Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B41: Metallic Hydrogen and HydridesInvited
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Sponsoring Units: DCMP Room: LACC 502A |
Monday, March 5, 2018 11:15AM - 11:51AM |
B41.00001: Molecular semimetallic hydrogen Invited Speaker: Mikhail Eremets Establishing the metallic hydrogen has been a goal of intensive theoretical and experimental work since 1935 when Wigner and Hungtinton predicted the dissociation of insulating molecular hydrogen into atomic metallic hydrogen at pressures above 25 GPa. The recent calculations give a value of ~450 GPa. Following the alternative scenario, metallization can be achieved prior the dissociation, in molecular state, through gradual overlapping of the electronic bands. |
Monday, March 5, 2018 11:51AM - 12:27PM |
B41.00002: Synthesis of superhydrides and metallization of hydrogen at high pressures Invited Speaker: Alexander Goncharov Hydrogen and hydrogen-rich materials are expected to possess spectacular properties such as high-temperature superconductivity at extreme pressure-temperature conditions where the crossovers to monatomic phases and formations of superhydrides would occur. Recent theoretical studies predicted the formations of such superhydrides at the experimentally accessible pressure range and some of them have been recently synthesized such as, for example, H3S. The transformation of molecular hydrogen to monatomic metallic hydrogen at high pressures in solid and fluid states remains controversial as the reported behaviors (both in theory and experiment) differ in the character and in the pressure-temperature range. I will review the latest results in the field obtained using different experimental and theoretical approaches and will present new results the plasma phase transition in hydrogen and synthesis of novel superhydrides including uranium hydrides. I will also discuss the complexities of theoretical and experimental (static and dynamic) studies of hydrogen-rich materials at extreme conditions which remain a challenge and continue inspiring researchers for further technical developments. |
Monday, March 5, 2018 12:27PM - 1:03PM |
B41.00003: Spectroscopic Evidence of a new Energy Scale in H3S. Invited Speaker: Thomas Timusk The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K by Drozdov et al. [1] has provided a new impetus to the search for even higher Tc. The observation of a sharp drop in resistance to zero at Tc, its downward shift with magnetic field and a Meissner effect confirm superconductivity but the mechanism involved remains to be determined. Using the AILES beamline at Soleil, we provide a first optical spectroscopy study of this new superconductor[2]. Experimental results for the optical reflectivity of H3S, under high pressure, for several temperatures and over the range 60 to 600 meV of photon energies, are compared with theoretical calculations based on Eliashberg theory using DFT results for the electron-phonon spectral density. Two significant features stand out: some remarkably strong infrared active phonons at approximately 160 meV and a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. The shape, magnitude, and energy dependence of this band at 150 K agrees with our calculations. This provides strong evidence of a conventional mechanism. However, the unusually strong optical phonon suggests a contribution of electronic degrees of freedom. |
Monday, March 5, 2018 1:03PM - 1:39PM |
B41.00004: van Hove Singularities and Strong Electron-Phonon Coupling: Superconductivity and H3S Invited Speaker: Warren Pickett van Hove singularities (vHs) have long been implicated in high temperature suuperconductivity. In A15 superconductors, with Tc up to 23K, Labe and coworkers proposed and studied a 1D vHs associated iwth the transition metal chains. In the cuprates, Newns et al introduced the role of the 2D vHs associated with the square lattice tight binding model. For H3S we have emphasized [3] the importance of vHs and the concept of spectral smearing due to strong electron-phonon coupling. vHs cause peaks and fine structure in the electronic density of states, whichiis the root of their interest. vHs "mirroring" has lately been proposed [4]as the mechanism behind the weak AFM emerging at low temperature in TiAu, which has a sharper and higher DOS peak than does H3S. Why is it magnetic rather than superconducting? The formalism for the smearing effect has been around a while,[5] with application to A15 compounds. vHs cause another complication, because electron velocities become less that phonon velocities near the vHs, violating the Midgal theorem premise of Eliashberg theory; this aspect was addressed by Drozhov[6] who found non-quasiparticle behavior near the vHs, but many questions remain. This talk will discuss the various aspects of vHs primarily in 3D materials, with special focus on H3S. Recent work has been done in collaboration iwth Yundi Quan. |
Monday, March 5, 2018 1:39PM - 2:15PM |
B41.00005: Quantum Motion and Anharmonicity in Superconducting Hydrides Invited Speaker: Ion Errea The recent discovery by Drozdov et al. [1] of superconductivity at 203 K in the hydrogen sulfide system compressed to about 150 GPa breaks the record of the cuprates and overturns the conventional wisdom that such a high critical temperatures cannot be obtained via phonon-mediated pairing. Exciting new prospects are now open to find room temperature superconductivity in other hydrogen-rich materials. |
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