TBA

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.

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.

Advancing the frontiers of science often requires the creation of new probes to uncover the

underlying microscopic mechanisms giving rise to exotic macroscopic phenomena, such as high-

temperature superconductivity. Can quantum entangled probes uncover the inherent

entanglement of the target matter? We have recently [1-3] developed an entangled neutron

beam where individual neutrons can be entangled in spin, trajectory, and energy. To

demonstrate entanglement in these beams we crafted neutron interferometric measurements

of contextuality inequalities whose violation provided an indication of the breakdown of

Einstein's local realism. In turn, the tunable entanglement (spin-echo) length of the neutron

beam from nanometers to microns and energy differences from peV to neV opens a pathway to

a future era of entangled neutron scattering in matter. What kind of information can be

extracted with this novel entangled probe? A recent general quantum many-body entangled-

probe scattering theory [4] provides a framework to respond to this question. Interestingly, by

carefully tuning the probe's entanglement and inherent coherence properties, one can directly

access the intrinsic entanglement of the target material. This theoretical framework supports

the view that our entangled beam can be used as a multipurpose scientific tool. We are

currently [5] pursuing several ideas and developing new spin-textured entangled beams with

OAM for future experiments in candidate quantum spin liquids, unconventional

superconductors, and chiral quantum materials.

[1] J. Shen et. al., Nature Commun. 11, 930 (2020).

[2] S. Lu et. al., Phys. Rev. A 101, 042318 (2020).

[3] S. J. Kuhn et. al., Phys. Rev. Research 3, 023227 (2021).

[4] A. A. Md. Irfan, P. Blackstone, R. Pynn, and G. Ortiz, New J. Phys. 23, 083022 (2021).

[5] Q. Le Thien, S. McKay, R. Pynn, and G. Ortiz, arXiv:2207.12419.

What physics does one hope to learn by looking closely at exotic quantum states of matter, esoteric to begin with, but even more so on curved surfaces?! In this talk first I will remind the audience why quantum hall states serve as paradigmatic topological quantum states of matter, highlighting their signature property: the quantization of Hall conductance, which is independent of sample-specific details to the extent that it is used for precise measurements of fundamental constants. Tracing the topological origin of this remarkable property, I will motivate by analogy the interplay between the geometry of these states and their response to “gravitational” perturbations, i.e., deformations to the real space manifold they are embedded in — on a cone, for example! This then naturally leads to a discussion of what, if any, universal signatures characterize this response. Finally, some reflections on the broader implications of these excursions, and connections to other branches of physics, including classical soft matter systems.



Short bio:

Dr. Biswas received his PhD from Harvard University, working with Prof. Subir Sachdev on experimentally relevant exotic quantum states of matter.  Prof. Bert Halperin served as mentor. While at Harvard Rudro held several fellowships including the James Mills Pierce Fellowship Award, the Purcell Fellowship and the Harvard Center for Energy and Environment Fellowship. Following postdoctoral research as an Institute of Condensed Matter Theory Fellow at UIUC, Rudro became an Assistant Professor of Physics at Purdue University, where he is currently.

Cuprate high-temperature superconductors feature rich phase diagrams due to the presence of various competing degrees of freedom, inevitable disorder and high anisotropy. In cuprates, the value of the upper critical magnetic field (H_c2) and the role of charge and spin orders have long been under debate. In order to reveal the nature of the quantum phases (T -> 0) and transitions between them, in a varied magnetic field, as well as to investigate the precise interplay of charge order, disorder, and high temperature superconductivity, we perform transport measurements on (1) underdoped Bi₂Sr_0.16La_0.84CuO_6+δ, which exhibits short-range charge order, (2) overdoped Bi_2.1Sr_1.9CuO_6+δ and (3) highly overdoped Bi₂Sr₂O_6+δ, where charge order has not been observed yet. Measurements of linear transport, non-linear transport and Hall effect were performed in magnetic fields (H) up to 45 T and temperatures (T) down to 17 mK.