An approach to identifying unconventional superconductivity in highly-compressed superconductors

Since the milestone experimental discovery by Drozdov et al( 2015 Nature 525 73-6) who reported the observation of near-room-temperature (NRT) superconductivity in highly-compressed sulphur hydride, the quest for room-temperature superconductivity is primarily focused on highly-compressed materials. Extreme conditions and space confinement inside a diamond anvil cell (DAC) dramatically limits the number of experimental techniques which can be applied to study highly-compressed superconductors. For this reason, the development of new approaches to characterize materials at extreme conditions is one of the central topics in the field of NRT superconductivity. In this paper, we describe an approach to categorize highly-compressed superconductors, including NRT superconductors, as unconventional superconductors. The primary idea for the classification is based on the empirical finding of Uemura (1997 Physica C 282-7 197) who showed that all unconventional superconductors have the ratio of the superconducting transition temperature, T-c, to the Fermi temperature, T-F, within a range of 0.01 <= T-c/T-F <= 0.05. To deduce the Fermi temperature in highly-compressed superconductors, we utilize temperature dependence of the upper critical field and the resistance data (which both can be more or less routinely measured for highly-compressed superconductors) and reported results by first principles calculations for these materials. We demonstrate the application of the approach for highly-compressed oxygen, sulphur, lithium, and recently discovered yttrium superhydride polymorphs, YHn( n = 4,6,7,9) (Troyan et al( 2019 arXiv:1908.01534) and Kong et al( 2019 arXiv:1909.10482)). We also show the application of the approach for the newly discovered uncompressed Nd2-xSrxNiO2 nickelate superconductor.