# Condensed Matter Seminars

**Condensed Matter**

Thursday, December 16, 2021

3:30 PM

Online, Room via Zoom

Note special room.

TBA

**Condensed Matter**

Thursday, December 9, 2021

3:30 PM

Physics Building, Room via Zoom

Note special room.

TBA

TBA

**Condensed Matter**

Thursday, November 18, 2021

3:30 PM

Online, Room via Zoom

Note special room.

TBA

**Condensed Matter**

Thursday, November 11, 2021

3:30 PM

Physics Building, Room 204

## "Quantum spin Hall effect in monolayer WTe2"

**Wenjin Zhao , Cornell University**

[Host: Prof. Dima Pesin]

WTe_{2} is an example of a two-dimensional semimetal. It shows incredibly diverse and intriguing behavior such as the quantum spin Hall effect (QSH), superconductivity, ferroelectricity, and excitonic insulator, providing a new platform for studying the interplay between topology and correlations. In this talk I will discuss the helical nature of the QSH edge state in monolayer WTe_{2} and the proximity effect of a magnet upon it. In the first part, I will describe how we explore the spin-momentum locking in the QSH edge state and determine the spin axis by studying the magnetic anisotropy. In the second part, I will discuss the magnetic coupling between a two-dimensional antiferromagnet, CrI_{3}, and the QSH edge state.

**Condensed Matter**

Wednesday, November 3, 2021

3:30 PM

Online, Room via Zoom

Note special date.

Note special room.

TBA

**Condensed Matter**

Thursday, October 28, 2021

3:30 PM

Online, Room via Zoom

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## "Triangular Gross-Pitaevskii breathers and Damski-Chandrasekhar shock waves"

**Professor Maxim Olchanyi , University of Massachusetts Boston**

[Host: Prof. Israel Klich]

The recently proposed map [arXiv:2011.01415] between the hydrodynamic equations governing the two-dimensional triangular cold-bosonic breathers [Phys. Rev. X 9, 021035 (2019)] and the high-density zero-temperature triangular free-fermionic clouds, both trapped harmonically, perfectly explains the former phenomenon but leaves uninterpreted the nature of the initial (t=0) singularity. This singularity is a density discontinuity that leads, in the bosonic case, to an infinite force at the cloud edge. The map itself becomes invalid at time t=T/4. Here, we first map -- using the scale invariance of the problem -- the trapped motion to an untrapped one. Then we show that in the new representation, the solution [arXiv:2011.01415] becomes, along a ray in the direction normal to one of the three edges of the initial cloud, a freely propagating one-dimensional shock wave of a class proposed by Damski in [Phys. Rev. A 69, 043610 (2004)]. There, for a broad class of initial conditions, the one-dimensional hydrodynamic equations can be mapped to the inviscid Burgers' equation, a nonlinear transport equation. More specifically, under the Damski map, the t=0 singularity of the original problem becomes, verbatim, the initial condition for the wave catastrophe solution found by Chandrasekhar in 1943 [Ballistic Research Laboratory Report No. 423 (1943)]. At t=T/8, our interpretation ceases to exist: at this instance, all three effectively one-dimensional shock waves emanating from each of the three sides of the initial triangle collide at the origin, and the 2D-1D correspondence between the solution of [arXiv:2011.01415] and the Damski-Chandrasekhar shock wave becomes invalid.

## "Predicted Nearly Room Temperature Superconductivity in Binary Metal Hydride Systems"

**Tianran Chen , NIST Center for Neutron Research**

[Host: Prof. Seung-Hun Lee]

Due to the low atomic mass and high electron-phonon coupling strength in hydrogen-rich materials, hydride compounds under extremely high pressures are most promising in the search of high-Tc superconductors. First-principles-based computational search has become extremely important not only in predicting new materials but also in guiding high-pressure experimental measurements. In this work, we have developed a super-efficient and fast method for searching high-T hydride superconductors. We introduce new "metrics" that are strongly correlated to strong electron-phonon coupling and T but it is much faster to calculate them. Using our new method, we have searched more than 100,000 binary hydride systems and discovered several new high-T superconductors. Among them, we report our prediction of high-temperature superconductivity at relatively low pressure in a novel binary metal hydride which may break the current record. A detailed mechanism of the superconductivity, phonons, and electron-phonon coupling, anharmonicity, as well as the abnormal T -pressure dependence, will be also discussed.

**Condensed Matter**

Thursday, September 30, 2021

3:30 PM

Physics Building, Room 204

## "Machine Learning for Material Properties and Design"

**Aravind Krishnamoorthy , University of Southern California**

[Host: Utpal Chatterjee]

The accelerated discovery and design of new quantum materials requires atomic-level information about chemical reactions, phase transformations, mechanical deformations and other collective and emergent quantum phenomena. Several techniques have been developed recently that can learn the potential energy surface (PES) of complex materials. Machine Learning (ML) models, particularly deep neural networks, have proven capable of learning highly complex non-linear relationships between atomic structure and properties and theory and experiments. In this talk, I will describe two examples of ML-driven MD called neural-network quantum molecular dynamics (NNQMD) to tackle problems related to large systems and long trajectories that cannot be investigated by Quantum Molecular Dynamics (QMD).

