Classifying Superconductivity in ThH-ThD Superhydrides

Abstract

Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) discovered that Th4H15-Th4D15 superhydrides exhibit superconductivity and have no isotope effect. The latter is fundamental contradiction with the concept of electron-phonon mediated superconductivity of Bardeen-Cooper-Schrieffer (BCS) theory. Soon after this work, Stritzker and Buckel (1972 Zeitschrift für Physik A Hadrons and nuclei 257 1-8) reported that superconductors in PdHx-PdDx system exhibit reverse isotope effect. Yussouff et al (1995 Solid State Communications 94 549) extended this finding on PdHx-PdDx-PdTx system. Recent interest to hydrogen- and deuterium-rich superconductors is based on 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). To date, there is no clarity about isotope effect in H3S-D3S system, because thorough examination of available experimental data reported by Drozdov et al (2015 Nature 525 73) shows that H3S-D3S system perhaps has reverse isotope effect. In attempt to reaffirm/disprove our primary idea that the mechanism for near-room-temperature superconductivity in hydrogen-rich superconductors is not BCS electron-phonon interaction, we analyse the upper critical field data, Bc2(T), in Th4H15-Th4D15 phases (Satterthwaite and Toepke 1970 Phys. Rev. Lett. 25 741) and two recently discovered high-pressure hydrogen-rich phases of ThH9 and ThH10 (Semenok et al 2019 arXiv:1902.10206). As the result, it is found that all known to date thorium super-hydrides/deuterides are unconventional superconductors which have Tc/TF ratios within a range of 0.008 < Tc/TF < 0.120, where Tc is the superconducting transition temperature and TF is the Fermi temperature.