High- T C Superconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions Between Hydrogen and Central-Atom El
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ORIGINAL PAPER
High-TC Superconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions Between Hydrogen and Central-Atom Electrons Dale R. Harshman 1
&
Anthony T. Fiory 2
Received: 11 May 2020 / Accepted: 27 May 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The uniquely characteristic macrostructures of binary hydrogen-clathrate compounds MHn formed at high pressure, a cage of hydrogens surrounding a central-atom host, is theoretically predicted in various studies to include structurally stable phononmediated superconductors. High superconductive transition temperatures TC have thus far been measured for syntheses with M = La, Y, and Th. In compressed LaH10, independent studies report TC of 250 K and over 260 K, a maximum in TC with pressure P, and normal-state resistance scaling with temperature (suggesting unconventional pairing). According to reported band structure calculations of Fm3m-phase LaH10, the La is anionic, with the chemical valence electrons appearing evenly split between La and H10. Thus, compressed LaH10 contains the combination of structure, charge separation, and optimal balanced allocation of valence electrons for supporting unconventional high-TC superconductivity mediated by Coulomb interactions between electronic charges associated with La and H10. A general expression for the optimal superconducting transition temperature for MHn clathrates is derived as TC0 = kB−1Λ[(n + v)/2A]1/2e2/ζ, where Λ is a universal constant, (n + v) is the chemical valence sum per formula unit, taking unity for H and v for atom M, A is the surface area of the H-polyhedron cage, and ζ is the mean distance between the M site and the centroids of the polyhedron faces. Applied to Fm3m LaH10, TC0 values of 249.8(1.3) K and 260.7(2.0) K are found for the two experiments. Associated attributes of charge allocation, structure, effective Coulomb potential, and H-D isotope effect in TC of Fm3m LaH10 and Im3m H3S are discussed, along with a generalized prospective for Coulombmediated superconductivity in MHn. Keywords LaH10 . Superhydrides . Superconductivity . TC . Coulomb mediation
1 Introduction Binary hydride clathrates MHn are a class of compounds comprising a central atom surrounded by a polyhedron hydrogen cage and have been designated as superhydrides. Following earlier theoretical results [1], superconductivity at high temperatures has been predicted in dozens of such (and other) hydride compounds under compression, assuming strong-coupled electron-
* Dale R. Harshman [email protected] 1
Department of Physics, The College of William and Mary, Williamsburg, VA 23187, USA
2
Department of Physics, New Jersey Institute of Technology, Newark, NJ 07102, USA
phonon theory [2–4]. Notably, the superconductors LaH10 [5, 6], YHn (n = 4, 6, 9) [5–9], and ThHn (n = 9, 10) [6, 10] have been experimentally verified in [11–13], [14, 15], and [16], respectively. The LaH10 clathrate, exhibiting superconductivity at temperatures of about 250 K [12] and above 260 K [13] for applie
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