Anomalous high-temperature superconductivity in YH$_6$

TL;DR

This study reports the synthesis of YH6 with a record high superconducting transition temperature of 224 K at high pressure, revealing unusual superconducting properties that challenge conventional theories.

Contribution

The paper presents the experimental realization of YH6 superconductivity at high pressure with a transition temperature exceeding 220 K, and explores its unconventional superconducting behavior.

Findings

YH6 exhibits superconductivity at 224 K under 166 GPa.

The upper critical magnetic field exceeds 116 T, larger than theoretical predictions.

Superconducting properties show significant deviations from conventional BCS and Migdal-Eliashberg theories.

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-YH$_6$ that demonstrates the superconducting transition with T$_c$ = 224 K at 166 GPa, much lower than the theoretically predicted (>270 K). The measured upper critical magnetic field B$_c$$_2$(0) of YH$_6$ was found to be 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T$_c$ in yttrium deuteride YD$_6$ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements showed that the critical current I$_c$ and its density J$_c$ may exceed 1.75 A and 3500 A/mm$^2$ 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 YH$_6$ from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.