Spectroscopic Evidence for Superconductivity of Elemental Metal Y under Pressures

Abstract Very high applied pressure induces superconductivity with the transition temperature (Tc) exceeding 19 K in elemental yttrium, but relatively little is known about the nature of that superconductivity. From point-contact spectroscopy (PCS) measurements in a diamond anvil cell (DAC), strong enhancement in the differential conductance is revealed near the zero-biased voltage owing to Andreev reflection, a hallmark of SC phase. Analysis of PCS spectra based on the extended Blonder-Tinkham-Klapwijk (BTK) model indicates two SC gaps at 48.6 GPa, where the large gap ΔL is 3.63 meV and the small gap ΔS is 0.46 meV. When scaled against the reduced temperature, both small and large SC gaps collapse on a single curve that follows the prediction from the BCS theory. The SC gap-to-Tc ratio is 8.2 for the larger gap and the initial slope of the upper critical field is -1.9 T/K, indicating that Y belongs to a family of the strongly coupled BCS superconductors. The successful application of PCS to Y in DAC environments demonstrates its utility for future research on other pressure-induced high-Tc superconductors.