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

概要

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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 $T_{\text{c}}$ 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 $T_{\text{c}}$ is ascribed to this structural transition, by which an enhancement in $T_{\text{c}}$ 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.
2010-11-15