
Thursday, February 22, 2024
11:00 AM
Physics, Room 323
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"Exploring emergent quantum phases in two-dimensional flat band systems"


Jiang-Xiazi Lin , Brown University
[Host: Seunghun Lee]
ABSTRACT:

Quantum phases such as superconductivity and ferromagnetism are among the most important topics in condensed matter physics research. Recently, a family of two-dimensional flat band systems, including magic-angle twisted graphene, uncovered an abundance of symmetry breaking and novel quantum phases.

In this talk, I will introduce the recent advances in these materials and give two examples of how we engineered and revealed new quantum phases of matter in twisted graphene. These include an orbital ferromagnetic state induced by spin-orbit coupling and a zero-field superconducting diode effect. Towards the end of the talk, I will mention our on-going effort of studying a new type of Coulomb-driven rotational symmetry breaking state in the moiré-less bilayer graphene. These examples establish the two-dimensional flat band systems as a versatile platform with multiple tuning knobs, where new physics emerges from the interplay between various quantum phases.

Condensed Matter Seminar
Monday, February 12, 2024
2:00 PM
Physics, Room 323
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A recording of this talk is available at this link (use passcode #D8FWkr?).


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"Ultranodal state in multiband spin-1/2 superconductors"


Peter Hirschfeld , University of Florida
[Host: Bellave Shivaram]
ABSTRACT:

Recent measurements on the tetragonal phase of the iron-based superconductor FeSe,S support the existence of a remarkable phase where the superconducting state supports a finite residual  density of states arising from patchlike nodal surfaces[1,2].  This ``ultranodal"> state can arise in situations where conventional intraband spin singlet pairing is highly anisotropic and coexists with time-reversal symmetry breaking  interband spin triplet interactions [3].  Here I present a  microscopic scenario including ferromagnetic interactions that can account for nonunitary pairing and C4 symmetry breaking in the superconducting state that is also observed in recent experiments.

 

1) Sato, Y. et al. Abrupt change of the superconducting gap structure at the nematic critical point in FeSe1-xSx. Proc. Natl Acad. Sci. 115, 1227??1231 (2018).

2) Hanaguri, T. et al. Two distinct superconducting pairing states divided by the nematic end point in FeSe1-xSx. Sci. Adv. 4, eaar6419 (2018).

3) ``Topologically protected ultranodal state in iron-based superonductors", S. Setty, S.

Bhattacharyya, Y. Cao, A. Kreisel and P.J. Hirschfeld,  Nat. Comm. 11, 523 (2020).

 

Condensed Matter Seminar
Thursday, February 8, 2024
3:30 PM
Physics, Room 323
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"Nanoscale quantum sensing of programmable quantum matter"


Shaowen Chen , Harvard University
[Host: Seunghun Lee]
ABSTRACT:

Characterization and quantum control of complex quantum matter is one of the shared goals for condensed matter and quantum information science research. Toward this end, my research uses van der Waals materials to synthesize topological and correlated states, and quantum sensors based on spin defects to uncover their microscopic picture. Focusing on superconductivity as the theme of this talk, I will first present pathways to program the electron correlation by exploiting the lattice degree of freedom, both in the planar and vertical directions of moiré materials. The challenges to fully characterize the moiré superconductivity will be discussed. In the second part, I will show new experimental observables unlocked by the nanoscale quantum sensing platform can uncover hidden physics. As an example, quantitative visualization of the super current flow in a Josephson junction is used to reveal electrically configurable ground states in the zero-resistance regime. A surprising role of the kinetic inductance and the implications for the Josephson diode effect will be discussed. Finally, I will share my vision to explore intertwined topology and correlation by integrating the programmable quantum materials with nanoscale quantum sensors.

Condensed Matter Seminar
Thursday, February 1, 2024
3:30 PM
Physics, Room 323
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A recording of this talk is available at this link (enter passcode *0m4DSym).


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"Novel Fabrication of Quantum Wires: Towards Fractionalized Excitations"


Tomoya Asaba , Kyoto University
[Host: Seunghun Lee]
ABSTRACT:

The quest for novel quantum states in condensed matter physics often hinges on the reduction of system dimensionality. In particular, one-dimensional systems are theoretically predicted to host a range of fractionalized excitations. These include the Tomonaga-Luttinger liquid, which exhibits spin and charge separation, and the Majorana particle, a cornerstone for fault-tolerant quantum computing. However, fabricating near-perfect one-dimensional quantum wires has been a significant challenge, especially those involving strongly correlated electrons.

In our research, we have developed a novel method to fabricate quantum wires of a Mott insulator on graphite substrates using pulsed-laser deposition, achieving structures such as stripes, junctions, and nanorings. These single-crystalline wires are one unit cell in thickness and precisely two to four unit cells in width, and can extend to several micrometers in length. The spectroscopy measurements along with theoretical calculations reveal the existence of strong electron correlations in this system. Moreover, our findings emphasize the importance of nonequilibrium reaction-diffusion processes in atomic-scale self-organization, opening up exciting avenues for the exploration of exotic fractionalized states in purely one-dimensional quantum wires.

