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Condensed Matter/Biophysics Seminars
Spring 2022

Condensed Matter/Biophysics Seminars
Mondays at 4:05pm in 2205 NPB

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

January 10 - Special Colloquium
	Speaker	Benjamin Geisler (U. Duisberg-Essen) via Zoom
	Title	Exploring the Physics of Complex Oxides: From First Principles to Deep Learning
	Abstract	Transition metal oxides host a variety of intriguing physical phenomena that result from electronic correlations and the strong coupling of charge, spin, orbital, and lattice degrees of freedom. Precise atomic-scale growth techniques yield complex oxides which exhibit novel effects that are absent in the constituent bulk compounds. High-performance computing proves to be essential for a fundamental understanding of the underlying mechanisms. In the first part of this talk, I will demonstrate in the spirit of a first-principles materials design how to devise artificial transition metal oxides with tailored thermoelectric properties for energy conversion applications [1, 2]. The second part is motivated by the recent observation of superconductivity in infinite-layer NdNiO2 films on SrTiO3(001) [3], which is absent in bulk NdNiO2. Simulations unraveled the key role of the interface: Polarity mismatch drives a surprising electronic reconstruction that results in the emergence of a correlated two-dimensional electron gas in the substrate. The concomitant depletion of the self-doping Nd 5d states renders infinite-layer nickelates close to cuprate superconductors [4]. Finally, I will provide a broader perspective on the infinite-layer materials class and illustrate how artificial intelligence, firmly rooted in fundamental physical principles, allows unconventional insights and, with its fast predictive power, complements modern materials design [5]. [1] B. Geisler, P. Yordanov, M. E. Gruner, B. Keimer, and R. Pentcheva, PSSB 2100270 (2021) [2] B. Geisler and R. Pentcheva, WO 2018/146269, Patent granted (2020) [3] D. Li et al., Nature 572, 624 (2019) [4] B. Geisler and R. Pentcheva, Phys. Rev. B 102, 020502(R) (2020) [5] A. Sahinovic and B. Geisler, Phys. Rev. Research 3, L042022 (2021)
	Host	Peter Hirschfeld

January 18 (Tuesday) - Special Colloquium
	Speaker	Yaxian Wang (Harvard University) via Zoom
	Title	Unconventional transport phenomena in quantum materials
	Abstract	An ever-increasing societal demand calls for sustainable energy and quantum information. Elucidating exotic transport and response phenomena in quantum materials is needed not only in designing next-generation modern devices, but also in discovering new physics. In this talk, I will present two of my recent theoretical discoveries in goniopolar materials and hydrodynamic transport. More specifically, I constructed the theory of axis-dependent conduction polarity, termed "goniopolar," where the same population of charge carriers can simultaneously conduct as n-type and p-type along orthogonal crystallographic axes, originating from the Fermi surface topology. I built a consolidated analytical model and proposed the chemical design principles in semimetals and semiconductors. My predictions enabled transverse thermoelectric devices with an unprecedentedly high figure of merit, which can serve as a promising route to improve energy conversion. Further, in quantum materials with nontrivial band topologies and strong interactions, electron scattering can give rise to unusual transport phenomena. For example, electrons can flow collectively, termed "hydrodynamic" exhibiting classical fluid phenomena such as vortices and Poiseuille flow. I utilized first principles tools to investigate the hierarchy of electron-scattering lifetimes in layered semimetal tungsten ditelluride (WTe2) at different temperatures, among which the phonon-mediated scattering mechanism could give rise to hydrodynamic behavior at intermediate temperatures. The theory also shows quantitative agreement with spatial electron current profiles measured using cryogenic scanning magnetometry within an exfoliated WTe2 sample. This microscopic mechanism opens up new possibilities in the search for hydrodynamic flow and strong interactions in high carrier density materials, which I will elaborate on with generalized fingerprints of electronic and phononic structures. Finally, perspectives for the future research directions into emerging energy and quantum materials will also be provided.
	Host	Hai Ping Cheng

