Condensed Matter Seminars

Condensed Matter
Thursday, September 2, 2021
3:30 PM
Physics Building, Room 204

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"TBA"


Genya Kolomeisky , University of Virginia - Department of Physics
ABSTRACT:

TBA

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Thursday, April 29, 2021
3:15 PM
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ABSTRACT:

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.

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Thursday, April 22, 2021
3:30 PM
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"The anomalous thermal relaxations in linear chemical reactions"


Saikat Bera , University of Virginia - Department of Physics
[Host: Marija Vucelja]
ABSTRACT:

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

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Tuesday, April 20, 2021
3:30 PM
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"Gutzwiller Quantum Molecular Dynamics Simulation in Liquid"


Chen Cheng , University of Virginia - Department of Physics
[Host: Gia-Wei Chern]
ABSTRACT:

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.

 

 

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Thursday, April 15, 2021
3:30 PM
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"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]
ABSTRACT:

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 105 lattice sites, we uncover an intriguing hidden sub-lattice symmetry breaking in the phase separation dynamics of FK model.

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Thursday, April 8, 2021
2:30 PM
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ABSTRACT:

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.

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Thursday, March 25, 2021
3:30 PM
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"Quantum Wakes and Measurement Induced Chirality"


Matthew Wampler , University of Virginia - Department of Physics
[Host: Israel Klich]
ABSTRACT:

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.    

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Thursday, March 11, 2021
3:30 PM
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"Ferrimagnetic materials for room temperature small skyrmions"


Wei Zhou , University of Virginia - Department of Physics
[Host: Joe Poon]
ABSTRACT:

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 Mn4N 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 Mn4N.

 

Reference:
[1] Fert, A., et al. Magnetic skyrmions: advances in physics and potential applications. Nat Rev Mater 2, 17031 (2017).
[2] Büttner, F., et al. Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications. Sci Rep 8, 4464 (2018).
[3] C.T. Ma., et al. Robust Formation of Ultrasmall Room-Temperature Neél Skyrmions in Amorphous Ferrimagnets from Atomistic Simulations. Sci Rep 9, 9964 (2019).
 
 

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