Condensed Matter Seminar
Monday, January 29, 2024
2:00 PM
Physics, Room 323
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A recording of this talk is available at this link (enter passcode ^Sa3J2OZ).


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"Next-generation artificial van der Waals quantum materials "


Dr. Bumho Kim , University of Pennsylvania
[Host: Seunghun Lee]
ABSTRACT:

If one can control the atomic symmetries of a material at will, the intrinsic properties of the material will be significantly modified. However, the atomic structures of conventional materials are often constrained by the equilibrium phase of matter. Here, we overcome this fundamental limitation using recent advances in twistronics, enabling precise control over all the individual point group symmetry elements – inversion, mirror, and rotational symmetries – in twisted van der Waals (vdW) material in a new 3D configuration [1]. The resulting 3D twisted materials exhibit emerging optical responses that are fundamentally different from those of natural vdW materials. This novel approach to control symmetries can enable nearly infinite vdW quasicrystalline phases, promising a practical platform to study less-explored structure-property relationships of quasicrystals. In addition, we will discuss an ultraclean vdW crystal synthesis method [2]. A self-flux synthesis method we developed has yielded vdW materials with ~ 2 orders of magnitude lower point defect density compared to commercial vdW materials grown by a chemical vapor transport method. These ultraclean vdW materials reveal intrinsic excitonic properties that were previously obscured by low-quality materials. The combination of these ultraclean materials with the symmetry design approach holds great promise for the development of high-performance artificial material systems for next-generation technologies.

 

References:

 

  1. Bumho Kim, Jicheng Jin, Zhi Wang, Li He, Thomas Christensen, Eugene J. Mele, and Bo Zhen, Nature Photonics 18, 91-98 (2024).
  2. Bumho Kim, Yue Luo, Daniel Rhodes, Yusong Bai, Jue Wang, Song Liu, Abraham Jordan, Baili Huang, Zhaochen Li, Takashi Taniguchi, Kenji Watanabe, Jonathan Owen, Stefan Strauf, Katayun Barmak, Xiaoyang Zhu, and James Hone, ACS Nano 16, 140-147 (2022).

 

Condensed Matter Seminar
Thursday, January 25, 2024
3:30 PM
Physics Building, Room 323
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A recording of this talk is available at this link (enter the passcode @mJua4k1).


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"Room-temperature and many-body quantum states in topological materials "


Md. Shafayat Hossain , Princeton University
[Host: Seunghun Lee]
ABSTRACT:

Topological states of matter combine quantum physics with topology—a branch of mathematics that explores geometric properties preserved under deformation. Quantum topology can lead to incredible properties. For instance, in a topological insulator, conducting edge states exist within an insulating bulk. Despite continuing progress, the search for such new quantum phases remains a central theme of condensed matter physics. In this talk, I will introduce two of the most sought-after quantum states—room-temperature topology and topological exciton insulator. I will first discuss our spectroscopic observation of topological edge states in Bi4Br4. I will show that the topological states, which typically can only be observed at temperatures around absolute zero, survive here at room temperature. I will also show how we probe the quantum transport response of this edge state using quantum interference. These observations mark the first steps in demonstrating the potential of topological materials for energy-saving applications. In the second part of my talk, I will discuss our discovery of a unique topological state in Ta2Pd3Te5. Here, the Coulomb interactions pair fermions (electrons and holes) into bosons (excitons), leading to a superfluid condensate state in the bulk while hosting topological edge states on the boundary. Finally, I will touch upon how these discoveries suggest exciting possibilities. This includes new devices and experimental techniques to discover the fundamental physics of topological quantum matter, opening doors for more efficient room-temperature devices and quantum information technology. 

Condensed Matter Seminar
Monday, January 22, 2024
2:00 PM
Physics, Room 323
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"Nonlinear Optical Spectroscopies for Resolution of Electronic Structure and Dynamics"


Veronica Policht , U.S. Naval Research Laboratory
[Host: Despina Louca]
ABSTRACT:

Rapid and efficient charge transfer following absorption of light is a process of intense interest from the
perspectives of both fundamental physics and optoelectronic applications. Among the exciting systems which
host charge transfer are photosynthetic reaction centers (RC), proteins packed with light-absorbing molecules
which yield a charge separated state with near unity quantum efficiency, and Transition Metal Dichalcogenide
Heterostructures (TMD HS), which host interlayer charge transfer to form spatially separated interlayer
excitons. Despite intense interest in understanding charge transfer in these systems, their complex electronic
structure and the ultrafast timescales of their dynamics have presented a significant challenge in clearly
resolving the underlying fundamental physics. Two-Dimensional Electronic Spectroscopy (2DES) is a nonlinear
optical spectroscopic technique with simultaneously high frequency and temporal resolution and is an ideal
tool for studying systems with complex electronic structure and femtosecond-timescale dynamics. In this talk I
will present on my work applying 2DES to resolving the excitonic structure of photosynthetic RCs as well as
resolving ultrafast interlayer exciton dynamics in TMD HS.

Condensed Matter Seminar
Thursday, January 18, 2024
3:30 PM
Gibson Hall, Room 211
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A recording of this talk is available at this link (enter the passcode UEd*U6Wr).


