Classifying superconductivity in ThH-ThD superhydrides/superdeuterides

TL;DR

This paper reviews the unconventional superconductivity observed in thorium hydrides and deuterides, highlighting their lack of isotope effect and comparing their properties to other unconventional superconductors.

Contribution

It provides a comprehensive analysis of thorium superhydrides/deuterides as unconventional superconductors, expanding understanding of their mechanisms and placing them within the broader context of high-temperature superconductivity.

Findings

Thorium superhydrides/deuterides are unconventional superconductors.

They exhibit Tc/Tf ratios between 0.008 and 0.120.

They show no isotope effect, indicating non-phonon mechanisms.

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 f\"ur 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.