TBA

Abstract: The thermodynamic interpretation of Schwarzschild black holes has a rich history spanning five decades. In 1973 Bardeen et al. derived the laws of black hole mechanics which suggested a correspondence between (i) temperature and black hole’s mass, specifically T=(8πM)^-1 and (ii) entropy and black hole area: S=A/4. In 1975 Hawking found that a Schwarzschild black hole radiates as if it were a blackbody with temperature T=(8πM)^-1. This meant that the temperature-mass relationship discovered earlier was more than a correspondence. In 1977 Gibbons and Hawking proposed the action of Euclidean Schwarzschild (which also has T=(8πM)^-1) as a means to understand black hole thermodynamics. They recovered S=A/4.  

In this talk we will test if Euclidean Schwarzschild behaves like a heat bath for quantum matter that propagates on it. For simplicity we will use matter with restricted excitations – Ising spins. We will discover that increasing M causes the spins to undergo a second order phase transition from disorder to order and that the phase transition occurs at sub-Planckian M. 

The role of quantum mechanical coherences or coherent superposition states in excited state processes has received considerable attention in the last two decades owing much to advances in ultrafast laser spectroscopy. The existence of coherence effects shows promise for enhancing the efficiency and robustness of functionally relevant processes, even when confronted with strong energy disorder and environmental fluctuations. Therefore, an in-depth understanding of coherence propels us to push the frontier to the grand challenge of using optical control of coherence to improve functions or create new ones in molecular and material systems. In this frontier, the role of electronic and vibrational interplay, or more specifically the role of vibrations in directing electronic dynamics, has emerged as the leading principle, where two energetically disparate quantum degrees of freedom work in-sync to dictate the trajectory of an excited state reaction. Moreover, with the vibrational degree being directly related to the structural composition of molecular or material systems, new molecular designs could be inspired by tailoring certain structural elements.

In this talk, I envision acquainting the quantum world of molecules to physicists. I will summarize essential aspects concerning the interplay of electronic and vibrational dynamics afforded from state-of-the-art ultrafast laser spectroscopy in three exemplary processes: photo-induced electron transfer, singlet-triplet intersystem crossing, and intramolecular vibrational energy-driven energy flow in molecular systems. More precisely, I will showcase crucial experimental signatures that offer deeper insights into the complex electronic-vibrational trajectories encompassing excited states. I will discuss rapid decoherence—loss of phase and amplitude correlations—of vibrational coherences along promoter vibrations in comparison to spectator coherences in a near-ballistic (~30-femtosecond time constant) electron transfer reaction in the Marcus-inverted region and during a sub-picosecond intersystem crossing dynamics in a series of binuclear platinum complexes. The rapid decoherence depicts the vibration-driven reactive pathways from Franck-Condon state to the curve crossing region. I will also discuss the generation of new vibrational coherences induced by impulsive reaction dynamics—not by the laser pulse—in these systems which informs on specific energy dissipation pathways and thereby on the progression of the reaction trajectory in the vicinity of the curve crossing on the product side. While the decoherence or impulsive generation of vibrational wavepackets has emerged as a prominent indicator of the interplay of electronic dynamics and vibrations, the other property of vibrational coherences, amplitude, also relays how energy can flow from one vibration to another vibration in the electronic excited state. 

From these studies, we learn that superposition states along vibrations that are part of the complex reactive trajectories are extremely sensitive to electronic dynamics. Thus, vibrational superposition states act as quantum mechanical windows for visualizing the interplay of electronic and vibrational dynamics. This frontier could change the outlook on how vibrations might soon become a control element in the hands of a chemist, influencing the outcome of a reaction.

R-Type Ferrites (RTF) constitute a large isostructural class of transition element oxides featuring quasi-two-dimensional Kagome sublattices.  We review a decade of experiments covering a large range of compositional variations, ATM₅O₁₁ (A= alkali earth, T= 3d element, M= Ru and 3d elements).  The physical properties of RTF are remarkably sensitive to atomic disorder, and crucial roles for spin-orbit interactions and magnetic frustration are indicated.  The availability of single-crystal samples has revealed a startling panorama of RTF ground states and exotic physical properties:  

Disordered iron-bearing RTF are narrow-gap semiconductors with colinear ferrimagnetic order well above 300 K with T_C proportional to the semiconducting gap, and properties suitable for spin injection applications.  In contrast, disordered Co- and Mn-bearing RTF exhibit canted, “all-in/all-out” ordering substantially below 300 K in the frustrated Kagome sublattice.  Atomically ordered RTF (T= Ni, Zn, M= Ru) do not order to below 4 K, and display spin fluctuations and unusual non-Fermi-liquid behavior.  Many RTF compositions display large anomalous topological Hall effects that involve Berry phase effects in the magnetization textures.  

In spite of the wide variations of ground states and physical properties, the lattice parameters of RTF vary by as little as 0.1%, which suggests fabrication of epitaxial RTF heterostructures with unique physical properties may be feasible for a range of applications.

*Research supported by U.S. DoE Grant #DE-FG02-97ER45653 and the Kentucky Science and Engineering Foundation.  
 

