Classifying superconductivity in ThH-ThD superhydrides/superdeuterides

Abstract

Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) discovered that Th _4 H _15 -Th _4 D _15 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 PdH _x -PdD _x system exhibit the reverse isotope effect. Yussouff et al (1995 Solid State Communications 94 549) extended this finding in PdH _x -PdD _x -PdT _x systems. Renewed interest in hydrogen- and deuterium-rich superconductors is driven by the discovery of near-room-temperature superconductivity in highly-compressed H _3 S (Drozdov et al 2015 Nature 525 73) and LaH _10 (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 Th _4 H _15 -Th _4 D _15 (Satterthwaite and Toepke 1970 Phys. Rev. Lett. 25 741) as well as two recently discovered high-pressure hydrogen-rich phases of ThH _9 and ThH _10 (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.

Keywords

• hydrogen-rich superconductors
• superconducting coherence length
• high pressure
• fermi temperature in superconductors