Condensed Matter/Biophysics Seminars
Fall 2021

Condensed Matter/Biophysics Seminars are in 2205 NPB
Mondays @ 4:05 pm to 4:55 pm

Committee: Yuxuan wang, Xiao-Xiao Zhang and Purushottam Dixit

August 30
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September 6 - No seminar (Labor Day holiday)
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September 13
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September 20
	Speaker	Dr Sarbajaya Kundu (UF)
	Title	Competing phases and critical behavior in three coupled spinless Luttinger liquids
	Abstract	In this talk, I will discuss our recent work involving electronic phase competition in a strongly correlated system of three coupled spinless Luttinger liquids - one of the simplest models where topologically nontrivial chiral orders may be realized. We study the problem as a coupled sine-Gordon model, using a perturbative renormalization group (RG) approach. In contrast with counterparts with fewer fermionic species, here the scaling procedure generates off-diagonal contributions to the phase stiffness matrix, which require both rescaling as well as large rotations of the bosonic fields. These rotations, generally non-abelian in nature, introduce a coupling between different interaction channels even at the tree-level order in the coupling constant scaling equations. We study competing phases in this system, taking into account the aforementioned rotations, and determine its critical behaviour in a variety of interaction parameter regimes where perturbative RG is possible. The phase boundaries are found to be of the Berezinskii-Kosterlitz-Thouless (BKT) type, and we specify the parameter regimes where valley-symmetry breaking, intervalley orders and chiral orders may be observed. Our approach and findings may be relevant for understanding phases and transitions at high magnetic fields in semimetals such as bismuth featuring three Fermi pockets. Ref: arXiv:1906.11053 Authors: Sarbajaya Kundu, Vikram Tripathi
	Host	Yuxuan Wang

September 27
	Speaker	Long Ju, MIT
	Title	Electron Correlation and Electron-Phonon coupling in a Trilayer Graphene/hBN Moire Superlattice
	Abstract	When two-dimensional materials with similar lattice constants are stacked vertically, spatial modulation can be induced in the form of moire superlattices. Such superlattices emerged as a novel platform to engineer interlayer interactions between electrons and phonons, which have resulted in correlated and topological electron phenomena. The experimental study of 2D moire superlattices, however, is quite challenging for conventional spectroscopy techniques. In this talk, I will show optical spectroscopy study of a particular moire superlattice that is formed between ABC trilayer graphene and hexagonal boron nitride. I will first show our FTIR photocurrent spectroscopy study of the bandstructure of moire mini-bands, and its implications on the formation of correlated electron ground states. Furthermore, I will show our observation of a strong interlayer electron-phonon coupling that is unique to moire superlattices. These results point to exciting opportunities in engineering and understanding of electronic and optical properties of 2D moire superlattices.
.	Host	Xiao-Xiao Zhang

October 4
	Speaker	Lex Kemper, NC State
	Title	Examining topology and thermodynamics using quantum computers
	Abstract	Quantum hardware has advanced to the point where it is now possible to perform simulations of physical systems and elucidate their topological and thermodynamic properties, which we will discuss in this talk. I will give a brief introduction to quantum computing and why they might be useful tools for solving problems in condensed matter physics and beyond. Following that, I will present a perspective on thermodynamics of quantum systems ideally suited to quantum computers, namely the zeros of the partition function, or Lee-Yang zeros. We developed quantum circuits to measure the Lee-Yang zeros, and used these to reconstruct the thermodynamic partition function of the XXZ model. The zeros qualitatively show the cross-over from an Ising-like regime to an XY-like regime, making this measurement ideally suitable in a NISQ environment. If time permits, I will discuss our demonstration of how topological properties of physical systems can be measured on quantum computers. We leverage the holonomy of the wavefunctions to obtain a noise-free measurement of the Chern number, which we apply to an interacting fermion model.
	Host	Peter Hirschfeld

October 11
	Speaker	Pilar Cossio, Flatiron Institute
	Title	Ligand binding and peptide design using molecular simulations
	Abstract	Molecular simulations enable the understanding of how small molecules bind to protein targets and can lead to an unsupervised design of better-binding molecules. In this seminar, first, I will present a funnel-like methodology developed for drug screening using computational methods, with particular interest in extracting the ligand-unbinding rates from molecular dynamics simulations. Then, I will present a method for designing peptides to bind with high affinity to the Major Histocompatibility complex class II, a key receptor involved in the immune response.
	Host	Purushottam Dixit

October 18
	Speaker	Jeetain Mittal, Texas A&M
	Title	Molecular organization in biology: What can computer simulations teach us?
	Abstract	The formation of membraneless organelles (MLOs) via phase separation of proteins and nucleic acids has emerged as an essential process with which cells can maintain spatiotemporal control. Despite enormous progress in understanding the role of MLOs in biological function in the last ten years or so, the molecular details of the underlying phenomena are only beginning to emerge recently. We use computer simulations of coarse-grained and all-atom models to complement experimental studies to achieve insights into the molecular driving forces underlying biomolecular phase separation. In this talk, I'll highlight results that demonstrate our approach's usefulness for identifying general principles and system-specific insights into biomolecular structure and function. These results also open up new avenues for the design of biomaterials with tunable properties. Short Bio: Jeetain Mittal is currently a Professor of Chemical Engineering at Texas A&M University. He received his doctorate in Chemical Engineering from the University of Texas, Austin, and worked as a postdoctoral research fellow at the Laboratory of Chemical Physics at the National Institutes of Health. His group is developing predictive physics-based computational tools to identify the fundamental rules that govern structural and compositional ordering in a wide variety of systems with a specific focus on the following active research projects: (1) biomolecular phase separation and (2) nanoparticle superlattice engineering by DNA-mediated interactions.
	Host	Purushottam Dixit