Two-dimensional (2D) van der Waals (vdW) materials are thin layers of crystals that are held together by the weak vdW forces. The recent discovery of long-range magnetic order in 2D vdW materials has triggered a renaissance in the study of 2D magnetism for both fundamental science and technological advances. In this talk, I will present our recent work on chromium
chalcogenides, a family of 2D self-intercalated vdW magnets which possess a broad spectrum of intriguing magnetic properties, including high temperature ferromagnetism, topological spin textures, and giant anomalous Hall conductivity. I will discuss the bottom-up synthesis of these 2D magnets, the correlation between their structural phases and emergent magnetism, as well as the new opportunities that they may offer and the challenges that need to be overcome for
spintronic applications.

Title:  Angle-resolved transport nonreciprocity as a new method for probing 2D electrons

Abstract: This talk focuses on a new aspect of electronic properties in a 2D confinement based on the angle-resolved transport nonreciprocity measurement. Nonreciprocity is defined as the difference in DC resistance between forward and reverse current bias. The angular dependence of nonreciprocity allows us to identify symmetry axes associated with electronic states under the influence of strong correlation. I will discuss the evolution of angle-resolved transport nonreciprocity in the phase space of multi-layer graphene samples, where the flat energy band enhances Coulomb correlation and stabilizes a host of exotic quantum phases. By investigating the interplay between transport nonreciprocity, ferromagnetism, and superconductivity, I will show that angle-resolved transport nonreciprocity offers an efficient new tool for probing 2D electrons. 

Chern insulator ferromagnets are a fascinating area of research in condensed matter physics. These materials exhibit a quantized anomalous Hall effect, which has been observed experimentally in various systems including magnetically-doped topological insulator (MTI) thin films and bilayer graphene moiré superlattices. Chern insulator ferromagnets are classified into two categories, spin or orbital, based on the origin of their orbital magnetization. In the case of spin Chern insulator (e.g. MTIs), the orbital magnetization arises from spontaneous spin-polarization combined with spin-orbit interactions. In contrast, orbital Chern insulators (e.g. graphene-based moiré superlattices) exhibit spontaneous orbital currents that give rise to the orbital magnetization. Understanding the differences between these two types of Chern insulators is crucial for developing new materials with interesting properties. In this talk, I will emphasise the curious magnetic properties of orbital Chern insulator using magic-angle twisted bilayer graphene as an example.

The two-dimensional electron gas (2DEG) is one of the simplest models in condensed matter physics to write down, yet has a rich phase diagram that exhibits a variety of interesting correlated phenomena. At weak coupling the system behaves essentially as a gas, while at strong coupling the system solidifies into the Wigner crystal state. However, there is a broad range of intermediate coupling where the system behaves as a strongly correlated quantum liquid, and in this regime the behavior is still not fully understood. I will discuss how 2D transitional-metal dichalcogenide (TMD) systems promise to be a fruitful platform for interrogating the behavior of the 2DEG at intermediate coupling and will describe experiments demonstrating the existence of Wigner crystal states in these systems, as well as unexpected intermediate phases between the liquid and solid. I will also present new theoretical results that suggest, in contrast to previous reports, the Wigner crystal phase over a broad range of density is likely to be a metallic charge-density wave state, better understood as a "quantum crystal” with a finite concentration of itinerant ground-state defects. 

Multiscale modeling methods are typically envisioned as precise and predictive
simulation tools to solve complex science and engineering problems. However, even
conventional atomistic models are often insufficient in terms of computational efficiency
and accuracy to provide reliable information for the large-scale continuum models. In this
seminar, I will focus on method developments aimed at overcoming these critical
limitations.
At the beginning of the talk, I will introduce how atomic models can help us understand
the experimental observation of crystal growth in 2D materials using empirical reactive
forcefield (FF). Although atomistic models can provide useful insight at the atomic scale,
developing reliable FFs is extremely limited due to the fixed potential expressions.
Recently, neural network (NN) potentials (or surrogate models) have emerged as a way to
overcome the long-standing limitations of empirical potentials.
I will present recent developments that integrate ab-initio level calculation (DFT and
DFTB) and a PyTorch implementation of NN potentials (TorchANI) into the LAMMPS
molecular dynamics software. I will discuss the pros and cons of NN potentials illustrated
by a simple carbon system, graphene. While NN potentials can provide higher accuracy
than other FFs, e.g., ReaxFF and AIREBO, and lower computational cost than quantum
calculations, efficient sampling or data generation arises as a critical issue.
In the end, I will present the ongoing development of active learning capabilities through
LAMMPS and NNPs as an efficient sampling method for rare events. The developed
capabilities will provide useful tools for fundamental understanding of the chemical
process and mechanistic insights into the predictive design and interpretive simulation
of materials properties and processes.

Bio
GS Jung is a Research Staff at Oak Ridge National Laboratory. His research interests are
on the multiscale modeling of materials to understand their fundamental properties from
synthesis and growth to performance and failures. Before joining ORNL, he earned his
Ph.D. in multiscale modeling for 2D materials from the Massachusetts Institute of
Technology.