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Condensed Matter Seminar

Joe Ross, TAMU
Speaker: Joe Ross, TAMU Title: Presentation for new-coming CM graduate students Abstract: Host: J. Ross

26 Aug 2016, 4:00PM | MIST M102
Hosted By: J. Ross

A. Govorov, Ohio University
Speaker: A. Govorov, Ohio University Title: Abstract: Host:

2 Sep 2016, 4:00PM | MIST 102

Zohar Nussinov, Washington University
Speaker: Zohar Nussinov, Washington University Title: Glassy dynamics as a quantum problem Abstract: We review the underpinning of the microcanonical ensemble and the more refined (and explicitly quantum) "Eigenstate Thermalization Hypothesis". We then find and apply a simple corollary of these to analyze the evolution of a liquid upon supercooling to form a structural glass. Simple theoretical considerations lead to a predictions for general properties of supercooled liquids. Amongst other things, a collapse of the viscosity of glass formers is predicted from this theory. This collapse indeed occurs over 16 decades of relaxation times for all known types of glass formers. Host: H. Katzgraber

16 Sep 2016, 4:00PM | MIST 102
Hosted By: H. Katzgraber

Mohammad H. Hamidian, Harvard University
Speaker: Mohammad H. Hamidian, Harvard University Title: Directly Imaging The Super States of Nature Abstract: A superconductor is a homogeneous quantum condensate of Cooper pairs, each formed by binding two electrons into a zero-spin, zero-momentum eigenstate. In 1964 Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) [1,2] proposed an alternative ground state wavefunction of Cooper pairs thus arriving at a new super state of electronic matter. The resulting pairs carry momentum Q requiring the superfluid density to modulate with wavevector Q. Remarkably, the same phenomenology appears in the theory of strong interactions described by quantum chromodynamics. At high baryonic densities, the quark-quark condensate, which forms the superfluid, is predicted to also develop spatial variations and the idea has found applicability in the physics of compact stars. The last 40 years has seen a proliferation of novel and exotic superconductors (organics, iron-based, ruthenates, heavy fermions, and cuprates) but, despite decades of effort, FFLO-like states have not been observed in any material. The challenge to detect modulated superfluids has become particularly urgent because of implications for the modern theory of high temperature superconductivity, and in particular for the class of materials with the highest of superconducting transition temperatures. The presence of a modulated super state in the cuprate compounds, referred to as a Pair Density Wave, provides a possible missing link to unify our understanding of the cuprate phase diagram and ultimately the mechanism for high temperature superconductivity. To search for a pair density wave state in the cuprates we developed a nanometer resolution, scanned Josephson (Cooper-pair) tunneling microscope (SJTM) [3]. We imaged Cooper-pair tunneling from a d-wave superconducting STM tip at millikelvin temperatures to the electronic superfluid state in the cuprate compound Bi2Sr2CaCu2O8. By visualizing the condensate with nanometer resolution we observed clear Cooper-pair density modulations. Our new technique for imaging Cooper-pair condensates, furthermore, opens the prospect of condensate visualizations in other cuprates, iron-based and unconventional superconductors, and will be especially advantageous in the study of topological superconductors. [1] Fulde, P. & Ferrell, R. A. Phys. Rev. 135, A550 (1964). [2] Larkin, A. I. & Ovchinnikov, Yu. N. Zh. Eksp. Teor. Fis. 37, 1146 (1964). [3] M. H. Hamidian et al. Nature, 532, 343-347 (2016). Host: Ar. Abanov

30 Sep 2016, 4:00PM | MIST 102
Hosted By: Ar. Abanov

Charles C. Hays, TAMU
Speaker: Charles C. Hays, TAMU Title: The effects of composition, and chemical-short-range-order, on the electrochemical properties of ultra-low platinum-group-metal content alloys for H2-Air fuel cell applications. Abstract: In this talk, microstructural, chemical, and electrochemical property measurements, for (111) crystallographically oriented Pt100-xMx (M = Zr, V) sputtered thin films are presented, together with some preliminary electronic structure calculations. These platinum-group-metal (PGM), valve metal based alloys are used as model surfaces to determine how the electrochemical properties evolve with changing chemical composition, and the moiety of the alloy constituents. The variation of the electronic structure is key design parameter, as the position of the Fermi Energy, Ef, dictates in part the voltage overpotential required to efficiently catalyze the oxygen-reduction-reaction (ORR) at the PEMFC cathode. The role of the valve-metal moiety, M = Zr, V, is of importance, in that the presence of the valve metal constituent may introduce a bi-functional character to the surface, to increase the normally sluggish ORR reaction kinetics, which typically limit the over-all cell performance (ORR current density). The influence of the alloy composition, and constituent moiety, was also examined for the other fuel cell reaction of interest, specifically the hydrogen-oxidation-reaction (HOR), operant at the PEMFC anode. While the kinetically fast HOR reaction does not typically limit the PEMFC performance, efforts to reduce the over-all PGM content would be enabled, if the new Pt100-xMx alloy surfaces exhibited a larger HOR active area, even though alloying reduces the number of platinum active surface sites. These results will aid in the development of candidate alloys, with even further reduced PGM contents, for use as the electrode materials in H2-Air polymer-electrolyte-membrane fuel cells (PEMFCs). This work is a continuation of research sponsored by the US Department of Energy, Office of Energy Efficiency & Renewable Energy (DOE-EERE), and was initially conducted at the California Institute of Technology’s Jet Propulsion Laboratory (JPL), in Pasadena, California. Host: Joe Ross

