Special Seminar
Thursday, January 16, 2020
12:45 PM
Physics Building, Room 313

Saso Grozdanov
[Host: Peter Arnold]
MIT
"From hydrodynamics to quantum chaos"

ABSTRACT:
Hydrodynamics is a theory of the collective properties of fluids and gases that can also be successfully applied to the description of the dynamics of quarkgluon plasma. It is an effective field theory formulated in terms of an infiniteorder gradient expansion. For any collective physical mode, hydrodynamics will predict a dispersion relation that expresses this mode’s frequency in terms of an infinite series in powers of momentum. By using the theory of complex spectral curves from the mathematical field of algebraic geometry, I will describe how these dispersion relations can be understood as Puiseux series in (fractional powers of) complex momentum. The series have finite radii of convergence determined by the critical points of the associated spectral curves. For theories that admit a dual gravitational description through holography, the critical points correspond to levelcrossings in the quasinormal spectrum of a dual black hole. Interestingly, holography implies that the convergence radii can be orders of magnitude larger than what may be naively expected. This fact could help explain the “unreasonable effectiveness of hydrodynamics” in describing the evolution of quarkgluon plasma. In the second part of my talk, I will discuss a recently discovered phenomenon called “poleskipping” that relates hydrodynamics to the underlying microscopic quantum manybody chaos. This new and special property of quantum correlation functions allows for a precise analytic connection between resummed, allorder hydrodynamics and the properties of quantum chaos (the Lyapunov exponent and the butterfly velocity).


Condensed Matter
Thursday, December 5, 2019
11:00 AM
Physics Building, Room 313

Professor Yi Li
[Host: Dima Pesin]
Johns Hopkins University
"Monopole Superconductivity and DensityWave Order in Weyl Semimetals"

ABSTRACT:
Although the existence of magnetic monopoles is admitted by the fundamental laws, the real monopoles in nature remain elusive. Nevertheless, variations of monopoles appear in realistic condensed matter systems, from quantum Hall effects to topological superconductivity, which spur a race to discover new exotic topological phases of matter. In this talk, we will present a dramatic effect arising from topological Fermi surfaces  a novel topological class of superconductivity and densitywave orders: When the ordered pairs acquire nontrivial twoparticle Berry phases, their pairing phases cannot be globally welldefined in the momentum space. Therefore, the conventional description of superconducting pairing symmetries in terms of spherical harmonics (e.g. s, p, dwaves) ceases to apply. Instead, they are characterized by topologically protected nodal gap functions represented by monopole harmonic functions. This socalled “monopole harmonic order” is expected to be realized and detected in Weyl semimetal materials.


Condensed Matter
Thursday, November 14, 2019
3:30 PM
Physics Building, Room 313

Leonid Petrov
[Host: Marija Vucelja]
UVa, Department of Mathematics
"Mapping TASEP back in time"

ABSTRACT:
We obtain a new relation between the distributions μ_t at different times t ≥ 0 of the continuoustime TASEP (Totally Asymmetric Simple Exclusion Process) started from the step initial configuration. Namely, we present a continuoustime Markov process with local interactions and particledependent rates which maps the TASEP distributions μ_t backwards in time. Under the backwards process, particles jump to the left, and the dynamics can be viewed as a ver sion of the discretespace Hammersley process. Combined with the forward TASEP evolution, this leads to a stationary Markov dynamics preserving μ_t which in turn brings new identities for expectations with respect to μ_t. Based on a joint work with Axel Saenz.


Condensed Matter
Thursday, August 22, 2019
2:30 PM
Physics Building, Room 204

Paul Fendley
[Host: Israel Klich]
Oxford University
"TBA"

ABSTRACT:


Condensed Matter
Thursday, May 30, 2019
11:00 AM
Physics Building, Room 313

Adam Wei Tsen
[Host: Seunghun Lee]
University of Waterloo
"Twodimensional magnetism and spintronics"

