Classifying superconductivity in ThH-ThD superhydrides/superdeuterides

Abstract

Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) discovered that Th4H15-Th4D15 superhydrides are superconducting but exhibit no isotope effect. As the isotope effect is a fundamental prediction of electron-phonon mediated superconductivity described by Bardeen, Cooper, and Schrieffer (BCS) its absence alludes to some other mechanism. Soon after this work, Stritzker and Buckel (1972 Zeitschrift für Physik A Hadrons and nuclei 257 1-8) reported that superconductors in the PdHx-PdDx system exhibit the reverse isotope effect. Yussouff et al (1995 Solid State Communications 94 549) extended this finding in PdHx-PdDx-PdTx systems. Renewed interest in hydrogen- and deuterium-rich superconductors is driven by the discovery of near-room-temperature superconductivity in highly-compressed H3S (Drozdov et al. 2015 Nature 525 73) and LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001) as well as two recently discovered high-pressure hydrogen-rich phases of ThH9 and ThH10 (Semenok et al 2019 Materials Today, DOI: 10.1016/j.mattod.2019.10.005). We conclude that all known thorium super-hydrides/deuterides, to date, are unconventional superconductors - along with the heavy fermions, fullerenes, pnictides, cuprates, where we find they have Tc/Tf ratios within a range of of 0.008 < Tc/Tf < 0.120, where Tc is the superconducting transition temperature and Tf is the Fermi temperature.

15 pages, 3 figures, 57 references

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• Universal Fermi velocity in highly compressed hydride superconductors
• Unconventional superconductivity in highly-compressed unannealed sulphur hydride
• A-15 type superconducting hydride $L{a}_4{H}_{23}$: Nanograined structure with low strain, strong electron-phonon interaction, and moderate level of nonadiabaticity
• Fermi-liquid nonadiabatic highly-compressed cesium iodide superconductor
• Advanced McMillan's equation and its application for the analysis of highly-compressed superconductors
• Debye temperature in LaHx-LaDy superconductors
• Unconventional superconductivity in $\zeta$-phase of oxygen compressed to megabar pressure
• Characterization of near-room-temperature superconductivity in yttrium superhydrides