Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B17: Matter in Extreme Environments: HydridesFocus
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Sponsoring Units: DCOMP Chair: Yanming Ma, Jilin Univ Room: BCEC 156A |
Monday, March 4, 2019 11:15AM - 11:51AM |
B17.00001: Near room temperature superconductivity in superhydrides at megabar pressures Invited Speaker: Russell Hemley Recent predictions and experimental observations of high Tc superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We confirmed the existence of a new class of such materials – superhydrides (MHx, with x > 6) – and developed preparation techniques for their syntheses and characterization, including measurements of structural and transport properties, at megabar pressures. Four-probe electrical transport measurements of lanthanum superhydride samples display signatures of superconductivity at temperatures ranging from 150 K to above 280 K near 200 GPa. The experiments are supported by pseudo-four probe conductivity measurements, critical current determinations, low-temperature x-ray diffraction, and magnetic susceptibility measurements. These measurements of near-room temperature superconductivity are in good agreement with density functional and BCS theory-based calculations. |
Monday, March 4, 2019 11:51AM - 12:03PM |
B17.00002: High-Tc Conventional Superconductivity in Clathrate Hydrides: What can we learn from Electronic Structure? Lilia Boeri, Christoph Heil, Simone Di Cataldo, Giovanni Bachelet The current record-holder for superconductivity is a lanthanum superhydride (LaH10), which crystallizes in a clathrate structure, in which hydrogen forms a dense lattice of interconnected cages. The highest Tc reported is close to room temperature (265 K). Tc's comparable, or even higher than this, have been predicted to occur in other clathrate hydrides with chemical formula XH6 and XH10, with X=Sc,Y,Mg,Ca. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B17.00003: Synthesis of cerium superhydride CeH9 with 3-dimensional atomic hydrogen sublattice Nilesh Salke, M. Mahdi Davari Esfahani, Youjun Zhang, Ivan Kruglov, Jianshi Zhou, Yaguo Wang, Eran Greenberg, Vitali Prakapenka, Artem Oganov, Jung-Fu Lin Hydrogen-rich super/polyhydrides were considered as an alternative to the monatomic metallic hydrogen to achieve superconductivity close to room temperature at relatively lower pressure. Concomitantly, superconductivity was reported in H3S at record high Tc of 203 K at 200 GPa. Synthesis of superhydrides is very challenging as most of these super/polyhydrides stabilize at very high pressure for e.g., FeH5 and LaH10 stabilized at 130 and 170 GPa respectively. However, it would be always useful if superhydrides are synthesized at lower possible pressure. With this motivation, we carried the experimental and theoretical studies on Ce-H system. We have successfully synthesized superhydride CeH9 around 100 GPa in the laser-heated diamond anvil cell. Theoretical calculations were carried to understand the structure, stability and superconductivity of CeH9. CeH9 crystallized in a P63/mmc clathrate structure with 3-dimensional atomic hydrogen sublattice. Synthesis of CeH9 at feasible pressure range is very promising and hints towards the future possibility to achieve higher Tc value with the lowest possible pressure in hydride superconductors. This study will also help to further investigate and understand hydride superconductivity at feasible pressure range. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B17.00004: Theoretical Investigation of Ternary Hydrogen-Rich Materials under Pressure Tiange Bi, Eva Zurek Investigations of chemical compounds under pressure has led to the discovery of novel materials with unique properties and chemical compositions that cannot form at ambient conditions. Hydrogen is believed to become metallic and even superconducting under pressure with a high transition temperature, but the metallization pressure has not been reached by current experiments. Our research mainly focuses on predicting the structures of novel superconducting hydrogen-rich materials using the open-source evolutionary algorithm XTALOPT coupled with density functional theory (DFT) calculations. Given only the chemical composition, this combination allows us to find the most stable structure of a solid under extreme conditions. Therefore, we present our predictions of ternary hydrides at high pressures including analysis of structural and electronic properties. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B17.00005: Strong Electron-Hole Symmetry Breaking in the High Temperature Superconductor H3S. Soham Ghosh, Warren E Pickett A favorable combination of strong electron-phonon coupling (EPC), high phonon frequency and substantial density of states (DOS) near the Fermi energy was predicted1 to produce a record superconducting critical temperature Tc around 200K at high pressure, quickly followed by the experimental discovery by Drozdov et al.2 up to 203K at 160 GPa pressure. Several theoretical papers have confirmed and elaborated on the new phenomena that arise in H3S. Here we employ first principles methods that reveal new aspects of H3S. The electron and phonon self energies are calculated using the EPW code, and we analyze effects of the two nearby van Hove singularities (vHs) that create a sharp and narrow peak of the electron DOS in this material. Several features emerge: The interacting DOS (the spectral density) is severely affected by the strong electron-phonon coupling, and the electronic spectrum displays extreme examples of band renormalization and broadening that have been suggested to arise from strong EPC. Phonon self-energy features will also be presented. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B17.00006: High Field Magnetotransport in Superconducting Hydrides under Pressure Shirin Mozaffari, Luis Balicas, Vasily S. Minkov, Dmitry Knyazev, Mikhail Eremets, Mari Einaga, Katsuya Shimizu, Dan Sun, Fedor Balakirev Hydrogen sulfide forms metallic phase under high pressure above one million atmospheres and becomes superconducting at temperatures as high as 203K at 160GPa. Despite record-breaking Tc the basic properties of superconducting sulphur hydride/deuteride are found to be consistent with conventional Bardeen-Cooper-Schrieffer (BCS) theory. Early measurements of the superconducting phase diagram in magnetic fields up to 7T estimate upper critical fields of the order of 70T. We report magnetotransport studies of superconducting sulphur hydride in DC fields up to 35T and pulsed fields up to 65T. We find that upper critical field generally follow the Werthamer, Helfand and Hohenberg (WHH) formalism at low fields, while noticeable deviations from WHH appear at experimental limit of 65T. |
