Condensed Matter/Biophysics
Seminars
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Condensed
Matter/Biophysics Seminars Committee:
Yuxuan wang, Xiao-Xiao Zhang and Purushottam Dixit |
January 10 - Special Colloquium
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Speaker |
Benjamin Geisler (U. Duisberg-Essen)
via Zoom |
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Title |
Exploring the Physics of Complex Oxides: From First Principles to Deep Learning |
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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) |
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Host |
Peter Hirschfeld |
January 18 (Tuesday) - Special Colloquium
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Speaker |
Yaxian Wang (Harvard University) via Zoom |
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Title |
Unconventional transport phenomena in quantum materials |
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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. |
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Host |
Hai Ping Cheng |
January 24 - Special Colloquium
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Speaker |
Qimin Yan (Temple University) via Zoom |
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Title |
Data-centric materials design in the quantum regime: motif learning and symmetry-guided discovery |
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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. |
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Host |
Greg Stewart |
January 31
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February 7
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February 14
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February 21
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Speaker |
John Barton (University of California Riverside) |
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Viral evolution through the lens of statistical physics |
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Abstract |
Pathogens such as HIV and SARS-CoV-2 pose serious threats to public health. Better understanding of how these pathogens evolve could inform efforts to control outbreaks and improve the design of vaccines. In this talk, I’ll discuss how we can model the evolution of viruses quantitatively. I’ll also show how we can use ideas from statistical physics to fit quantitative evolutionary models to data, allowing us to understand how different mutations affect viral replication or transmission. We’ll explore two examples: understanding how HIV evolves within patients to escape from the immune system and how SARS-CoV-2 has evolved to become more transmissible over the course of the pandemic. |
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Host |
Purushottam Dixit |
February 28
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Speaker |
Xiaoxing Xi (Temple University) |
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Challenges Facing Academics of Chinese Descent and Those Who Collaborate with Scientists in China |
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Abstract |
Collaborating with scientists in China was once encouraged by the US government and universities. As tension between the two countries rises rapidly, those who did, especially scientists of Chinese descent, are under heightened scrutiny by the federal government. Law enforcement officials consider collaborating with Chinese colleagues "by definition conveying sensitive information to the Chinese." In 2015, I became a casualty of this campaign despite being innocent. In 2018, the Department of Justice established the "China Initiative," which has resulted in numerous prosecutions of university professors for allegedly failing to disclose China ties. In this talk, I will discuss the criminal prosecutions of Professor Anming Hu of University of Tennessee, Professor Charles Lieber of Harvard University, and Professor Gang Chen of MIT as well as government policies and congressional actions on research security. Collectively, they have created an environment overtly hostile to academics of Chinese descent and scientific exchanges with China. It is a tall order to convince the public and policy makers that this is not in America's interest, but the scientific community must try lest the American leadership in science and technology will be irreparably damaged. |
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Peter Hirschfeld |
March 7
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March 14
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March 21
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Speaker |
Ellen Kang (UCF) |
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Regulation of actin cytoskeleton mechanics and structure by intracellular environmental factors |
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The mechanical and structural properties of the actin cytoskeleton drive many cellular processes, including structural support of the plasma membrane, cell motility, and force generation. Actin monomers assemble into double-stranded helical filaments as well as higher-ordered structures such as bundles and networks. Cells incorporate macromolecular crowding, cation interactions, and actin-crosslinking proteins to regulate the organization of actin bundles and networks. Although the roles of each of these intracellular environmental factors in actin assembly dynamics have been shown individually, how combined factors contribute to the organization and mechanics of actin cytoskeleton is not well established. We investigate the mechanisms of how intracellular environmental factors influence actin cytoskeleton mechanics and structure. In particular, the effects of macromolecular crowding, cation interactions, and actin-crosslinking proteins on actin bundle organization, structure, and mechanics will be highlighted. Understanding these molecular mechanisms will allow us to better understand how changes in actin mechanics and structure are linked to cell physiology as well as human disease states. |
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Juan Guan and Yoonseok Lee |
March 28
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Speaker |
Silas Hoffman (Lab of Physical Sciences, College Park, Maryland) |
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Localized states in superconductors from magnetic adatoms, chains, and latices |
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Magnetic adatoms on the surface of superconductors support localized states, known as Yu-Shiba-Rusinov states, that can be used as building blocks for larger structures. Experimental evidence has suggested that chains of such adatoms support topological superconductivity and thus support Majorana zero modes at their ends. After introducing recent experimental and theoretical efforts interfacing magnetic adatoms and superconductors, I propose a lattice of adatoms with a specific magnetic texture, a chain of antiferromagnetic skyrmions, as a realization of topological superconductivity. Upon calculating the phase diagram of this system, we find that such a texture can also host Majorana zero modes at its ends. |
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April 4
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April 6 - 3:00pm - special day and time
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Chun Hung (Joshua) Lui, Department of Physics and Astronomy, University of California, Riverside |
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Optical spectroscopy of novel excitonic states and electronic phases in 2D semiconductors and moiré superlattices |
