Condensed Matter Seminars

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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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Thursday, November 5, 2020
9:30 AM
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"Probing the Universality of Topological Defect Formation in a Quantum Annealer"


Adolfo del Campo , Ikerbasque & DIPC
[Host: Israel Klich]
ABSTRACT:

The number of topological defects created in a system driven through a quantum phase transition exhibits a power-law scaling with the driving time. This universal scaling law is the key prediction of the Kibble-Zurek mechanism (KZM), and testing it using a hardware-based quantum simulator is a coveted goal of quantum information science. Here we provide such a test using quantum annealing. Specifically, we report on extensive experimental tests of topological defect 
formation via the one-dimensional transverse-field Ising model on two 
different D-Wave quantum annealing devices. We find that the quantum simulator results can indeed be explained by the KZM for open-system quantum dynamics with phase-flip errors, with certain  quantitative deviations from the theory likely caused by factors such as random control errors and transient effects. In addition, we probe  physics beyond the KZM by identifying signatures of universality in the distribution and cumulants of the number of kinks and their decay, and again find agreement with the quantum simulator results. This implies that the theoretical predictions of the generalized KZM theory, which assumes isolation from the environment, applies beyond its original scope to an open system. 

We support this result by extensive numerical computations. To check whether an alternative, classical interpretation of these results is possible, we used the spin-vector Monte Carlo model, a candidate classical description of the D-Wave device. We find that the degree of agreement with the experimental data from the D-Wave annealing devices is better for the KZM, a quantum theory, than for the classical spin-vector Monte Carlo model, thus favoring a quantum description of the device. Our work provides an experimental test of quantum critical dynamics in an open quantum system, and paves the way to new directions in quantum simulation experiments.

Ref.:
Yuki Bando, Yuki Susa, Hiroki Oshiyama, Naokazu Shibata, Masayuki 
Ohzeki, Fernando Javier Gómez-Ruiz, Daniel A. Lidar, Sei Suzuki, Adolfo 
del Campo, and Hidetoshi Nishimori, Phys. Rev. Research 2, 033369 (2020)

Special Seminar
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Wednesday, November 4, 2020
10:00 AM
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"Crystal structure, rotational dynamics and vibrational dynamics of a two-dimensional perovskite"


Xiao Hu , University of Virginia - Department of Physics
[Host: Seung-Hun Lee]
ABSTRACT:

Metal halide perovskites (MHPs) have come to the forefront in the photovoltaic and light-emitting devices due to their attractive optoelectronic properties, such as tunable bandgaps, long charge carrier lifetime, and bright luminescence. The interactions between charge carriers and crystal lattice, such as electron-phonon coupling, polaron formation and low thermal conductivity, are proposed to be the major reasons for their extraordinary device performance. Our previous studies have demonstrated the significant role of the organic cation mobility in screening the excited charge carriers and further extending the charge carrier lifetime in three-dimensional (3D) perovskites. Compared with 3D perovskites, however, 2D perovskites exhibit a one-order-of-magnitude longer degradation time and some of them could have extremely high photoluminescence quantum yields, which make them popular in the LED field. This talk will summarize the experimental investigations on a 2D MHP and examine the relationships between the lattice dynamics and optoelectronic properties in this type of 2D perovskites.

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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]
ABSTRACT:

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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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]
ABSTRACT:

Studies on transition metal dichalcogenides (TMD) is of great significance due to their interesting topological properties and remarkable electronic behavior. Among these materials,1T- TaX2 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-TaS2 and 1T-TaSe2 and why they differ from one another despite having the same trigonal symmetry. At very low temperatures, 1T-TaS2 undergoes a metal - insulator transition and is proposed to harbor a quantum spin liquid behavior whereas 1T-TaSe2 remains metallic. Investigating the local structure of pristine 1T-TaS2 and 1T-TaSe2 in the CDW regime could tell us the differences in local atomic correlations in these compounds.

Webinar

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Thursday, March 26, 2020
11:00 AM
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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]
ABSTRACT:

Mo1-xWxTe2 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, MoTe2 and WTe2 are Weyl semimetals upon cooling to the orthorhombic Td phase. Mo1-xWxTe2 undergoes a structural phase transition from a high-temperature monoclinic 1T' phase, to a non-centrosymmetric orthorhombic Td phase at low temperatures through a first-order structural phase transition. Both 1T' and Td 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 Mo1-xWxTe2 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 Mo1-xWxTe2. 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 Td* phase was discovered for the first time across the Td-1T’ phase boundary in Mo1-xWxTe2 with x up to ~0.2. In WTe2, a sharp transition from Td 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

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"Design principles of biological and chemical intelligence"


Zhiyue Lu , University of North Carolina at Chapel Hill
[Host: Marija Vucelja]
ABSTRACT:

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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"Generalized 't Hooft anomalies"


Mohamed Anber , Lewis & Clark University
[Host: Peter Arnold]
ABSTRACT:

'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]
ABSTRACT:

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.

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