October 25
	Speaker	Jia Leo Li, Brown University
	Title	Engineering graphene moire structures using proximity effect
	Abstract	The discovery of magic-angle twisted bilayer graphene marks the beginning of a new chapter in quantum science research and material engineering in the 2D limit. In a short 2-3 years, a wide variety of material properties and novel physical constructions are realized in different moire structures, which cover almost all functionalities of solid-state systems including Mott-like insulators, superconductors, ferro- and antiferromagnets. In this talk, I will discuss a new method to engineer the band structure and associated emergent phenomena in graphene moire structures using proximity effect. I will describe two different types of proximity effect: proximity Coulomb screening and proximity-induced spin-orbit coupling (SOC). For example, SOC can be introduced into the moire flatband by creating an atomic interface between magic-angle twisted bilayer graphene and a tungsten diselenide crystal. I will show that proximity induced SOC not only modifies the energy band structure, but it also provides novel experimental knobs to probe and control the ground state order. In addition, I will discuss the influence of proximity effect in twisted trilayer graphene structures.
	Host	Xiao-Xiao Zhang

October 29 at 2:00pm - Special Day & Time
	Speaker	Jonathan Friedman (The Hebrew University of Jerusalem)
	Title	Synthetic Ecology: Building Microbial Communities From The Bottom Up
	Abstract	Ecosystems are arguably the most complex but least understood level of biological organization. Microbial communities, composed of numerous interacting species, are of particular importance as they play key roles in numerous application areas, including biotechnology, agriculture, and medicine. In this talk, I will discuss our recent theoretical and experimental efforts towards developing a predictive understanding of the structure and function of microbial communities. We found that community structure can be well predicted from pairwise interactions in laboratory microbial communities both on short, ecological time scales and on longer, evolutionary time scales when pairwise interactions vary over time. These results indicate that higher-order interactions among species often do not play a significant role in shaping microbial communities. These findings provide the first step towards "synthetic ecology" - the rational design of and management of microbial communities.
	Host	BingKan Xue

November 1
	Speaker	Ajit Srivastava, Emory University
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	Host	Xiao-Xiao Zhang

November 8
	Speaker	Emanuel Tutuc from University of Texas at Austin
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	Host	Dominique Laroche

November 15
	Speaker	Luiz Santos, Emory University
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	Host	Yuxuan Wang

November 22
	Speaker	Naween Anand
	Title	Room Temperature Spintronic Studies of Topological Weyl Antiferromagnet Mn₃Ge
	Abstract	In recent times, spintronics has attracted a lot of attention among the research and the industrial communities. Improving the efficiency and adding functionalities to modern electronic devices by incorporating the spin degree of freedom of electrons is the central goal of spintronics. Key aspects to realize this goal include generation, propagation, processing and detection of spin-polarized currents in suitably chosen materials or heterostructures. Towards this end, a new interest has emerged to investigate conductive antiferromagnets. Mn₃Ge, a room temperature noncollinear antiferromagnet, belongs to a class of materials commonly known as Weyl semimetals (WSMs). The WSMs are of particular interest not only because of their exotic Fermi-arc-type surface states, but also because of their appealing bulk chiral magneto-transport properties. Owing to the topological nature of the band structure of Mn₃Ge, very large anomalous Hall effect (AHE) and spin Hall effect (SHE) have been found in the bulk Mn₃Ge single crystals. We have synthesized hexagonal single phase, epitaxial, homogeneous and continuous films of Mn₃Ge using the magnetron sputtering and the molecular beam epitaxy techniques. Anomalous charge transport properties were measured using Nernst measurement which shows very strong topological thermoelectric effects in the films. The Seebeck coefficient per unit saturation magnetization for this system is one of the largest experimentally measured figure of merit so far. Moreover, the topological nature of the band structure also results in a peculiar spin transport which we characterize using the spin-pumping ferromagnetic resonance (SP-FMR) and spin-torque ferromagnetic resonance (ST-FMR) techniques. Our analysis confirms that the Mn₃Ge films show excellent spin-current generation and detection capabilities with a large spin diffusion length scale. The estimated figure of merit, the spin-Hall angle, is found to be an order of magnitude larger than the prototypical industry standard in Platinum. This promising result suggests that the power consumption for the data writing process in a magnetic data storage device could be reduced by two orders of magnitude. In addition, the spin-Hall angle shows a high degree of tunability with the applied magnetic field. In general, antiferromagnets have much faster spin reorientation dynamics which essentially lifts any limitation on the rate at which the data writing step could be carried out. Such large transport effects and tunability are highly desired for ultrafast and robust MRAM devices with larger areal density and reduced power consumption for data storage.
	Host	David Tanner

November 29
	Speaker	Yizhuang You, UCSD
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	Host	Yuxuan Wang and Xiao-Xiao Zhang

December 6
	Speaker	Shenshen Wang, UCLA
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	Host	Purushottam Dixit