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Two-dimensional (2D) valley semiconductors, such as MoS2, MoSe2, WS2 and WSe2, host robust excitonic states with remarkable optical, optoelectronic, and valleytronic properties for novel applications. We investigate the electronic and excitonic states in the 2D semiconductors and moiré superlattices by optical spectroscopy. In the monolayer, we observe a panoply of excitonic states, including bright and dark, ground and excited excitonic states. When two monolayers are stacked together, the bilayer can host excitons with vertical dipole, which enables effective control of the excitons by an electric field. Remarkably, if the two layers have different lattice constants and/or twist angle, they can form moiré superlattices. The moiré superlattices can significantly modify the characteristics of electrons, excitons and trions, giving rise to moiré trions and numerous correlated electronic phases with distinctive optical signatures. These rich results motivate further experimental and theoretical research on the novel correlated phenomena in 2D systems. |
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April 11
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Speaker |
Rafael Fernandes (UMN) |
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Electronic nematicity and phason excitations in twisted moiré systems |
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Four years ago, the discovery of superconductivity and correlated insulating states in twisted bilayer graphene unveiled a new platform to investigate strongly-correlated and topological quantum matter. Since then, several tunable twisted moiré systems displaying nearly flat bands have been discovered, and interesting symmetry-breaking electronic states have been observed. A prominent one is electronic nematic order, which consists of the spontaneous breaking of the threefold rotational symmetry of the emergent moiré superlattice. In this talk, we present a comprehensive model for the electronic nematicity of twisted moiré systems. Described by a Potts-like order parameter, it is fundamentally different from the Ising-nematic state typically observed in bulk unconventional superconductors. This difference is rooted in the strong coupling between the electronic nematic degrees of freedom and the elastic excitations of the underlying lattice. The key point is that, unlike those bulk systems, the moiré superlattice is not a rigid crystal. As a result, its collective low-energy excitations are not the usual propagating acoustic phonons, but diffusive and gapped phasons. We show how phason-mediated interactions qualitatively alter the properties of the Potts-nematic state in twisted moiré systems, promoting novel behavior that cannot be realized in crystalline lattices. We also briefly discuss the relationship between electronic nematicity and the superconductivity of twisted moiré systems. |
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Yuxuan Wang |
Condensed Matter/Biophysics
Seminars
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Condensed
Matter/Biophysics Seminars are in 2205 NPB
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 |
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Dr Sarbajaya Kundu (UF) |
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Competing phases and critical behavior in three coupled spinless Luttinger liquids |
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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
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Host |
Yuxuan Wang |
September 27 |
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Speaker |
Long Ju, MIT |
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Electron Correlation and Electron-Phonon coupling in a Trilayer Graphene/hBN Moire Superlattice |
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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. |
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Host |
Xiao-Xiao Zhang |
October 4 |
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Speaker |
Lex Kemper, NC State |
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Examining topology and thermodynamics using quantum computers |
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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.
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Host |
Peter Hirschfeld |
October 11 |
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Speaker |
Pilar Cossio, Flatiron Institute |
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Ligand binding and peptide design using molecular simulations |
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Host |
Purushottam Dixit |
October 18 |
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Molecular organization in biology: What can computer simulations teach us? |
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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.
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Host |
Purushottam Dixit |
October 25 |
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Speaker |
Jia Leo Li, Brown University |
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Title |
Engineering graphene moire structures using proximity effect |
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Abstract |
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Host |
Xiao-Xiao Zhang |
October 29 at 2:00pm - Special Day & Time |
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Speaker |
Jonathan Friedman (The Hebrew University of Jerusalem) |
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Title |
Synthetic Ecology: Building Microbial Communities From The Bottom Up |
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Abstract |
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Host |
BingKan Xue |
November 1 |
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Speaker |
Ajit Srivastava, Emory University |
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Title |
2D Materials: A New Platform to Realize "Quantum Light-Matter" |
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Abstract |
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Host |
Xiao-Xiao Zhang |
November 8 |
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Speaker |
Emanuel Tutuc from University of Texas at Austin |
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Title |
Twist-Controlled van der Waals Heterostructures |
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Host |
Dominique Laroche |
November 15 |
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Speaker |
Luiz Santos, Emory University |
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Title |
Hofstadter Superconductors |
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Abstract |
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Host |
Yuxuan Wang |
November 22 |
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Speaker |
Naween Anand |
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Title |
Room Temperature Spintronic Studies of Topological Weyl Antiferromagnet Mn3Ge |
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Abstract |
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Host |
David Tanner |
November 29 |
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Speaker |
Yizhuang You, UCSD |
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Title |
Kohn-Luttinger Superconductivity and Inter-Valley Coherence in Rhombohedral Trilayer Graphene |
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Abstract |
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Host |
Yuxuan Wang and Xiao-Xiao Zhang |
December 6 |
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Speaker |
Shenshen Wang, UCLA |
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Title |
Physical constraints and driving forces of adaptive immunity |
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Abstract |
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Host |
BingKan Xue |