High-Tc Superconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions between Hydrogen and Central-Atom Electrons

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.

Measured transition temperatures TC for LaH10 and H3S are plotted against (ℓζ)−1. The solid line represents the interlayer Coulomb pairing theory for Tc0. Results for the mixed-phase, non-optimal samples of YH9, YH6, ThH10, and ThH9 are also presented for comparison; the red dashed trend line is a fit to these data as described in the text. Horizontal error bars correspond to the lower and upper pressure limits.

Dale R. Harshman and Anthony T. Fiory, Journal of Superconductivity and Novel Magnetism 33, 2945 (2020).

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