Superconducting and Structural Transitions in the {\beta}-Pyrochlore Oxide KOs2O6 under High Pressure

TL;DR

This study explores how high pressure affects superconductivity and structure in KOs2O6, revealing a transition from cubic to lower symmetry phases that suppresses rattling-induced superconductivity.

Contribution

It provides new insights into the pressure-induced structural transition and its impact on superconductivity in {eta}-pyrochlore oxides, highlighting the role of rattling vibrations.

Findings

Superconducting Tc decreases with pressure, then drops sharply at 3.6 GPa.

Structural transition from cubic to monoclinic/triclinic occurs near 3.6 GPa.

Rattling vibrations are suppressed in the high-pressure phase.

Abstract

Rattling-induced superconductivity in the {\beta}-pyrochlore oxide KOs2O6 is investigated under high pressure up to 5 GPa. Resistivity measurements in a high-quality single crystal reveal a gradual decrease in the superconducting transition temperature Tc from 9.7 K at 1.0 GPa to 6.5 K at 3.5 GPa, followed by a sudden drop to 3.3 K at 3.6 GPa. Powder X-ray diffraction experiments show a structural transition from cubic to monoclinic or triclinic at a similar pressure. The sudden drop in Tc is ascribed to this structural tran-sition, by which an enhancement in Tc due to a strong electron-rattler interaction present in the low-pressure cubic phase is abrogated as the rattling of the K ion is completely suppressed or weakened in the high-pressure phase of reduced symmetry. In addition, we find two anomalies in the temperature dependence of resistivity in the low-pressure phase, which may be due to subtle changes in rattling vibration.