Classifying superconductivity in ThH-ThD superhydrides/superdeuterides

Abstract

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). Here we attempt to reaffirm or disprove our primary idea that the mechanism for near-room-temperature superconductivity in hydrogen-rich superconductors is not BCS electron-phonon mediated. To that end, we analyse the upper critical field data, B c2(T), in Th4H15-Th4D15 (Satterthwaite and Toepke 1970 Phys. Rev. Lett. 25 741) 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 T c/T F ratios within a range of 0.008 < T c/T F < 0.120, where T c is the superconducting transition temperature and T F is the Fermi temperature.