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"Ising Superconductivity and Nematicity in Bernal Bilayer Graphene with Strong Spin-Orbit Coupling"


Ludwig Holleis , University of California Santa Barbara
[Host: Bellave Shivaram]
ABSTRACT:

Superconductivity is an almost ubiquitous feature in the low temperature phase diagram of multilayer graphene allotropes – moire or crystalline. While the microscopic electronic structures of these systems differ, supporting devices with monolayer WSe2 has been shown to increase superconductivity along many axes of the phase space like density, magnetic field and temperature. Here, we study two superconducting domes (SC1 and SC2) in Bernal Bilayer graphene on WSe2 in light of their resilience to in-plane magnetic fields. While SC1 appears in a symmetry unbroken phase, quantum oscillation measurements show that the normal state of SC2 is nematic, breaking C3 symmetry. Despite this difference, both superconductors violate the Pauli limit consistent with spin singlet pairing between opposite valleys protected from de-pairing by Ising SOC. Our results suggest that the induced SOC is central to the observed enhancement of superconductivity in many graphene multilayer systems - favoring pairing between time reversal symmetric partners.

Condensed Matter Seminar
Thursday, November 30, 2023
3:30 PM
Clark Hall, Room G004
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https://virginia.zoom.us/j/94608914338?pwd=a2xtYTNSMFpubThmRTA4Q1dkT29KZz09

 Meeting ID: 946 0891 4338

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"TOPOLOGY IS EVERYWHERE"


Maia Vergniory , DIPC and MPI for Chemical Physics of Solids
[Host: Dmytro Pesin]
ABSTRACT:

Quantum materials are a collection of atoms with interacting electrons and nuclei displaying emergent behaviour and topological properties—properties robust to local defects. The past two decades has witnessed an explosion in the field of topological materials: from weak interacting electrons to strongly correlated ones, topological materials represent one of the most exciting discoveries bot hat fundamental and application leve. High performance electronics, quantum information or ultrafast spintronics are just a few of the possible technologies that can be developed based on these materials. In this talk I will discuss the route to go from pure mathematical prediction of topological properties, through high through-put materials search to experimental realization. I will discuss both topological insulators, in non magnetic and magnetic phases as well as topological (chiral) semimetals using the the modern theory of topological band structure—Topological Quantum Chemistry — built upon symmetry-based considerations and complemented with chemical theories of bonding, ionization, and covalence. Consequently, it describes the universal global properties of all possible band structures and materials. Going beyond the single-particle perspective, I will introduce our formalism grounded in Green’s functions. This approach is designed to uncover topologically correlated phases in materials exhibiting electronic entanglement, such as Mott phases. Additionally, I will discuss recent results centered on Green’s function zeros, which are aimed at diagnosing topology in correlated materials.

Condensed Matter Seminar
Thursday, November 16, 2023
3:30 PM
Clark Hall, Room G004
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"Recent insights into the metal-insulator transition of vanadium dioxide (VO2) "


Mumtaz Qazilbash , William and Mary
[Host: Bellave Shivaram]
ABSTRACT:

Metal-insulator transitions are among the most fascinating and least understood phenomena in condensed matter physics. Metal-insulator transitions lead to significant changes in the electronic conductivity and optical properties, and are generally accompanied by structural and magnetic transformations due to a complex interplay between charge, spin, orbital, and lattice degrees of freedom. The thermally-driven metal-insulator transition (MIT) in bulk vanadium dioxide (VO2) is accompanied by a structural distortion that leads to pairing of all the vanadium atoms in the insulating phase. This V-V pairing has long been thought critical to the emergence of insulating behavior. We shall present our latest experiments on ultrathin VO2 films grown on TiO2 substrates. We demonstrate that the MIT in ultrathin VO2 films occurs without the V-V structural distortion. Our results establish a route to a purely electronic MIT that is driven by electron-electron interactions. We shall also present our recent experiments and results on infrared nano-imaging and nano-spectroscopy of VO2 films. The development of table-top, broadband infrared light sources in my lab has enabled nano-spectroscopy experiments on VO2 and other materials.

Condensed Matter Seminar
Thursday, November 2, 2023
3:30 PM
Clark Hall, Room G004
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"Giant microwave absorption in superconductors"


Boris Spivak , University of Wisconsin
[Host: Dima Pesin]
ABSTRACT:

 I will discuss a mechanism of microwave absorption in conventional and unconventional  superconductors which is similar to the Debye absorption mechanism in molecular gases. The contribution of this mechanism to AC conductivity is proportional to the inelastic quasiparticle relaxation time rather than the elastic one and therefore it can be much larger than the conventional one. The Debye contribution to the linear conductivity arises only in the presence of a DC supercurrent in the system and its magnitude depends strongly on the orientation of the microwave field relative to the supercurrent. The Debye contribution to the non-linear conductivity exists even in the absence of the supercurrent. It provides an anomalously low non-linear threshold.

 I will also discuss a closely related problems of resistance of superconductor-normal metal-superconductor junctions, and the resistance of superconductors in the magnetic flux flow regime.

