615 research outputs found
Semimetallic molecular hydrogen at pressure above 350 GPa
According to the theoretical predictions, insulating molecular hydrogen
dissociates and transforms to an atomic metal at pressures P~370-500 GPa. In
another scenario, the metallization first occurs in the 250-500 GPa pressure
range in molecular hydrogen through overlapping of electronic bands. The
calculations are not accurate enough to predict which option is realized. Here
we show that at a pressure of ~360 GPa and temperatures <200 K the hydrogen
starts to conduct, and that temperature dependence of the electrical
conductivity is typical of a semimetal. The conductivity, measured up to 440
GPa, increases strongly with pressure. Raman spectra, measured up to 480 GPa,
indicate that hydrogen remains a molecular solid at pressures up to 440 GPa,
while at higher pressures the Raman signal vanishes, likely indicating further
transformation to a good molecular metal or to an atomic state
Black metal hydrogen above 360 GPa driven by proton quantum fluctuations
Hydrogen metallization under stable conditions is a major quest for realizing the first room temperature su-
perconductor. Recent low-temperature experiments report different metallization pressures, varying from 360GPa to 490GPa. In this work, we simulate structural properties, vibrational Raman, IR and optical spectra of hydrogen phase III accounting for proton quantum effects. We demonstrate that nuclear quantum fluctuations downshift the vibron frequencies by 25%, introduce a broad line-shape in the Raman spectra, and reduce the optical gap by 3eV. We show that hydrogen metallization occurs at 380GPa in phase III due to band overlap, in good agreement with transport data. Our simulations predict this state is a black metal - transparent in the IR - so that the shiny metal observed at 490GPa is not phase III. We predict the conductivity onset and the optical gap will substantially increase if hydrogen is replaced by deuterium, underlining that metallization is driven by quantum fluctuations and is thus isotope dependent. We
show how hydrogen acquires conductivity and brightness at different pressures, explaining the apparent contradictions in existing experimental scenarios.European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (grant agreement No. 802533
Structure and metallicity of phase V of hydrogen
A new phase V of hydrogen was recently claimed in experiments above 325 GPa and 300 K. Due to the extremely small sample size at such record pressures the measurements were limited to Raman spectroscopy. The experimental data on increase of pressure shows decreasing Raman activity and darkening of the sample, which suggests band-gap closure and impending molecular dissociation, but no definite conclusions could be reached. Furthermore, the available data is insufficient to determine the structure of phase V, which remains unknown. Introducing saddle-point ab initio random structure searching (sp-AIRSS), we find several new structural candidates of hydrogen which could describe the observed properties of phase V. We investigate hydrogen metallisation in the proposed candidate structures, and demonstrate that smaller band gaps are associated with longer bond lengths. We conclude that phase V is a stepping stone towards metallisation
Perspective: Role of structure prediction in materials discovery and design
Materials informatics owes much to bioinformatics and the Materials Genome Initiative has been inspired by the Human Genome Project. But there is more to bioinformatics than genomes, and the same is true for materials informatics. Here we describe the rapidly expanding role of searching for structures of materials using first-principles electronic-structure methods. Structure searching has played an important part in unraveling structures of dense hydrogen and in identifying the record-high-temperature superconducting component in hydrogen sulfide at high pressures. We suggest that first-principles structure searching has already demonstrated its ability to determine structures of a wide range of materials and that it will play a central and increasing part in materials discovery and design.This is the final version of the article. It first appeared from the American Institute of Physics via http://dx.doi.org/10.1063/1.494936
A Perspective on Conventional High-Temperature Superconductors at High Pressure: Methods and Materials
Two hydrogen-rich materials, HS and LaH, synthesized at megabar
pressures, have revolutionized the field of condensed matter physics providing
the first glimpse to the solution of the hundred-year-old problem of room
temperature superconductivity. The mechanism underlying superconductivity in
these exceptional compounds is the conventional electron-phonon coupling. Here
we describe recent advances in experimental techniques, superconductivity
theory and first-principles computational methods which have made possible
these discoveries. This work aims to provide an up-to-date compendium of the
available results on superconducting hydrides and explain how the synergy of
different methodologies led to extraordinary discoveries in the field. Besides,
in an attempt to evidence empirical rules governing superconductivity in binary
hydrides under pressure, we discuss general trends in the electronic structure
and chemical bonding. The last part of the Review introduces possible
strategies to optimize pressure and transition temperatures in conventional
superconducting materials as well as future directions in theoretical,
computational and experimental research.Comment: 68 pages, 30 figures, Preprint submitted to Physics Report
Toward the nonmetal-to-metal phase transition of helium
Properties of helium under high pressure are of great importance for a variety of astrophysical objects. This work presents results of extensive ab initio calculations for high-pressure helium regarding the equation of state, the melting line, the electrical conductivity, the reflectivity, and the band gap. The calculations determined a higher-order phase transition from an insulating to a conducting phase which has been discussed for decades. This work finally proposes an update of the phase diagram of helium and discusses its implications on dwarf stars and giant planets.Die Eigenschaften von Helium unter hohem Druck sind wichtig für eine Vielzahl von astrophysikalischen Objekten. Diese Arbeit präsentiert die Ergebnisse umfangreicher Ab-initio-Berechnungen für Helium. Insbesondere wurde die Zustandsgleichung, die elektrische Leitfähigkeit, das Reflexionsvermögen und die Bandlücke berechnet. Die Ergebnisse weisen einen kontinuierlichen Phasenübergang zwischen isolierendem und elektrisch leitfähigem Helium nach. Schließlich zeigt diese Arbeit ein aktualisiertes Phasendiagramm von Helium und diskutiert die Konsequenzen für Zwergsterne und Riesenplaneten
Hydrogen and lithium under high pressure.
192 p.In this thesis we present a first-principles analysis of the electronic, vibrational and superconducting properties of solid hydrogen and lithium under high pressure based on density functional theory (DFT). The main goal of this thesis is to provide an accurate theoretical description of lithium and hydrogen focusing on the regions of their respective phase diagrams where the superconducting properties, which are still potential in the case of hydrogen, emerge. With this in mind, we have presented electronic and vibrational spectra along with the coupling between the electronic and nuclear subsystems in order to obtain the superconducting properties of the analyzed materials. While our calculations rely on the DFT framework, we have accurately included the quantum behavior of the nuclei and anharmonic effects arising due to the lightness of the elements. For that purpose, we have used the stochastic self-consistent harmonic approximation, which accounts for quantum nuclear and anharmonic effects in a non- perturvative variational way
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