7 Oct 2016, 4:00PM | MIST 102
Hosted By: Joe Ross

Javad Shabani, City College, City University of New York
Speaker: Javad Shabani, City College, City University of New York Title: Epitaxial superconductor-semiconductor two-dimensional systems: platforms for quantum circuits Abstract: A key challenge in fabrication of hybrid semiconductor-superconductor devices is forming highly transparent contacts between the active electrons in the semiconductor and the superconducting metal. It has been shown that a near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on InAs nanowires [1]. Recently, this method was extended to two-dimensional systems and epitaxial aluminum on top of our near-surface InAs 2DESs were grown [2]. Quantum devices fabricated by selective etching of Al exhibited unprecedented quality for hybrid superconductor-semiconductor weak links. This has sparked a great deal of interest in low power classical and quantum computation as gate-controlled properties of the semiconductor allows scaling of such devices. In this work, we present recent progress in optimization of the growth of Al on InAs near surface quantum wells. We show the growth of InGaAs layers on top of InAs facilitates lower strain energy at the interface with Al and results in a flat and smooth growth allowing ultra thin superconducting Al films. We further extend this work to growth of higher Tc superconductors (such as Nb) on these thin film Al and compare the results to direct growth of these superconductors. InAs is a particularly interesting material as it has strong spin orbit coupling that can lead to engineering novel states such as topological superconductivity. However, this method can be extended to topological insulators (in-situ growth of superconductor) and silicon for superconducting qubits. We discuss the challenges and our preliminary results in these areas. These exciting developments might lead to a number of useful applications ranging from novel low-power classical to quantum computing. [1] P. Krogstrup et al. Nature Materials (2015) [2] J. Shabani et al. Phys Rev B (2016) Host: Ar. Abanov

14 Oct 2016, 4:00PM | MIST 102
Hosted By: Ar. Abanov

Vladimir Zyuzin, TAMU
Speaker: Vladimir Zyuzin, TAMU Title: Magnon mediated spin Nernst effect in a collinear antiferromagnet Abstract: We predict that a temperature gradient can induce a magnon-mediated spin Hall response (a response of transverse spin density current) in insulating collinearly ordered antiferromagnet with Dzyaloshinskii-Moriya interaction. To study the response we applied a linear response theory based on the Luttinger approach of the gravitational scalar potential. We then applied our theory to honeycomb lattice antiferromagnet with Dzyaloshinskii-Moriya interaction. We also identified the magnon edge modes in finite geometry of such system. A discussion of spin Nernst effect in ordered ferromagnet with Dzyaloshinskii-Moriya interaction will be made. If time permits, magnetization dynamics driven responses (for example generation of heat current) of ordered magnets will be discussed. Host: A. Finkelstein

21 Oct 2016, 4:00PM | MIST 102
Hosted By: A. Finkelstein

Elaine Li, U. of Texas
Speaker: Elaine Li, U. of Texas Title: Coherent Quantum Dynamics of Quasiparticles in Atomically Thin Semiconductors Abstract: The near band-edge optical response of an emerging class of semiconductors, known as the transitional metal dichalcogenides (TMDs), is dominated by tightly-bound excitons and charged excitons (i.e. trions). A fundamental property of these quasiparticles (excitons and trions) is dephasing time, which reflects irreversible quantum dissipation arising from system (excitons and trions) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons and trions can be coherently manipulated. Using a powerful coherent spectroscopy method known as the two-dimensional Fourier transform spectroscopy, we investigate the ultrafast coherent dynamics of excitons, trions, and valley pseudo-spins in these monolayer semiconductors. Host: A. Belyanin

28 Oct 2016, 4:00PM | MIST 102
Hosted By: A. Belyanin

Erik Henriksen, Washington University
Speaker: Erik Henriksen, Washington University Title: Electronic transport properties of adatom-decorated graphene Abstract: The fact that the 2D electron system in graphene is inherently unprotected from the environment is often detrimental: while the electron mobility is superior to most semiconductors, it is generally far lower than it could be due to scattering from extrinsic disorder. While this can be greatly improved by encapsulating graphene within layers of insulating material, there is on the other hand much interest in viewing the interactions between graphene's quasi-relativistic electrons and various surface adsorbates as an advantage. The question immediately arises: can the electronic structure of graphene be controllably altered? We are starting to explore the possibility of adatom-induced spin-orbit couplings in graphene in order to realize the original topological insulator predicted by Kane and Mele in 2005. Initial experiments on dilute coatings of indium atoms on graphene will be presented, as well as our current efforts with tungsten & osmium adatoms. Host: Ar. Abanov