ABSTRACT:
The recent discoveries of ferromagnetism in single atomic layers have opened a new avenue for twodimensional (2D) materials research. Not only do they raise fundamental questions regarding the requirements for longrange magnetic order in lowdimensional systems, but they also provide a new platform for the development of spintronic devices. In this talk, I will present a series of studies on the family of layered ferromagnetic semiconductors, CrX_{3} (X = I, Br, Cl), in the atomically thin limit. By incorporating these materials as tunnel barriers between graphene electrodes, we are able to achieve extremely large tunnel magnetoresistance as well as robust memritive switching that is tunable with magnetic field. Tunneling spectroscopy further allows for direct observation of their spin wave excitations, or magnons, from which we are able to derive a simple microscopic Hamiltonian for all three spin systems. These results show that strong exchange anisotropy is not necessary to stabilize ferromagnetism in the monolayer limit.


Condensed Matter
Tuesday, May 28, 2019
11:00 AM
Physics Building, Room 313

Joseph Avron
[Host: Israel Klich ]
Technion
"Flexible Sganac interferometers for the neophytes"

ABSTRACT:
I shall review the history of Sagnac interferometers and give a geometric description of light rays propagation in flexible optical fibers in Minkowski space. Based on joint works with Amos Ori and Oded Kenneth.


Condensed Matter
Thursday, May 16, 2019
11:00 AM
Physics Building, Room 313

Professor YingJer Kao
[Host: GiaWei Chern]
National Taiwan University
"Tunnelinginduced restoration of classical degeneracy in quantum kagome ice"

ABSTRACT:
Quantum effect is expected to dictate the behavior of physical systems at low temperature. For quantum magnets with geometrical frustration, quantum fluctuation usually lifts the macroscopic classical degeneracy, and exotic quantum states emerge. However, how different types of quantum processes entangle wave functions in a constrained Hilbert space is not well understood. Here, we study the topological entanglement entropy (TEE) and the thermal entropy of a quantum ice model on a geometrically frustrated kagome lattice. We find that the system does not show a Z2 topological order down to extremely low temperature, yet continues to behave like a classical kagome ice with finite residual entropy. Our theoretical analysis indicates an intricate competition of offdiagonal and diagonal quantum processes leading to the quasidegeneracy of states and effectively, the classical degeneracy is restored.


Condensed Matter
Thursday, May 9, 2019
11:00 AM
Physics Building, Room 313

Professor Carlos Silva
[Host: Seunghun Lee]
Georgia Tech
"Exciton polarons in twodimensional organicinorganic hybrid perovskites"

ABSTRACT:
Owing to both electronic and dielectric confinement effects, twodimensional organicinorganic hybrid perovskites sustain strongly bound excitons at room temperature. In this seminar, we demonstrate that there are nonnegligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic countercation. In these systems, organic cations not only serve as spacers between slabs consisting of cornersharing metalhalide octahedra, but also determine lattice structure by inducing varying degree of distortion of the octahedra via the organicinorganic interactions. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in singlelayer organicinorganic hybrid perovskites, based on the twodimensional Wannier formalism. We include four distinct excitonic states separated by 35±5 meV, and additional vibronic progressions. Intriguingly, the associated HuangRhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the line shape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Furthermore, by means of highresolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various lowfrequency (≲50 cm^{−1}) optical phonons involving motion in the lead iodide layers. This supports our conclusion that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. Excitonic correlations (exciton and biexciton binding energies) and exciton dynamics (e.g. uni and bimolecular population decay mechanisms, pure dephasing processes, excitationinduced dephasing, etc.) reflect the polar solvationlike processes induced by organic cation components of the hybrid lattice in a broad structural space. I will address how ultrafast nonlinear spectroscopies yield deep insight on the multiparticle properties in compelx semiconductor materials.