Condensed Matter Seminar
Thursday, October 26, 2023
3:30 PM
Clark Hall, Room G004
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"Probe and Control of Coherent Phonons in Multifunctional Materials"


Giti Khodaparast , Virginia Tech
[Host: Olivia Pfister]
ABSTRACT:

The desire for multifunctional devices has driven significant research toward exploring multiferroics, where the coupling between electric, magnetic, optical, and structural order parameters can provide new functionality. While BiFeO3 is a well-studied multiferroic, recent research has shown that the addition of BaTiO3 can improve material properties.1 In this talk we focus on coherent phonon (CP) generation in BaTiO3-BiFeO3 (BTO-BFO) layered structure as well as nanorod arrays.  Usually, CPs are used to provide detailed spectroscopy information and to characterize surfaces and buried interfaces.  However, the ability to generate strain via ultrafast optics offers the intriguing possibility of dynamically manipulating the strain with ultrashort optical pulses and opens the possibility of creating a new class of devices, where the strain is manipulated in time to control the properties and operation of a device. In our nanorod arrays, we demonstrated several coherent modes, with possible signatures of the coexistence of CPs and magnons.2 While magnons, in general, are hard to manipulate and control, a strong magneto-elastic interaction between phonons and magnons can be important for a variety of reasons: (i) Coherent Acoustic Phonons generated with ultrafast optical pulses can propagate long distances from the surface, into the sample.   With strong magneto-elastic coupling, they can carry the spin information along with them into the sample, perhaps between different regions of a chip. (ii) Strong interactions with phonons can enhance the excitation, manipulation, and detection of the magnons for possible applications in memory devices.

In this talk, I will present our observations in several BTO-BFO films and nanorod arrays with different interfaces to demonstrate the tunability of CPs and discuss the possibility of the co-existence of CPs and magnons. I will also discuss the possibility of controlling these coherent states using external magnetic fields which have been demonstrated to increase the sensitivity of the CPs’ detection in other systems.3

 

References:

[1] S.-C. Yang, A. Kumar, V. Petkov, and S. Priya, J. of Appl. Phys., 113, 144101 (2013).

[2] R. R. H. H. Mudiyanselage, B. A. Magill, J. Burton, M. Miller, J. Spencer, K. McMillan, G. A. Khodaparast, H.-B. Kang, M.-G. Kang, D. Maurya, S. Priya, J. Holleman, S. McGill, and C. J. Stanton, J. Mater. Chem. C, 7, 14212 (2019).

[3] B. A. Magill, S. Thapa, J. Holleman, S. McGill, H. Munekata, C. J. Stanton, and G. A. Khodaparast, Phys. Rev. B 102, 045306 (2020).

 

Acknowledgment: This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-17-1-0341 and DURIP funding (FA9550-16-1-0358). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.

Condensed Matter Seminar
Thursday, October 19, 2023
3:30 PM
Clark Hall, Room G004
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"Ultraslow dynamics, fragile fragmentation, and geometric group theory"


Shankar Balasubramanian , MIT - Massachusetts Institute of Technology
[Host: Israel Klich ]
ABSTRACT:

A recurring theme in classical and quantum dynamics is finding examples of systems which fail to thermalize.  We introduce a class of 1D translationally invariant classical and quantum dynamics that have an unusual approach to equilibrium.  For certain examples, expectation values of local operators relax in a time which is exponentially large in system size, implying an unusual kind of hydrodynamics.  In other examples, thermalization only occurs when the system is connected to a bath which is at least exponentially large in system size, a phenomenon that we call fragile fragmentation.  A field of mathematics called geometric group theory plays an important role in constructing these examples, and we discuss extensions of these results to 2D.

Condensed Matter Seminar
Thursday, October 12, 2023
3:30 PM
Clark Hall, Room G004
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"Spin Vortices and Phase Transition in 2D Janus Colloidal Crystal"


Myeonggon Park , Brandeis University
[Host: Marija Vucelja]
ABSTRACT:

Colloids have provided versatile model systems to investigate rudimentary characteristics of material phases and have been employed as elementary units for spontaneous and directed self-assembly, mimicking atoms. Their size observable in the microscopy is feasible to track individual positions and orientations, so many phenomena including melting and nucleation have been studied at the single-particle level for better understanding atomic scale dynamics. In the same manner, we used Janus colloids to imitate spins in the observable scale under the microscope. We proved the phase transition of Janus spheres’ spin (orientation) order in Janus colloidal crystal, where the spin interaction is controlled by the external electric field. The spin configuration evolves from a random pattern to vortex and zigzag patterns, as increased the electric field. During this process, we measured the spin arrangement is changed from the short-range order to the quasi-long-range order through the power law decay of spatial correlation functions. Furthermore, we found the density of topological defects, which are vortex and anti-vortex, is correlated with the spin phase transition that is corroborated with the susceptibility of the spin order parameter. To describe the spin phase transition, we suggested the 2D Heisenberg model. Therefore, we expect that the designed system provides a platform to help understand spin behaviors in the single-particle level as well as plenty of phenomena induced by orientational interaction in atomic and molecular materials.