First, we use NNQMD for quantitatively characterizing the intermediate range order, manifested as first sharp diffraction peak in GeSe_{2}. In the second example, we compute the dielectric constant, ε_{0}, and its temperature dependence for liquid water using fluctuations in macroscopic polarization using two coupled neural network models. The first network, NNQMD, learns the PES of liquid water from QMD training data. The second network, neural-network maximally localized Wannier functions, NNMLWF, is trained to predict dipole moments.

I will also briefly discuss applications of ML to discovery of new dielectric polymer materials with high breakdown strengths and to optimization of chemical vapor deposition synthesis of quantum materials.

**Joint Condensed Matter and Gravity Seminar**

Thursday, September 2, 2021

3:30 PM

Physics Building, Room 204

## "Coulomb Universe in a Jellium Droplet"

**Professor Genya Kolomeisky , University of Virginia - Department of Physics**

[Host: Prof. Israel Klich]

Analogy between the Coulomb law of interaction between charges and the Newton law of gravitational attraction between masses is familiar to every physics student. In this talk I demonstrate that this analogy implies that a system of identical charges can evolve with time in a manner that parallels cosmological evolution of the physical Universe with hallmarks such as Hubble's law and Friedmann-type dynamics present. The Coulomb and Newton laws are also dissimilar because the electrostatic force is many orders of magnitude larger than the gravitational force whose manifestations are only noticeable on astronomical scale. On the other hand, analog cosmological evolutions driven by Coulomb interactions are predicted to be observable in laboratory experiments involving Coulomb explosions and electron density oscillations in conductors.

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

**Meeting ID:** 970 0122 0790

**Password:** 498831

**Meeting ID:**

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

3:15 PM

Online, Room via Zoom

Note special time.

Note special room.

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

**Dr. Eva Zurek , SUNY Buffalo**

[Host: Bellave Shivaram]

The pressure variable opens the door towards the synthesis of materials with unique properties, e.g. superconductivity, hydrogen storage media, high-energy density and superhard materials. Under pressure elements that would not normally combine may form stable compounds or they may adopt novel stoichiometries. As a result, we cannot use our chemical intuition developed at 1 atm to predict phases that become stable when compressed.

To facilitate the prediction of the crystal structures of novel materials, without any experimental information, we have deve loped XtalOpt, an evolutionary algorithm for crystal structure prediction. XtalOpt has been applied to predict the structures of hydrides with unique compositions that become stable at pressures attainable in diamond anvil cells. In the ternary hydride system two different classes of superconductors composed of S and H atoms have been discovered - methane intercalated H3S perovskites with the CSH7 stoichiometry, and phases containing SH honeyco mb sheets. We also predict a superconducting RbB3Si3 phase in the bipartite sodalite structure that could be synthesized at mild pressures and quenched to 1 atm.

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

**Meeting ID:** 952 5366 8001

**Password:** 656540

**Meeting ID:**952 5366 8001

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

3:30 PM

Online, Room via Zoom

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

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

[Host: Marija Vucelja]

Thermal quenching is the process of rapidly cooling or heating a material. It has been practiced since ancient times to obtain desirable mechanical properties in materials, especially metals. The dynamics in play during quenching fall in the regime of non-equilibrium dynamics and is a subject of interest as most processes in nature happen out of equilibrium. A curious phenomenon during out of equilibrium processes is the so called Mpemba effect. The Mpemba effect is a phenomenon where a system prepared at a hot temperature (Thot) “overtakes” an identical system prepared at a warm temperature (Twarm) and cools down faster to be in equilibrium with a cold environment (Thot > Twarm > Tenvironment). My project involves studying the dynamics and behavior of linear chemical reaction networks during this kind of out of equilibrium process. Chemical reaction networks are a good model to study various biochemical processes, which are integral to the study of biochemical pathways and thus the functioning of cells. I am especially searching for the existence of a Mpemba like behavior in these kinds of systems and trying to characterize their behavior and dependence on the different parameters of the linear chemical reaction network. In this seminar I will be detailing on the methods used to study the out of equilibrium dynamics of linear chemical reaction networks and will be presenting the preliminary results which indicates the existence of Mpemba like behavior. This understanding will eventually lead to the optimization of chemical production for industrial application and characterization of biochemical pathways.

**Special Condensed Matter Seminar**

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

**Password:** No Password Required

**Meeting ID:**

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

3:30 PM

Online, Room via Zoom

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Note special room.

## "Gutzwiller Quantum Molecular Dynamics Simulation in Liquid"

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

[Host: Gia-Wei Chern]

The Gutzwiller approximation is a method for strongly-correlated systems, it is the simplest theory that successfully captures the correlated induced metal-insulator transition, i.e. mott transition. Density function theory (DFT) is a very efficient method to deal with many-electron systems, thus currently quantum molecular dynamics (QMD) simulations are dominantly based on DFT, however DFT fails to describe many strong electron correlation phenomenon, for example the mott transition.