Condensed Matter
Thursday, May 2, 2019
3:30 PM
Physics Building, Room 313

Yuchen Du
[Host: Diana Vaman]
University of Virginia  Physics
"Worldline approach"

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


Condensed Matter
Wednesday, May 1, 2019
1:00 PM
Physics Building, Room 314

Xixiao Hu
[Host: Joe Poon]
University of Virginia  Physics
"Thermoelectric transport properties of topological BiSb cryogenic materials"

ABSTRACT:
BiSb alloys have shown promising thermoelectric (TE) properties at cryogenic temperature (<200 K). Over six decades, the figure of merit zT of ntype polycrystalline BiSb has plateaued at ~0.4, while its ptype counterpart has remained even lower at ~0.1. We have studied the TE properties of meltspun and spark plasma sintered (SPS) BiSb alloys. We obtained a zT of 0.55 @100150 K for ntype undoped Bi_{85}Sb_{15 }based on a low thermal conductivity 1.5 W/(m*K) measured with the hotdisk method. For ptype BiSb, doping effects of Ge, Sn, and Pb were investigated. A high doping level of Ge and a high doping efficiency of Pb were obtained with the help of a lowtemperature SPS processing. The transport properties (resistivity and Seebeck coefficient) of ntype undoped and ptype doped Bi_{85}Sb_{15} were analyzed using the twoband effective mass model within the Boltzmann transport theory. A band gap decreasing phenomenon was observed which poses challenges to the improvement of ptype BiSb’s zT.


Condensed Matter
Friday, April 26, 2019
1:00 PM
Physics Building, Room 205

Meng Hua
[Host: Jeffrey Teo]
University of Virginia  Physics
"Topological phases with twofold spatial antiunitary symmetries"

ABSTRACT:
An interesting theme in topological materials has been classification and prediction of symmetry protected topological(SPT) phases. Despite the AltlandZirnbauer(AZ) classification under timereversal symmetry, particlehole symmetry and chiral symmetry, a system can also be invariant under a combined symmetry composed by two distinct operations. In this talk I will discuss the classification of nodal topological phases under twofold spatial antiunitary symmetries. We also generalize SPT phases to nonHermitian system with twofold spatial antiunitary symmetries and give an example of dissipative topological superconductors.


Condensed Matter
Thursday, April 25, 2019
11:00 AM
Physics Building, Room 313

Milovan Suvakov
[Host: Marija Vucelja ]
Institute of Physics Belgrade
"The threebody problem: periodic solutions, topological classification"

ABSTRACT:
The threebody 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 twobody orbit is always an
ellipse (circle). For two centuries, scientists tried different tacks
to find similar solution for threebody problem, until the German
mathematician Heinrich Bruns pointed out that the search for a general
solution for the threebody 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 LagrangeEuler (1772); 2) the BrouckeHenon
(1975); and 3) Cris Moore's (1993) periodic orbit of three bodies
moving on a "figure8" 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 threebody systems.
https://scholar.google.com/citations?hl=en&user=dEJ0ThoAAAAJ&view_op=list_works&sortby=pubdate


Condensed Matter
Wednesday, April 24, 2019
10:00 AM
Physics Building, Room 313

Hamed Vakili
[Host: Avik Ghosh]
University of Virginia  Physics
"Magnetic Skyrmions on a racetrack"

ABSTRACT:
Skyrmions are topologically protected magnetic quasiparticles. An isolate skyrmion is a metastable state of ferromagnet. The metastable state of skyrmions have a finite lifetime at non zero temperature which depends on energy barrier and attempt frequency. I will talk about how we are trying to calculate lifetime of skyrmion in candidate Heuslers compounds. Materials with different symmetry groups can support different kind of skyrmions (Bloch, Neel, Antiskyrmion). We will see how this different types of symmetries can be used to control movements of a skyrmion. Also, we will look at how presence of point defects can effect dynamics of skyrmion, either for movement or nucleation. The ultimate goal is to figure out a compact analytical model for describing skyrmion movement and critical spin current needed for nucleation.