Condensed Matter Seminar
Thursday, October 5, 2023
3:30 PM
Clark Hall, Room G004
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"Criticality at the quantum Hall-superconductor interface "


Vlad Kurilovich , Yale
[Host: Dima Pesin]
ABSTRACT:

Topological superconductors provide a promising route to fault-tolerant quantum computing; however, it proved hard to find or engineer them. Recently, topological superconductivity was predicted to arise at the interface between quantum Hall and conventional superconducting states. Since both ingredients are readily available in the lab, topological superconductivity seemed to be within the reach. The predictions, however, focus on the idealized “clean” case, whereas only strongly disordered superconductors are compatible with high magnetic fields needed for the quantum Hall effect. Can topological superconductivity survive the presence of disorder?

 

We develop a theory of two counter-propagating quantum Hall edge states coupled via a narrow disordered superconductor. We show that, in contrast to the clean-case predictions, the edge states do not turn into a topological superconductor. Instead, the disorder tunes them to the critical point between the trivial insulating phase and the topological phase. We determine the manifestations of this criticality in the charge transport, finding that the critical conductance is a random, sample-specific quantity with a zero average and unusual bias dependence. The developed theory of disordered superconductor-quantum Hall interfaces offers an interpretation of recent experiments.

Condensed Matter Seminar
Thursday, September 28, 2023
3:30 PM
Clark Hall, Room G004
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ABSTRACT:

UTe2 has drawn a lot of attention among the superconductivity (SC) community recently for the promising secondary evidence of spin-triplet Cooper pairing shown in this system through temperature-independent NMR Knight shift, the highly anisotropic and unusually large upper critical field, power-law behavior of the specific heat, and possible ferromagnetic spin fluctuation hinted by the magnetic susceptibility. Under an extremely large magnetic field (~30 T above Hc2 in the bc-plane) the system shows re-entrance of the superconducting phase. Our previous experiments at CNCS on a co-aligned UTe2 sample in its 0KL and HK0 plane showed two types of rod-like excitations at Brillouin zone boundary, and one of them develops a spin gap and a spin resonance upon entering the SC state. SEM study shows development of charge density wave (CDW) in the (011) cleaved edge at the Brillouin zone corner, which is similar to the other excitation we observed with neutron scattering. Because the resonance has only been found in spin-singlet unconventional superconductors near an AF instability, its observation in UTe2 suggests that AF spin fluctuations may also induce spin-triplet pairing24 or that electron pairing in UTe2 has a spin-singlet component.

Condensed Matter Seminar
Friday, July 7, 2023
4:00 PM
Ridley, Room 127
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"Hydrogen-atom Spin Liquid Mechanism of Hot Superconductivity in Metal Hydrides"


Prof. G. Baskaran , MatScience Institute, IIT Madras and Perimeter Institute, Toronto
[Host: Shivaram]
ABSTRACT:

Conventional BCS mechanism of HTSC in hydrides, a la' Ashcroft works remarkably well in H_3S, LaH_{10} etc. We pointed out (GB 2015) [1] that H-atom networks in hydrides is a Mott insulating subsystem, which supports RVB superconductivity via internal doping. We will present a case for occurrence of both BCS and RVB pairing to varying degrees, in different hydrides. It resolves several existing puzzles. In our theory, H-C-S and Lu-N-H systems have dominant RVB pairing, which favors competing orders create, encouraged by structural variations and disorder, wide variation in Tc (like in cuprates). On the other hand, H_3S and LaH_{10} is a dominant phonon mediated BCS system; consequently Tc is protected by Anderson theorem and varies less. We provide support for our electronic mechanism from the works of Ng et al. (1996) and Eder et al., (1997) on emergence of Kondo insulating states in LaH_3 and YH_3, via Kondo coupling of band electrons to localized H atom spins.

Condensed Matter Seminar
Thursday, May 25, 2023
11:00 AM
Ridley Hall, Room 123
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ABSTRACT:

Superconducting qubits have emerged as the prominent building block of a scalable quantum computer. While inventing novel qubit designs has enabled the progress towards multiqubit systems that have demonstrated quantum advantage, there much more to be gained by properly engineering the qubit materials. Coherence time of the qubit, the relevant figure of merit, is limited by materials losses. In particular, the main limiting factor is the presence of “parasitic” two-level systems (TLS) that manifest in increased losses of the superconducting circuits at millikelvin temperatures and single-photon powers. Amorphous oxides, residues and transition layers at interfaces and grain boundaries are identified as sites harboring such TLS states. In this talk, I will present techniques to carefully address each interface, superconducting films, and substrate, and ultimately reduce the losses in the superconducting circuit in the quantum regime (millikelvin temperatures, single-photon occupancy). This will ultimately allow to push the envelope regarding coherence times and allow for quantum processors with larger number of qubits and larger quantum volume.

