Anomalous high-temperature superconductivity in YH6
Abstract
Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here we report the synthesis of yttrium hexahydride Im3m-YH6 that demonstrates the superconducting transition with Tc = 224 K at 166 GPa, much lower than the theoretically predicted (>270 K). The measured upper critical magnetic field Bc2(0) of YH6 was found to be 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of Tc in yttrium deuteride YD6 with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements showed that the critical current Ic and its density Jc may exceed 1.75 A and 3500 A/mm2 at 0 K, respectively, which is comparable with the parameters of commercial superconductors, such as NbTi and YBCO. The superconducting density functional theory (SCDFT) and anharmonic calculations suggest unusually large impact of the Coulomb repulsion in this compound. The results indicate notable departures of the superconducting properties of the discovered YH6 from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.