# Condensed Matter Seminars

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Thursday, April 16, 2020

3:30 PM

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## "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.

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

Tuesday, April 14, 2020

2:00 PM

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## "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.

**Webinar**### Join Zoom Meeting

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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)

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## "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.

**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

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'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.

**Special Seminar**

Thursday, February 13, 2020

12:45 PM

Physics Building, Room 313

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## "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.

**Special Seminar**

Thursday, February 6, 2020

12:45 PM

Physics Building, Room 313

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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.

**Special Seminar**

Thursday, January 30, 2020

12:45 PM

Physics Building, Room 313

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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.

**Special Seminar**

Thursday, January 23, 2020

12:45 PM

Physics Building, Room 313

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## "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.

**Special Seminar**

Thursday, January 16, 2020

12:45 PM

Physics Building, Room 313

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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).

**Condensed Matter**

Thursday, December 5, 2019

11:00 AM

Physics Building, Room 313

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## "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 14, 2019

3:30 PM

Physics Building, Room 313

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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, August 22, 2019

2:30 PM

Physics Building, Room 204

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TBA

**Condensed Matter**

Thursday, May 30, 2019

11:00 AM

Physics Building, Room 313

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## "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.

**Condensed Matter**

Tuesday, May 28, 2019

11:00 AM

Physics Building, Room 313

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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

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## "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.

**Condensed Matter**

Thursday, May 9, 2019

11:00 AM

Physics Building, Room 313

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## "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.

**Condensed Matter**

Thursday, May 2, 2019

3:30 PM

Physics Building, Room 313

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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".

**Condensed Matter**

Wednesday, May 1, 2019

1:00 PM

Physics Building, Room 314

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## "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*.

**Condensed Matter**

Friday, April 26, 2019

1:00 PM

Physics Building, Room 205

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## "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.

**Condensed Matter**

Thursday, April 25, 2019

11:00 AM

Physics Building, Room 313

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## "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

**Condensed Matter**

Wednesday, April 24, 2019

10:00 AM

Physics Building, Room 313

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## "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.

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.]