 

Biography:

Nikolay is a research assistant professor in the department of Physics and Astronomy at Northwestern University. He is also part of the Superconducting Quantum Materials and Systems (SQMS) center, one of the five US Department of Energy quantum information science research centers. His current research involves studying mechanism of losses in superconducting quantum circuits, aiming to engineer superconducting qubits with longer coherence times. He is also in charge of the Quantum Science Engineering and Technology (QSET) laboratory at Northwestern University – a cryogenic measurement hub supporting superconducting quantum materials and devices research at Northwestern University.

Condensed Matter Seminar
Friday, May 12, 2023
2:00 PM
Thorton, Room C311
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"The quantum Mpemba effect"


Sara Murciano , Caltech
[Host: Israel Klich ]
ABSTRACT:

What is the connection between Aristotle and an ice cream? The answer is encoded in the Mpemba effect, the counterintuitive and controversial phenomenon that hot water cools faster than cold one. Here I will provide an analog quantum definition of this phenomenon, which has been studied in a quantum quench of a spin chain whose initially broken global U(1) symmetry is restored dynamically. Studying this evolution, we find that the more the symmetry is initially broken, the faster it is restored. We dub the measure detecting this effect entanglement asymmetry, which is a measure of symmetry breaking inspired by the theory of entanglement in many-body states. Although symmetry breaking is a pillar of modern quantum physics, quantifying how much a symmetry is broken is an issue that has received little attention so far: The merit of the entanglement asymmetry is to provide a quantitative tool for this.

Condensed Matter Seminar
Thursday, April 27, 2023
4:00 PM
Ridley Hall, Room 177
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"Coherence Properties of Spin-Orbit Coupled Condensates"


Supriyo Ghosh , UVA
[Host: Dima Pesin]
ABSTRACT:

Realization of artificial spin-orbit (SO) coupling in the cold atom systems has opened a broad spectrum of phenomena to be observed experimentally in bosonic systems. The physics of spontaneous symmetry breaking leads to the several exotic quantum phases such as superfluid and more recently the supersolid. In the latter case, both diagonal and off-diagonal long-range order, although being mutually exclusive, are present and it breaks two U(1) symmetries. One of the ways to realize this supersolid phase is using the condensates of Alkali atoms in a double well optical lattice. Such SO coupled systems have interesting phase diagrams depending on the interaction and SO coupling strength. In presence of weak coupling and negligible inter-species interaction the system remains in the phase known as striped phase or miscible phase. In this case, the Raman beam couples two condensates and provides a momentum kick along a perpendicular direction, which indeed leads to a one-dimensional density modulation due to interference.  Two continuous symmetries are broken by the superfluid phase and the breaking of translation symmetry along one direction, satisfying the definition of a supersolid. Spinor BECs and their relative phase coherence has been a topic of interest for quite some time. The dephasing of the relative phase has important implications in the density modulation and its experimental observation.  I will consider a specific realization of a pseudo-spin-1/2 BEC, in which the two spin components are represented by spatially separated low-lying states in a double-well potential. I will demonstrate that even if the system is initialized with a given relative phase between the two components, there is an apparent dephasing of the relative phase. The dephasing is a consequence of a very weak interaction between the spin species. Furthermore, I will demonstrate that the dephasing rate is a non-monotonic function of the Raman coupling strength. The minimum of the dephasing rate corresponds to the critical value of the Raman coupling, above which miscible to immiscible phase transition occurs.

Condensed Matter Seminar
Tuesday, April 25, 2023
12:00 PM
Rotunda, Room 102
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"Taming energy dissipation in driven topological systems"


Iliya Esin , Caltech
[Host: Israel Klich ]
ABSTRACT:

In this talk, I will discuss energy dissipation and heating in slowly driven quantum systems, focusing on topological driving schemes. In the first part of my talk, I will present a system in which many-body dynamics leads to the emergence of a quasi-steady state with a high entropy density and yet robust topological transport. I will explain the mechanisms behind this phenomenon and demonstrate the emergence of the quasi-steady state on an exactly solvable strongly coupled fermionic model. In the second part of my talk, I will show that the dissipation of energy in nearly adiabatic quantum systems is linked to the quantum geometry of the problem. Interestingly, this result implies a topological bound on the energy dissipation rate in a class of topological systems. Our findings uncover new connections between topology and dissipation in slowly driven quantum systems, shedding light on their fundamental properties and potential for practical applications, such as the development of optimized driving protocols for topological drives.

Condensed Matter Seminar
Thursday, April 20, 2023
4:00 PM
Ridley Hall, Room 177
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ABSTRACT:

Every animal cell is filled with a cytoskeleton, a dynamic gel made of inextensible filaments / bio-polymers, such as microtubules, actin filaments, and intermediate filaments, all suspended in a viscous fluid. Similar suspensions of elastic filaments or polymers are widely used in materials processing. Numerical simulation of such gels is challenging because the filament aspect ratios are very large.

We have recently developed new methods for rapidly computing the dynamics of non-Brownian and Brownian inextensible slender filaments in periodically-sheared Stokes flow [1,2,4]. We apply our formulation to a permanently1 and dynamically cross-linked actin mesh3 in a background oscillatory shear flow. We find that nonlocal hydrodynamics can change the visco-elastic moduli by as much as 40% at certain frequencies, especially in partially bundled networks [3,4].

