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  1. Boeri, L. ; Hennig, R. ; Hirschfeld, P. ; Profeta, G. ; Sanna, A. ; Zurek, E. (Ed.)
    Last year, the report of Room-Temperature Superconductivity in high-pressure carbonaceous sulfur hydride marked a major milestone in the history of physics: one of the holy grails of condensed matter research was reached after more than one century of continuing efforts. This long path started with Neil Ashcroft’s and Vitaly Ginzburg’s visionary insights on high-temperature superconductivity in metallic hydrogen in the 60’s and 70’s, and has led to the current hydride fever, following the report of high-Tc high-pressure superconductivity in H3S in 2014. This Roadmap collects selected contributions from many of the main actors in this exciting chapter of condensed matter history. Key for the rapid progress of this field has been a new course for materials discovery, where experimental and theoretical discoveries proceed hand in hand. The aim of this Roadmap is not only to offer a snapshot of the current status of superconductor materials research, but also to define the theoretical and experimental obstacles that must be overcome for us to realize fully exploitable room temperature superconductors, and foresee future strategies and research directions. This means improving synthesis techniques, extending first-principles methods for superconductors and structural search algorithms for crystal structure predictions, but also identifying new approaches to material discovery based on artificial intelligence. 
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  2. null (Ed.)
    X-ray diffraction indicates that the structure of the recently discovered carbonaceous sulfur hydride (C-S-H) room temperature superconductor is derived from previously established van der Waals compounds found in the H2S-H2 and CH4-H2 systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique leading to the superconducting critical temperature Tc of 288 K at 267 GPa. X-ray diffraction patterns measured from 124 to 178 GPa, within the pressure range of the superconducting phase, are consistent with an orthorhombic structure derived from the Al2Cu-type determined for (H2S)2H2 and (CH4)2H2 that differs from those predicted and observed for the S-H system to these pressures. The formation and stability of the C-S-H compound can be understood in terms of the close similarity in effective volumes of the H2S and CH4 components, and denser carbon-bearing S-H phases may form at higher pressures. The results are crucial for understanding the very high superconducting Tc found in the C-S-H system at megabar pressures. 
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  6. The anomalous nondipolar and nonaxisymmetric magnetic fields of Uranus and Neptune have long challenged conventional views of planetary dynamos. A thin-shell dynamo conjecture captures the observed phenomena but leaves unexplained the fundamental material basis and underlying mechanism. Here we report extensive quantum-mechanical calculations of polymorphism in the hydrogen–oxygen system at the pressures and temperatures of the deep interiors of these ice giant planets (to >600 GPa and 7,000 K). The results reveal the surprising stability of solid and fluid trihydrogen oxide (H 3 O) at these extreme conditions. Fluid H 3 O is metallic and calculated to be stable near the cores of Uranus and Neptune. As a convecting fluid, the material could give rise to the magnetic field consistent with the thin-shell dynamo model proposed for these planets. H 3 O could also be a major component in both solid and superionic forms in other (e.g., nonconvecting) layers. The results thus provide a materials basis for understanding the enigmatic magnetic-field anomalies and other aspects of the interiors of Uranus and Neptune. These findings have direct implications for the internal structure, composition, and dynamos of related exoplanets. 
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  7. Hydrogen-containing materials are of fundamental as well as technological interest. An outstanding question for both is the amount of hydrogen that can be incorporated in such materials, because that determines dramatically their physical properties such as electronic and crystalline structure. The number of hydrogen atoms in a metal is controlled by the interaction of hydrogens with the metal and by the hydrogen–hydrogen interactions. It is well established that the minimal possible hydrogen–hydrogen distances in conventional metal hydrides are around 2.1 Å under ambient conditions, although closer H–H distances are possible for materials under high pressure. We present inelastic neutron scattering measurements on hydrogen in Z r V 2 H x showing nonexpected scattering at low-energy transfer. The analysis of the spectra reveals that these spectral features in part originate from hydrogen vibrations confined by neighboring hydrogen at distances as short as 1.6 Å. These distances are much smaller than those found in related hydrides, thereby violating the so-called Switendick criterion. The results have implications for the design and creation of hydrides with additional properties and applications. 
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