Superconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions between Hydrogen and Central-Atom Electrons, D. R. Harshman and A. T. Fiory [arXiv]
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 phonon-mediated 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 LaH$10 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 Coulomb-mediated superconductivity in MHn.
Dale R. Harshman and Anthony T. Fiory, Journal of Superconductivity and Novel Magnetism 33, 2945 (2020).