I will focus on accounting for bending thermal fluctuations of the filaments by first establishing a mathematical formulation and numerical methods for simulating the dynamics of stiff but not rigid Brownian fibers in Stokes flow [4]. I will emphasize open questions for the community such as whether there is a continuum limit of the Brownian contribution to the stress tensor from the filaments.

References:

1. O. Maxian et al, Integral-based spectral method for inextensible slender fibers in Stokes flow,. Phys. Rev. Fluids, 6:014102, 2021
2. O. Maxian et al,. Hydrodynamics of a twisting, bending, inextensible fiber in Stokes flow, Phys. Rev. Fluids, 7:074101, 2022
3. O. Maxian et al, Interplay between Brownian motion and cross-linking controls bundling dynamics in actin networks, Biophysical J., 121:1230\u20131245, 2022.
4. O. Maxian et al., Bending fluctuations in semiflexible, inextensible, slender filaments in Stokes flow: towards a spectral discretization, ArXiv:2301.11123, to appear in J. Chem. Phys., 2023.

Condensed Matter Seminar
Thursday, April 13, 2023
4:00 PM
Ridley Hall, Room 177
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ABSTRACT:

Two-dimensional hybrid organic-inorganic perovskites (HOIPs) have shown promising progress in light-emitting diodes applications. The three-dimensional (3D) HOIPs, commonly used as novel solar cells, exhibit extended charge carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection from defects [1]. It has been experimentally shown that the reorientation of the polarized organic molecules can facilitate the polaron formation, where polaron is referred as a quasiparticle formed by the Coulomb interaction between an excess charge (an electron or a hole) and the ionic lattice, enhance the screening effects on charge carriers, and thus prolong the charge carrier lifetime [2-3]. Whereas the pure inorganic 3D perovskites without organic molecules, such as CsPbI3, also show a moderate photovoltaic performance, which brings researcher’s efforts into investigations on dynamics of the inorganic perovskite framework [4]. An understanding of the microscopical mechanisms behind extended charge carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection in HOIPs is lacking. We performed time-of-flight neutron spectroscopy for two perovskites, butylammonium lead iodide (BA)2PbI4 (BA) and phenethyl-ammonium lead iodide (PEA)2PbI4 (PEA). From the obtained spectra we identified and quantitatively separated the rotational and phonon contribution for BA. Similar analysis would be performed for the PEA spectra in future. We try to understand how both inorganic vibrational dynamics and organic molecule rotational dynamics contribute to charge carrier lifetime and hence power conversion efficiency of solar cells. The study is important to get an idea on how to engineer new HOIPs by exploiting these dynamics for higher device performance. By examining the corresponding temperature dependence, we revealed that the rotational dynamics of organic molecules in these materials tends to suppress their photoluminescence quantum yield [5] while the vibrational dynamics did not show predominant correlations with their optoelectronic properties.

 

[1] Mei, Anyi, et al. science 345.6194 (2014): 295-298.

[2] Miyata, Kiyoshi, Timothy L. Atallah, and X-Y. Zhu. Science Advances 3.10 (2017): e1701469.

[3] Chen, Tianran, et al. Proceedings of the National Academy of Sciences 114.29 (2017): 7519-  7524.

[4] Wang, Kang, et al. Nature communications 9.1 (2018): 4544.

[5] Gong, Xiwen, et al. Nature materials 17.6 (2018): 550-556.

 

Condensed Matter Seminar
Wednesday, April 12, 2023
3:00 PM
Physics, Room 120
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ABSTRACT:

In recent years anomalous cooling and heating effects in the far from equilibrium limit have gained attention. One anomaly is the so called Mpemba Effect, in which the time to relax towards thermal equilibrium does not grow monotonically as a function of distance to the target. Instead, it has been proposed that there exist shortcuts in the relaxation process that allow both faster, and even exponentially faster heating and cooling. In this talk I will discuss recent works [1,2] that have progressed our understanding of such shortcuts by studying the Mpemba effect using Overdamped Langevin dynamics. I will show when and where you can get the effect, and that our models are in good agreement with experimental findings. Lastly, I will touch upon current works where we study the effect using Markovian jump processes on linear reaction networks.  

 

  1. Anomalous thermal relaxation of Langevin particles in a piecewise-constant potential

Matthew R Walker and Marija Vucelja J. Stat. Mech. (2021) 113105

  1. Mpemba effect in terms of mean first passage times for overdamped Langevin dynamics

Matthew R Walker and Marija Vucelja arXiv preprint arXiv:2212.07496 (2022)

 

Condensed Matter Seminar
Thursday, April 6, 2023
4:00 PM
Ridley, Room 177
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"Scanning tunneling spectroscopy of unconventional superconductors"


Pavlo Sukhachov , Yale
[Host: Dmytro Pesin]
ABSTRACT:

Motivated by recent experimental observations of unconventional superconductivity in twisted bilayer and trilayer graphenes, we develop a theory describing the differential conductance between a normal STM tip and a 2D superconductor with an arbitrary gap structure. Our analytical scattering theory accounts for Andreev reflections, which become prominent at larger transmission between the tip and the superconductor. Exploiting the dependence of Andreev reflection on the relative position of the STM tip with respect to the lattice symmetry points, we show that the structure of the superconducting gap can be extracted by combining weak- and strong-tunneling limits of differential conductance. Furthermore, the theory incorporates a tip/impurity-induced scattering potential within the 2D material, which allows us to describe subgap resonances.