Monday, March 4, 2019 12:51PM - 1:03PM |
B17.00007: " High superconductivity in hydrides at high pressures " Xue Li, Hanyu Liu, Yanming Ma The mechanisms for the strong electron-phonon coupling predicted for hydrogen-rich compounds with high superconducting critical temperature were examined within the Bardeen-Schrieffer-Cooper and Midgal-Eliashberg theory. In this study, we have explored some candidate structures for hydrides at high pressures. Electron-phonon coupling calculations predict the existence of new superconducting phases, some even exhibiting superconductivity in the range of room temperature. Further analysis shows that hydrogen-hydrogen vibrations played acritical role in enhancing electron-phonon coupling parameters. Moreover, the calculated stabilities indicate the materials are likely to be synthesized at pressures that are currently accessible in the laboratory. The results open the prospect for the design, synthesis, and recovery of new high-temperature superconductors with potential practical applications. |
Monday, March 4, 2019 1:03PM - 1:15PM |
B17.00008: Electronic Structure and Properties of Calcium and Iron Polyhydrides Under Pressure Eva Zurek The recent synthesis of the calcium and iron polyhydride phases under pressure has inspired theoretical investigations of their structures and electronic structures. First principles calculations have helped to characterize a newly synthesized C2/m symmetry Ca2H5 phase at 25 GPa and an I4/mmm symmetry CaH4 phase at 120 GPa. Both of these phases contain atomic and molecular hydrogen. The H-H distances in the molecular units are elongated as compared to those in elemental H2 at the same pressure. Electronic structure calculations illustrate that the bond-lengthening mechanism under pressure in the solid state resembles the one responsible for lengthening the H-H bond in Kubas like molecualr complexes. Crystal structure prediction techniques have also been employed to predict stable and metastable high hydrides of iron between 150-300 GPa that have not been discussed in other studies. Density functional theory calculations show that neither the I4/mmm nor the Cmca symmetry FeH5 phases found to be stable are superconducting. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B17.00009: Bipartite character of electronic structure in compressed H3S and its relation to the high-Tc superconductivity Ryosuke Akashi The sulfur superhydride H3S has attracted enormous interest since the discovery of the 200K superconductivity in this material under extreme pressure. According to the previous studies, its Tc is boosted by the peaked structure in the density of states (DOS) and its presence is robust against substitution of sulfur atoms to phosphorus etc.. Its persistence has been thought to emerge through some complicated interplay of the sulfur 3s, 3p and hydrogen 1s orbitals but clear explanation on it has yet been established. We disentangle the “complicated interplay” to clarify how the DOS peak emerges. In particular, we find that a sublattice model with decorated simple cubic structure well represents the electronic structure in this system near a two-dimensional manifold in the Brillouin zone, which is due to small inter-sublattice coupling dependent on the crystal wavenumber. We discuss this point and its possible relation to the robust DOS peak. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B17.00010: A New Analysis of Strong Electron-Phonon Coupling in Compressed Metal Hydrides Yundi Quan, Warren E Pickett The experimental discovery that H3S becomes superconducting at 200 K under 160 GPa pressure has re-invigorated interest in metal hydrides under compression. Understanding the role of hydrogen relative to that of metal atom and the crystal structure is essential for designing materials with even higher Tc. Importantly for H3S and other hydrides, various physical quantities including the phonon spectra, the coupling strength λ, and matrix elements can be clearly identified as contributions from either hydrogen or the metal atom. We systematically study a few classes of binary hydrides, viz. H3S, CaH6, MgH6 and recently reported lanthanum hydride to carry out this separation and gain a unique type of insight into the origin of strong electron-phonon coupling and high Tc in hydrides. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B17.00011: Probing Hydrogen in Metal Hydrides:
Proton-NMR study of FeH up to 1.9 Mbar Thomas Meier, Saiana Khandarkhaeva, Sylvain Petitgirard, Stella Chariton, Timofey Fedotenko, Natalia Dubrovinskaia, L. S. Dubrovinsky, Florian Trybel, Gerd Steinle-Neumann Investigation of electronic structures of metal hydrides remains an elusive challenge in high pressure laboratories. Using a novel high pressure NMR technique [1,2,3] we study face centred cubic iron hydride, FeHx with x=1.0(1), synthesized in-situ in a diamond anvil cell at 30 GPa and temperatures above 1000 K by reaction of iron powder and paraffin. Signals stemming from metallic FeH were identified at protonic Knight shifts of about -1200 ppm. Measurement of the Korringa relation shows that FeH preserve Fermi-liquid like electronic behaviour between 30 and 63 GPa and from 154 GPa towards the highest pressures in this study. Between 63 GPa and 154 GPa the volume dependence of the Proton Knight shift shows an unexpected deviation of the free-electron V2/3 Knight Shift curvature, indicating an electronic topological transition of the Fermi surface. This study demonstrates the possibility to investigate the electronic structures of hydrogen under chemical pre-compression in metal hydride systems at pressures not accessible by comparable methods. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B17.00012: Spin-Lattice Model of Plutonium Hydride Nucleation Ryan Mullen, Nir Goldman Plutonium hydride forms when plutonium is exposed to hydrogen gas or water vapor. The mechanism of plutonium hydride nucleation is a key material property in this process. Conducting experiments with plutonium is difficult and expensive due to its toxicity and radioactivity. Computer models allow us to probe the phase transition to plutonium hydride without physically handling plutonium. |
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