Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D52: Invited Session: DCMP Prize Session |
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Sponsoring Units: DCMP Chair: Laura Greene, Universirty of Illinois-Urbana Room: Grand Ballroom C2 |
Monday, March 2, 2015 2:30PM - 3:06PM |
D52.00001: Davisson-Germer Prize Talk: Atomically Uniform Thin Films as Quantum Wells and Device Components Invited Speaker: Tai C. Chiang Atomically uniform films can be made for various overlayer-substrate combinations (such as Ag, Pb, Sb, \textellipsis on Si, Ge, Fe, \textellipsis ), many of which are not even lattice matched. These films show remarkable property variations as the film thickness is built up in atomic-layer increments. The thermal stability of the film, its work function, electron-phonon coupling, superconducting transition temperature, etc. exhibit damped and modulated oscillations as the film thickness increases toward the bulk limit. The underlying physics can be understood generally in terms of the energetics of a coarsened electronic structure of thin films and more specifically in terms of a "one-dimensional shell effect" -- the quantized electronic levels in the film are progressively filled at increasing film thicknesses just like the elemental atomic shells in going through the periodic table. The phase and the amplitude of the oscillations can be tailored by surface/interface engineering that leads to changes in the surface potential and the interface Schottky barrier or band mismatch. These quantum size and confinement effects are important and observable at film thicknesses well in the realm of practical device dimensions and at room temperature, suggesting opportunities for applications. When the films are made of topologically nontrivial materials, the electron spin and its transport become relevant parameters. This talk will discuss issues related to uniform film growth, general trends in connection with reduced dimensions, surprising findings including phonon-mediated pseudogaps, and technology potential. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:42PM |
D52.00002: Davisson-Germer Prize Talk: Structure and Reactivity of Surfaces in Vacuum and Under Ambient Gas Pressures Invited Speaker: Miquel Salmeron The goal of surface science research is to provide atomic level understanding of the structural and dynamic properties of surfaces, a goal particularly relevant for chemical applications, including catalysis, photochemistry, batteries and fuel cells. With X-ray Photoemision Spectroscopy (XPS) we can determine the composition and electronic structure. With Scanning Tunneling Microscopy (STM) we can image atoms and molecules as they adsorb, diffuse and react on single crystal surfaces. STM uniquely permits to visualize and determine adsorbate-adsorbate interactions by making movies of their motion. I will show how water molecules diffuse, H-bond to each other, and wet the surface forming 2D films. We also imaged how H2 molecules adsorb and dissociate on a Pd surface, and how the movies revealed that a particular arrangement of substrate atoms is required to generate the active sites through fluctuations. To study surfaces in the presence of gases, in the Torr to Atmospheres range, which is relevant to practical catalysis, new instrumentation is needed. Over the last years we developed high pressure STM and XPS, which allowed us to study surfaces under high coverage of adsorbates in equilibrium with gases near ambient pressures and temperature. I will show how under these conditions the structure of surfaces can be very different from that at low coverage, or even at high coverage but at low temperature. Adsorbates can induce dramatic restructuring of the surface, as I will show in the case of CO on Pt and Cu. Equally important, reactions on catalyst surfaces can now be followed in real time, by measuring composition with XPS, and structure with STM, during the reactions to extract kinetic parameters. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 4:18PM |
D52.00003: Lars Onsager Prize Talk: Flow Equations for Hamiltonians Invited Speaker: Franz Wegner The equation $dH(l)=GH(l)$ can describe the renormalization group equation for the Hamiltonian/Lagrangian $H(l)$ and the generator $G$ of the group. But it can also describe a Hamiltonian flow for bosons/fermions, which eliminates the off-diagonal matrix elements or (quasi)-particle violating terms by means of unitary transformations $dH(l)/dl=[\eta(l),H(l)]$. Typically off-diagonal matrix elements are eliminated for $\Delta E > l^{-1/2}$. The flow equation has been applied to numerous systems (F. Wegner, J. Phys. A: Math. Gen. 39 (2006) 8221, arXiv: cond-mat/0511660; S. Kehrein, The Flow Equation Approach to Many-Particle Systems, Springer 2006), among them to the two-dimensional Hubbard-model, spin-boson models, the Anderson impurity model, QED and QCD. A simple and surprising result (as compared to that by Frohlich) is obtained for the elimination of the electron-phonon interaction yielding an attractive interaction for all energies. (P. Lenz and F. Wegner, Nucl. Phys. B482 (1996) 693; arXiv: cond-mat/9604087). [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:54PM |