Condensed Matter Seminar
Thursday, March 30, 2023
4:00 PM
Ridley Hall, Room 177
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"Altermagnetism: a third type of ordered collinear magnetism"


Igor Mazin , George Mason University
[Host: Dima Pesin]
ABSTRACT:

Since many years, the canonincal classification of ordered magnets included noncollinear (with many further subdivisions) and two collinear

types: antiferromagnets (AF), which have net magnetization zero by symmetry, and ferro/ferrimagnets (FM), which do not have this property.

The two have distinctly different micro- and macroscopic properties. It was supposed, for instance, that only FM can exhibit spin-splitting of the electronic bands in absence of spin-orbit coupling AND lack of inversion symmetry, have anomalous Hall effect (i.e., Hall effect driven by variation of the Berry phase), magnetooptical effects, suppressed Andreev scattering in contact with a singlet superconductor etc.

A surprisingly recent development (~2019) is that this classification is

incomplete: there are collinear magnets that would belong to AF by this classification, but show all characteristics of FM, *except the net spin polarization*! They were recently dubbed by Mainz group "altermagnets", AM. Incidentally, what has also not been fully appreciated was that there are also materials that have strictly zero net magnetization, but enforced not by symmetry, but by the Luttinger's theorem, and therefore truly belonging to the FM class ("Luttinger-compensated ferrimagnets").

In this talk I will present the new classification and explain, in specific examples, what are the symmetry conditions for AM, why these are a truly new class deserving a new name, and how their unusual properties appear.

Condensed Matter Seminar
Monday, March 27, 2023
3:30 PM
Physics Building, Room 313
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"The Mpemba effect for phase transitions by Landau theory"


Roi Holtzmann , Weizmann Institute of Science
[Host: Marija Vucelja]
ABSTRACT:

The Mpemba effect describes the situation in which a hot system cools faster than an identical copy that is initiated at a colder temperature. In many of the experimental observations of the effect, e.g. in water and clathrate hydrates, it is defined by the phase transition timing. However, none of the theoretical investigations so far considered the timing of the phase transition, and most of the abstract models used to explore the Mpemba effect do not have a phase transition. In this talk, I will suggest a definition for the phase transition time in a non-equilibrium state using the Landau theory for phase transitions. Using this definition, I will show that a Mpemba effect with respect to phase transitions can exist in such models, namely that the hotter system undergoes the transition before the colder one when quenched to a cold temperature.

Condensed Matter Seminar
Thursday, March 23, 2023
4:00 PM
Ridley Hall, Room 177
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"Machine Learning Meets Quantum Many-body Physics"


Di Luo , MIT
[Host: Gia-Wei Chern]
ABSTRACT:

Simulation of quantum many-body physics, such as looking for ground state properties and real time dynamics, plays an important role in the study of condensed matter physics, high energy physics and quantum information science. The recent advancement of machine learning provides new opportunities for tackling challenges in simulating quantum many-body physics. In this talk, I will first discuss a class of wave functions via neural network transformation, neural network backflow,  which can fulfill the anti-symmetry property and capture the correlation and the sign structure for strongly-interacting fermionic physics. Next, I will talk about recent progress of simulating continuum quantum field theories with neural quantum field state [2], and lattice gauge theories such as 2+1D quantum electrodynamics with finite density dynamical fermions using gauge symmetric neural networks [3,4]. Finally, I will present a neural network representation based on positive-value-operator measurements for quantum circuit and open quantum system dynamics simulation [5].

Reference:
[1] Di Luo, Bryan K. Clark, Backflow Transformations via Neural Networks for Quantum Many-Body WaveFunctions, Phys. Rev. Lett. 122, 226401.
[2] John M. Martyn, Khadijeh Najafi, Di Luo, Variational Neural-Network Ansatz for Continuum Quantum Field Theory, https://arxiv.org/abs/2212.00782.
[3]  Di Luo, Giuseppe Carleo, Bryan K. Clark, James Stokes, Gauge Equivariant Neural Networks for Quantum Lattice Gauge Theories, Phys. Rev. Lett. 127, 276402.
[4] Zhuo Chen†, Di Luo†, Kaiwen Hu, Bryan K. Clark, Simulating 2+1D Lattice Quantum Electrodynamics at Finite Density with Neural Flow Wavefunctions, https://arxiv.org/abs/2212.06835.
[5] Di Luo†, Zhuo Chen†, Juan Carrasquilla, Bryan K. Clark, Autoregressive Neural Network for Simulating Open Quantum Systems via a Probabilistic Formulation, Phys. Rev. Lett. 128, 090501.


 

Condensed Matter Seminar
Monday, March 20, 2023
4:00 PM
Physics Building, Room 313
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