Condensed Matter
Thursday, April 18, 2019
11:00 AM
Physics Building, Room 313

Preetha Saha
[Host: GiaWei Chern]
University of Virginia  Physics
"Spin dynamics in two distinct types of classical spin liquids "

ABSTRACT:
Unconventional magnetic states such as spin liquids and spin glasses continue to attract the interest of researchers in magnetism.These materials retain their magnetic disorder even at zero temperatures. We study two different cases of frustrated systems 1)In the case of Kitaevtype models frustration originates from highly anisotropic exchange interactions. We report a new classical spin liquid in which the collective flux degrees of freedom break the translation symmetry of the honeycomb lattice. This exotic phase exists in frustrated spinorbit magnets where a dominant offdiagonal exchange, the socalled Γ term, results in a macroscopic groundstate degeneracy at the classical level [1]. We show that this phase transition actually corresponds to plaquette ordering of hexagonal fluxes. We also study the dynamical behavior of fluxes. 2) We study the deterministic spin precession dynamics using energy conserving LandauLifshitz equation on a geometrically frustrated magnet. The lattice constitutes of a triangular arrangement of bipyramids with classical antiferromagnetic Heisenberg interaction. Such a lattice structure is realized in frustrated SrCr9Ga129pO19 [SCGO(p)] compounds [2]. Monte Carlo simulations are used to thermalize the system, which is then used as the initial state for the dynamical studies. We explore the temperature, wave vector and frequency dependence in the dynamical structure factor and the corresponding time dependent correlation functions of the model. Dynamics simulations is further used to estimate the extent to which transport of spin excitations in the lattice conform with phenomenological concept of spin diffusion [1]I. Rousochatzakis and N. B. Perkins Phys. Rev. Lett. 118, 147204 (2017). [2]T. Arimori and H. Kawamura J. Phys. Soc. Jpn. 70, 3695 (2001)


Special Presentation
Thursday, April 4, 2019
11:00 AM
Physics Building, Room 313

UVa's Advanced Research Computing Services
[Host: Bryan Wright]
University of Virginia
"Introduction to Rivanna"

ABSTRACT:
Members of UVa's Advanced Research Computing Services group will be presenting the second of two information/Q&A sessions about rivanna (UVa's supercomputing cluster) on Thursday, April 4 at 11am in Physics 313.
Rivanna is a 7,000core cluster with more than a petabyte of storage. It includes a subset of GPUequipped nodes. Please drop in if you have any interest in highperformance computing.
Slides from the talk can be found here:
http://galileo.phys.virginia.edu/compfac/faq/IntroductionToRivanna.pdf


Condensed Matter
Thursday, March 28, 2019
11:00 AM
Physics Building, Room 313

Peter Schauss
[Host: Dmytro Pesin]
University of Virginia  Physics
"Quantum gas microscopy of manybody dynamics in FermiHubbard and Ising systems"

ABSTRACT:
The ability to probe and manipulate cold atoms in optical lattices at the atomic level using quantum gas microscopes enables quantitative studies of quantum manybody dynamics. While there are many welldeveloped theoretical tools to study manybody quantum systems in equilibrium, gaining insight into dynamics is challenging with available techniques. Approximate methods need to be benchmarked, creating an urgent need for measurements in experimental model systems. In this talk, I will discuss two such measurements.
First, I will present a study that probes the relaxation of density modulations in the doped FermiHubbard 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 finitetemperature 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 lowlying Rydberg state, we observe quench dynamics in the resulting Ising model and prepare states with antiferromagnetic correlations.


Condensed Matter
Thursday, March 21, 2019
11:00 AM
Physics Building, Room 313

Zhenyang Xu
[Host: Despina Louca]
University of Virginia  Physics
"MetalInsulator Transition in Phase Change Material Ge2Sb2Se5xTe55x"

ABSTRACT:
Ge2Sb2Te5 (GST225) is a phase change material which has wide use in fabricating random access memories. With different quenching process, the GST225 could have three different phases: an amorphous phase, an intermediate cubic phase, a crystalline hexagonal phase. The fast transition between amorphous and crystalline phase makes GST225 an ideal material for RAM with fast speed. In our project, the Sedoped GST225 materials are grown and studied. At x=0.9 in liquid nitrogen quenched samples, we observe a phase transition from crystalline to amorphous. The transport measurement also confirm that there are metalinsulator transition happen for both furnace cooled samples and liquid N2 quenched samples at this limit. A tentative hypothesis is proposed to explain this metalinsulator transition.