D52.00004: Julius Edgar Lilienfeld Prize Talk: Quantum spintronics: abandoning perfection for new technologies Invited Speaker: David D. Awschalom There is a growing interest in exploiting the quantum properties of electronic and nuclear spins for the manipulation and storage of information in the solid state. Such schemes offer qualitatively new scientific and technological opportunities by leveraging elements of standard electronics to precisely control coherent interactions between electrons, nuclei, and electromagnetic fields. We provide an overview of the field, including a discussion of temporally- and spatially-resolved magneto-optical measurements designed for probing local moment dynamics in electrically and magnetically doped semiconductor nanostructures. These early studies provided a surprising proof-of-concept that quantum spin states can be created and controlled with high-speed optoelectronic techniques. However, as electronic structures approach the atomic scale, small amounts of disorder begin to have outsized negative effects. An intriguing solution to this conundrum is emerging from recent efforts to embrace semiconductor defects themselves as a route towards quantum machines. Individual defects in carbon-based materials possess an electronic spin state that can be employed as a solid state quantum bit at and above room temperature. Developments at the frontier of this field include gigahertz coherent control, nanofabricated spin arrays, nuclear spin quantum memories, and nanometer-scale sensing. We will describe advances towards quantum information processing driven by both physics and materials science to explore electronic, photonic, and magnetic control of spin. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:18PM |
D52.00005: Experimental observation of high-temperature superconductivity in H$_x$S at P$\sim$150 GPa Invited Speaker: M. Eremets We found that sulfur hydride transforms at P$\sim$90 GPa to metal and superconductor with T$_c$ increasing with pressure to 150 K at $\approx$ 200 GPa. Moreover we found superconductivity with T$_c$ $\approx$ 190 K in a H2S sample pressurized to P $>$ 150 GPa at T $>$ 220 K. This superconductivity likely associates with the dissociation of H2S, and formation of SHn (n $>$ 2) hydrides. We proved occurrence of superconductivity by the drop of the resistivity at least 50 times lower than the copper resistivity, the decrease of T$_c$ with magnetic field, and the strong isotope shift of T$_c$ in D2S which evidences a major role of phonons in the superconductivity. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:42PM |
D52.00006: High $T_{\mathrm{c}}$ phase of (H$_{2}$S)$_{2}$H$_{2}$ at high pressures Invited Speaker: Tian Cui Hydrogen was predicted to metalize at high pressures and believed to be a room-temperature superconductor. However, metallization of hydrogen is still under debates. As an alternative, hydrogen dominated materials were extensively explored because of their lower metallization pressure. Here I present the high-pressure studies on structures, metallization, and superconductivity of (H$_{2}$S)$_{2}$H$_{2}$ from \textit{ab initio} calculations [1]. At lower pressures, two phases containing H$_{2}$ units are stable with $P$1 (\textless 37 GPa) and \textit{Cccm} (37-111 GPa) symmetries, which are still insulators. Upon further compression, H$_{2}$ units disappear and two intriguing metallic structures with $R$3m and \textit{Im-}3$m$ symmetries are reconstructive above 111 GPa and 180 GPa, respectively. Remarkably, the estimated $T_{\mathrm{c}}$ of \textit{Im-}3$m$ phase at 200 GPa achieves a very high value of 191 $\sim$ 204 K. Moreover, $T_{\mathrm{c}}$ decreases with pressure at an approximate rate (\textit{dT}$_{\mathrm{c}}$/\textit{dP}) of -0.12~K/GPa. Our predicted high $T_{\mathrm{c}}$ and its pressure dependence in \textit{Im-}3$m$ phase are subsequently verified by recent experiments [2]. Our findings support the conjecture that hydrogen-rich materials are a way to achieve a metallic phase with high $T_{\mathrm{c}}$ at accessibly experimental pressures and represent a significant step toward the understanding of high-pressure behavior of metallic hydrogen. \\[4pt] [1] D. Duan, Y. Liu, T. Cui, et al. Sci. Rep., 4, 6968 (2014)\\[0pt] [2] A. P. Drozdov, M. I. Eremets, and I. A. Troyan, arXiv:1412.0460, (2014) [Preview Abstract] |
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