We proposed a new scheme of quantum molecular dynamics based on the Gutzwiller method, the Gutzwiller quantum molecular dynamics (GQMD). A liquid Hubbard model is studied by GQMD, two schemes of mott metal-insulator transition is found at different densities, based on which a phase diagram can be given to describe different states of the Hubbard liquid system. An effort to apply GQMD to real materials is also made on hydrogen system at high temperature and pressure conditions.

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

**Meeting ID:** N/A

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**Meeting ID:**

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

3:30 PM

Online, Room via Zoom

Note special room.

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

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

[Host: Gia-Wei Chern]

The Falicov-Kimball (FK) model was initially introduced as a statistical model for metal-insulator transition in correlated electron systems. It can be exactly solved by combining the classical Monte Carlo method for the lattice gas and exact diagonalization (ED) for the itinerant electrons. However, direct ED calculation, which is required in each time-step of dynamical simulations of the FK model, is very time-consuming. Here we apply the modern machine learning (ML) technique to enable the first-ever large-scale kinetic Monte Carlo (kMC) simulations of FK model. Using our neural-network model on a system of unprecedented 10^{5} lattice sites, we uncover an intriguing hidden sub-lattice symmetry breaking in the phase separation dynamics of FK model.

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

**Meeting ID:** N/A

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**Meeting ID:**

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

2:30 PM

via Zoom, Room Online

Note special time.

Note special room.

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

**Adrian Feiguin , Northeastern University**

[Host: Israel Klich ]

I present a new computational paradigm to simulate time and momentum resolved inelastic scattering spectroscopies in correlated systems. The conventional calculation of scattering cross sections relies on a treatment based on time-dependent perturbation theory, that provides formulation in terms of Green’s functions. In equilibrium, it boils down to evaluating a simple spectral function equivalent to Fermi’s golden rule, which can be solved efficiently by a number of numerical methods. However, away from equilibrium, the resulting expressions require a full knowledge of the excitation spectrum and eigenvectors to account for all the possible allowed transitions, a seemingly unsurmountable complication. Similar problems arise when the quantity of interest originates from higher order processes, such as in Auger, Raman, or resonant inelastic X-ray scattering (RIXS). To circumvent these hurdles, we introduce a time-dependent approach that does not require a full diagonalization of the Hamiltonian: we simulate the full scattering process, including the incident and outgoing particles (neutron, electron, photon) and the interaction terms with the sample, and we solve the time-dependent Schrödinger equation. The spectrum is recovered by measuring the momentum and energy lost by the scattered particles, akin an actual energy-loss experiment. The method can be used to study transient dynamics and spectral signatures of correlation-driven non-equilibrium processes, as I illustrate with several examples and experimental proposals using the time-dependent density matrix renormalization group method as a solver. Even in equilibrium, we find higher order contributions to the spectra that can potentially be detected by modern instruments.

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

**Meeting ID:** 921 7069 3950

**Password:** 942161

**Meeting ID:**

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

3:30 PM

Online, Room via Zoom

Note special room.

## "Quantum Wakes and Measurement Induced Chirality"

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

[Host: Israel Klich]

We study the long term behavior of lattice fermions undergoing repeated particle detection, extraction, or injection interspersed with unitary evolution in two specific regimes. First, we investigate the wake pattern formed behind a moving probe performing these operations. These disturbances show dramatically different behavior where, notably, at half-filling the “measurement wake” vanishes and the “extraction wake” becomes temperature independent. Second, in analogy with the edge modes found in topologically trivial systems when undergoing floquet driving, we provide a protocol of repeated local density measurements that induces edge modes in a topologically trivial system while the hamiltonian remains time independent. In the limit of rapid measurements, the so-called Zeno limit, we connect this system to a novel stochastic dynamical system and discover an interesting double step structure in the charge transport in this regime.

**Join Zoom Meeting:**

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

**Meeting ID:** 953 0813 2506

**Password: ** 253352

**Meeting ID:**

**Password:**

Thursday, March 11, 2021

3:30 PM

Online, Room via Zoom

Note special room.

## "Ferrimagnetic materials for room temperature small skyrmions"

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

[Host: Joe Poon]

The magnetic skyrmions are topologically protected spin configuration, which stabilized by Dzyaloshinskii-Moriya interaction (DMI). Due to skyrmions’ ability to be small, stable, and controllable by electric current [1], they have considerable potential for high-density data storage applications. It is theoretically predicted that ferrimagnetic materials prefer holding small skyrmions at room temperature (RT) [2,3]. 10-15nm ferrimagnetic CoGd heterostructures and 10-15nm ferrimagnetic Mn_{4}N heterostructures were fabricated by magnetron sputtering for holding small skyrmions at RT. Magnetic force microscope images show skyrmions. A designed compound layer is capping on the top of the magnetic layer to adjust the interfacial DMI, thus tune the size of skyrmions. The micromagnetic simulation was performed to study the effect of DMI on the size of skyrmions Mn_{4}N.

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.]