Special Condensed Matter Seminar
Wednesday, March 13, 2019
11:00 AM
Physics Building, Room 313

Rafael Alexander
[Host: Israel Klich]
UNM
"Walks, tiles, and zippers: exact holographic tensor networks for Motzkin spin chains"

ABSTRACT:
The study of lowdimensional quantum systems has proven to be a particularly fertile field for discovering novel types of quantum matter. The tensor network's utility in studying short range correlated states in 1D have been thoroughly investigated. Yet, despite the large number of works investigating these networks and their relations to physical models, examples of exact correspondence between the ground state of a quantum critical system and an appropriate scaleinvariant tensor network have eluded us so far. Here we show that the features of the quantumcritical Motzkin model can be faithfully captured by an analytic tensor network that exactly represents the ground state of the physical Hamiltonian. In particular, our network offers a twodimensional representation of this state by a correspondence between walks and a type of tiling of a square lattice. We discuss connections to renormalization and holography.


Condensed Matter
Thursday, February 14, 2019
11:00 AM
Physics Building, Room 313

Maxim Dzero
[Host: Israel Klich]
Kent State
"Thermodynamic properties of disordered unconventional superconductors with competing interactions"

ABSTRACT:
A topic of interplay between disorder and competing electronic phases in multiband superconductors have recently got renewed interest in the context of ironbased superconductors. In my talk I will present a theory of disordered unconventional superconductor with competing magnetic order. My discussion will be based on the results obtained for on a twoband model with quasitwodimensional Fermi surfaces, which allows for the coexistence region in the phase diagram between magnetic and superconducting states in the presence of intraband and interband scattering induced by doping. Within the quasiclassical approximation I will present the analysis of the quasiclassical Eilenberger’s equations which include weak external magnetic field. I will demonstrate that disorder has a crucial effect on the temperature dependence of the magnetic penetration depth as well as critical current, which is especially pronounced in the coexistence phase.


Condensed Matter
Thursday, November 8, 2018
11:00 AM
Physics Building, Room 313

Arnab Banerjee
[Host: Bellave Shivaram]
Oak Ridge National Laboratory
"Fractionalized excitations towards a nonAbelian phase in a Kitaev honeycomb magnet"

ABSTRACT:
The Kitaev model on a honeycomb lattice predicts a special quantum spin liquid (QSL) ground state with excitations resembling Majorana Fermions and gauge flux excitations. These emergent features are exciting prospects to both basic physics and applications towards a lossless technology for quantum qubits. In this talk, I will describe our recent range of experiments on the magnetic Mott insulator alphaRuCl3 which has honeycomb layers held together with weak vanderWaals interactions. A strong spinorbit coupling and an octahedral crystal field makes the Kitaev interactions arguably the leading order term in the Hamiltonian. Prominently, despite a longrange ordered ground state, our neutron scattering measurements reveal a continuum of fractionalized excitations resembling predictions from Majorana Fermions, confirming that the material is proximate to a QSL. In a 8T magnetic field the longrange order vanishes and the continuum becomes gapped, supporting a state where a direct evidence of nonAbelian excitations can be measured. I will describe the present and future endeavors that may help to stabilize the coherent quantum excitations allow a better understanding of the underlying physics, as well as experiments to complete the understanding of the phase diagram of this material.


Condensed Matter
Thursday, October 18, 2018
11:00 AM
Physics Building, Room 313

Erhai Zhao
[Host: Bellave Shivaram]
George Mason University
"Competing orders in a quantum spin model with longrange interactions"

ABSTRACT:
Quantum spin liquids evade longrange magnetic order down to absolute zero temperature. These anarchic, yet highly entangled states break no symmetry but have remarkable properties such as fractional excitations. In this talk, I will first give an example of spin liquid using a compass model relevant to recently discovered honeycomb antiferromagnet NaNi2BiO6. Then I will introduce a new model, the dipolar Heisenberg model, motivated by recent experiments on artificial manyspin systems based on interacting dipoles. I will argue that longrange magnetic order can be suppressed by simply tuning the direction of the dipoles using an external field. The classical, semiclassical, and quantum phase diagram of this frustrated spin model will be presented to show an extended region where the ground state is a quantum paramagnet. By comparing to DMRG, I will argue that it is likely a quantum spin liquid.