January 24 - Special Colloquium
	Speaker	Qimin Yan (Temple University) via Zoom
	Title	Data-centric materials design in the quantum regime: motif learning and symmetry-guided discovery
	Abstract	Materials design in the quantum regime call for the integration of multi-tier materials information that go beyond atomic structures. Many quantum behaviors are greatly controlled by local symmetries and local bonding environments. In this talk, motivated by Pauling’s rules, I will show that local bonding environments (motifs) can be incorporated in a graph-based machine learning architecture to make reliable property predictions for solid-state quantum materials including complex metal oxides. I will demonstrate that the unsupervised machine (Atom2Vec and Motif2Vec) can learn the basic properties of atoms and motifs by themselves from the extensive database of known materials. Clustering of atoms and motifs in vector space classifies them into meaningful groups consistent with human knowledge. The proposed atom-motif dual network model demonstrates the feasibility to incorporate beyond-atom materials information in a graph network framework and achieves the state-of-the-art performance in predicting the complex properties of solid-state quantum materials. With these tools developed, I will discuss the potential application of artificial intelligence in the field of quantum materials design, including two-dimensional quantum materials and defect qubit-based quantum information science. I will also discuss the continued development of AI-driven technologies for quantum phenomena, with the consideration of symmetries, orbital interactions, and physical constraints.
	Host	Greg Stewart

January 31
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February 7
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February 14
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February 21
	Speaker	John Barton (University or California Riverside)
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February 28
	Speaker	Rick Greene (University of Maryland)
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	Host	Peter Hirschfeld

March 7
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March 14
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March 21
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March 28
	Speaker	Silas Hoffman (UF Physics)
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April 4
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April 11
	Speaker	Rafael Fernandes (UMN)
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	Host	Yuxuan Wang

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
	Title	2D Materials: A New Platform to Realize "Quantum Light-Matter"
	Abstract	Atomically thin materials, such as graphene and transitional metal dichalcogenides (TMDs), have recently come to the forefront of research in materials physics. This is largely due to the ease with which they can be combined into artificially engineered heterostructures that exhibit emergent electronic and optical properties. Enhanced Coulomb interactions in the truly 2D limit makes TMDs, such as MoSe2/WSe2, promising to explore correlated quantum phases of matter. Moreover, the same interactions also lead to very strong light-matter couplings, resulting in rich exciton physics and half-light, half-matter polaritonic states. Finally, the presence of non-trivial geometry and topology in electronic and optical states of these materials is an additional ingredient to realize coupled phases of quantum light and matter – quantum light-matter – that are not only interesting from a fundamental perspective but can also have applications in quantum information processing applications. In this talk, I will begin by highlighting some unique properties of optical excitations in TMDs which result from the chiral nature of constituent single-particle electronic states. The confinement of these optical excitations in TMD heterostructures results in quantum emitters whose emission energy can be tuned. Moreover, few-body and many-body dipolar interactions amongst them is promising for realizing on-demand quantum matter in a driven-dissipative setting.
	Host	Xiao-Xiao Zhang

November 8
	Speaker	Emanuel Tutuc from University of Texas at Austin
	Title	Twist-Controlled van der Waals Heterostructures
	Abstract	Heterostructures of atomic layers such as graphene, hexagonal boron-nitride, and transition metal dichalcogenides can serve as testbed for novel quantum phenomena in two-dimensions, and potential device applications. A key ingredient that has added a new dimension to the atomic layer heterostructures palette is the rotational control, and alignment of different two-dimensional (2D) layers. We review here the experimental techniques that enable rotationally controlled heterostructures with accurate alignment of the individual layer crystal axes [1], and illustrate the applicability of this technique to rotationally aligned double layers of graphene separated by a tunnel barrier which display resonant, energy- and momentum-conserving tunneling in vertical transport, consistent with theoretical expectations [2]. When two 2D layers are overlaid with a relative twist, the resulting heterostructure shows a new type of periodicity associated with the moiré superlattice. We discuss the transport and thermodynamic properties of twisted double bilayer graphene heterostructures, which reveal correlated insulators at fractional fillings of the moiré Brillouin zone, and the emergence of a time-reversal-invariant Chern insulator at charge neutrality in the presence of an applied transverse electric field [3]. [1] K. Kim et al., Nano Lett. 16, 1989 (2016); K. Kim et al., Proc. Natl. Acad. Sci. USA 114, 3364 (2017). [2] G. W. Burg et al., Nano Lett. 17, 3919 (2017); G. W. Burg et al., Phys. Rev. Lett. 120, 177702 (2018). [3] G. W. Burg et al., Phys. Rev. Lett. 123, 197702 (2019); Y. Wang et al., arXiv:2105.07104 (2021).
	Host	Dominique Laroche

