Rattling-Induced Superconductiviy in the Beta-Pyrochlore Oxides AOs2O6

TL;DR

This study investigates how rattling modes of alkali metal ions influence the superconducting properties of beta-pyrochlore oxides CsOs2O6 and RbOs2O6, revealing a transition from weak to strong coupling with increasing Tc.

Contribution

It demonstrates that low-energy rattling modes of alkali ions are key to the superconductivity mechanism and explains the variation in Tc across different compounds.

Findings

Superconductivity varies from weak to strong coupling among the compounds.

Rattling modes of alkali ions are responsible for the superconducting mechanism.

Tc increases with stronger electron-rattler coupling and anharmonicity.

Abstract

The superconducting properties of two beta-pyrochlore oxides, CsOs2O6 and RbOs2O6, are studied by thermodynamic and transport measurements using high-quality single crystals. It is shown that the character of superconductivity changes systematically from weak coupling for CsOs2O6 to moderately strong coupling for RbOs2O6, and finally to extremely strong coupling with BCS-type superconductivity for KOs2O6, with increasing Tc. Strong-coupling correction analyses of the superconducting properties reveal that a low-energy rattling mode of the alkali metal ions is responsible for the mechanism of the superconductivity in each compound. The large enhancement of Tc from Cs to K is attributed to the increase in the electron-rattler coupling with decreasing characteristic energy of the rattling and with increasing anharmonicity. The existence of weak anisotropy in the superconducting gap or in the electron-rattler interactions is found for the Cs and Rb compounds.