4 Nov 2016, 4:00PM | MIST 102
Hosted By: Ar. Abanov

Yong Chen, Purdue University
Speaker: Yong Chen, Purdue University Title: Quantum transport in topological insulators Abstract: Three-dimensional (3D) topological insulators (TI) are a novel class of electronic materials with topologically-nontrivial band structure such that the bulk is gapped and insulating yet the surface has topologically protected gapless conducting states. Such “topological surface states” (TSS) give helically spin polarized Dirac fermions, and offer a promising platform to realize various other novel physics such as topological magnetoelectric effects and Majorana fermions, and may enable technological applications in areas such as spintronics and thermoelectrics. However, it is often challenging to unambiguously access and study the transport properties of TSS in many practical TI materials due to non-negligible bulk conducting states. I will discuss some of our experiments on high-quality “intrinsic” TIs with insulating bulk and surface-dominated conduction that allow us to reveal a number of characteristic quantum transport properties of spin-helical Dirac fermion topological surface states, such as the “half-integer” quantum Hall effect [1-2] and “half-integer” Aharonov-Bohm effect [3]. I will also discuss some issues and cautions related to interpreting commonly measured magnetotransport signatures such as “2D” quantum oscillations, weak antilocalization and linear magnetoresistance that may or may not arise from topological surface states or be unique to topological insulators [4-6]. If time permits, I may discuss other ways of probing TI transport using magnetic (spin-dependent) or superconducting (phase sensitive) electrodes. References: [1] Y. Xu et al., Nature Physics 10, 956 (2014) [2] Y. Xu et al., Nature Communications 7, 11434 (2016) [3] L. A. Jauregui et al., Nature Nanotechnology 11, 345 (2016) [4] H. Cao et al., Physical Review Letters 108, 216803 (2012) [5] L. A. Jauregui et al., Scientific Reports 5, 8452 (2015) [6] J. Tian et al., Scientific Reports 4, 4859 (2014) Host: Ar. Abanov

11 Nov 2016, 4:00PM | MIST 102
Hosted By: Ar. Abanov

Siddharth A. Parameswaran, University of California, Irvine
Speaker: Siddharth A. Parameswaran, University of California, Irvine Title: Topology and Response in Semimetals Abstract: Topological ideas have revitalized the study of gapped insulating phases of matter. My talk will examine the role of such ideas in understanding semimetals, gapless phases whose Fermi surfaces may be shrunk to isolated point or line nodes. First, I will review the theory of ‘Weyl semimetals’ that arise from accidental degeneracies in 3D crystals, and explore the consequences of the resulting band topology. Then, I will show that the interplay of crystal symmetry and topology can aid in identifying “filling enforced” semimetals. I will discuss how the topological underpinnings of these gapless phases may be probed via transport or spectroscopy. Finally, if time permits, I will sketch ongoing work in using these ideas to identify novel phases in heavy fermion systems. References: arXiv:1508.01546; Nat. Phys. 9, 299 (2013); PRX 4, 031035 ; PRX 5, 041046; Related work: arXiv:1609.04023 Host: Ar. Abanov

18 Nov 2016, 4:00PM | MIST 102
Hosted By: Ar. Abanov

Rafael Fernandes, University of Minnesota
Speaker: Rafael Fernandes, University of Minnesota Title: Superconductivity mediated by quantum critical antiferromagnetic fluctuations: the rise and fall of hot spots. Abstract: The maximum transition temperature Tc observed in the phase diagrams of several unconventional superconductors takes place in the vicinity of a putative antiferromagnetic quantum critical point. This observation motivated the theoretical proposal that superconductivity in these systems may be driven by quantum critical fluctuations, which in turn can also promote non-Fermi liquid behavior. In this talk, we present a combined analytical and sign-problem-free Quantum Monte Carlo investigation of the spin-fermion model – a widely studied low-energy model for the interplay between superconductivity and magnetic fluctuations. By engineering a series of band dispersions that interpolate between near-nested and open Fermi surfaces, and by also varying the strength of the spin-fermion interaction, we find that the hot spots of the Fermi surface provide the dominant contribution to the pairing instability in this model. We show that the analytical expressions for Tc and for the pairing susceptibility, obtained within a large-N Eliashberg approximation to the spin-fermion model, agree well with the Quantum Monte Carlo data, even in the regime of interactions comparable to the electronic bandwidth. Host: Ar. Abanov

2 Dec 2016, 4:00PM | MIST 102
Hosted By: Ar. Abanov

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