November 15
	Speaker	Luiz Santos, Emory University
	Title	Hofstadter Superconductors
	Abstract	The spectrum of electrons subject to a 2D periodic potential and a constant magnetic field is formed by Hofstadter bands characterized by fractal and topological properties that are traditionally associated with the quantum Hall effect. In recent years, the ability to realize such appealing band structures, both in AMO and in moiré superlattices, has renewed interest in exploring novel quantum orders in Hofstadter systems. In this talk, I will discuss our recent work [1], which investigates the properties of mean-field states resulting from the pairing of electrons in time-reversal broken Hofstadter bands in 2D lattices where the unit cell traps magnetic flux Ø = (p/q)Ø0 comparable to the flux quantum Ø0 = h/e. It will be described how the dimension and degeneracy of the irreducible representations of the magnetic translation group (MTG) furnished by the charge 2e pairing fields have different properties from those furnished by single particle Bloch states, and in particular are shown to depend on the parity of the denominator q. This symmetry analysis will set the stage for a phenomenological Ginzburg-Landau theory describing the thermodynamic properties of Hofstadter superconductors in terms of a multicomponent order parameter that describes the finite momentum pairing of electrons across different Fermi surface patches. I will describe the characteristic properties of the phase diagram resulting from different symmetry breaking patterns of the MTG, and show that it can support several interesting phases such as topological superconductors with tunable Chern numbers as well as Bogoliubov Fermi surfaces protected by parity and MTG symmetries that are captured by a new topological invariant. Consequently, Hofstadter superconductors emerge as a new setting to explore the rich interplay of symmetry broken and topological orders. [1] Theory of Hofstadter Superconductors, D. Shaffer, J. Wang and L.H. Santos, Phys. Rev. B 104, 184501 (2021)
	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
	Title	Kohn-Luttinger Superconductivity and Inter-Valley Coherence in Rhombohedral Trilayer Graphene
	Abstract	Motivated by recent experiments on ABC-stacked rhombohedral trilayer graphene (RTG) which observed spin-valley symmetry-breaking and superconductivity, we study instabilities of the RTG metallic state to symmetry breaking orders. We find that interactions select the inter-valley coherent order (IVC) as the preferred ordering channel over a wide range, whose theoretically determined phase boundaries agree well with experiments on both the hole and electron doped sides. The Fermi surfaces near van Hove singularities admit partial nesting between valleys, which promotes both inter-valley superconductivity and IVC fluctuations. We investigate the interplay between these fluctuations and the Hunds (intervalley spin) interaction using a renormalization group approach. For antiferromagnetic Hund's coupling, intervalley pairing appears in the spin-singlet channel with enhanced T_c, that scales with the dimensionless coupling g as T_c\sim\exp(-1/\sqrt{g}) , compared to the standard \exp(-1/g) scaling. In its simplest form, this scenario assumes a sign change in the Hund's coupling on increasing hole doping. On the other hand, the calculation incorporates breaking of the independent spin rotations between valleys from the start, and strongly selects spin singlet over spin triplet pairing, and naturally occurs in proximity to the IVC, consistent with observations.
	Host	Yuxuan Wang and Xiao-Xiao Zhang

December 6
	Speaker	Shenshen Wang, UCLA
	Title	Physical constraints and driving forces of adaptive immunity
	Abstract	The adaptive immune system of jawed vertebrates must protect the host against a vast and changing multitude of microscopic invaders. Given the immense diversity and variability of the pathogen universe, how is this possible? An immune response starts with physical engagement of immune receptors and antigenic ligands at cell-cell interfaces, and ends with the formation of immune memory which allows vaccines to work. In between, a remarkable evolutionary process takes place inside us at a striking speed. Using antigen recognition and antibody evolution as examples, I will present a view that driving forces of diverse nature keep the immune system out of equilibrium, allowing it to exploit physical and evolutionary constraints to discover adaptable solutions to unforeseen challenges. I hope to demonstrate that statistical mechanics offers frameworks and tools essential for understanding living systems as complex as the immune system, and uncovers ways to steer our natural defense when needed.
	Host	BingKan Xue