Rattling-Induced Superconductiviy in the Beta-Pyrochlore Oxides AOs2O6

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

Superconductivity in alkaline earth metal and Yb intercalated group VI layered dichalcogenides

Alkaline earth metals and Yb have been intercalated into Group VI layered dichalcogenides, e.g., MoS<SUB>2</SUB>, etc., by the liquid ammonia method at low temperatures. Chemical analysis and X-ray data indicate that ammonia is also intercalated along with the metal and that the c axis increases by ~3 Å in all the intercalated materials. In most cases, these compounds are only partially intercalated in that some pure unintercalated phase is present. Superconductivity has been observed in these intercalated compounds with the highest value T<SUB>onset</SUB> of 5.7°K for Ba<SUB>x</SUB>(NH<SUB>3</SUB>)<SUB>y</SUB>MoS<SUB>2</SUB>; the T<SUB>onset</SUB> temperatures depend on the nature of the metal and the host dichalcogenides but not on the concentration of the intercalated metal ion. Fine-particle (1-10 μm size) materials which are fully intercalated do not exhibit the superconductivity behavior as measured by the ac susceptibility method. The results are discussed in terms of the current band models for the Group VI MCh<SUB>2</SUB> materials and compared with alkali metal intercalated MoS<SUB>2</SUB> as reported by Somoano et al. (4)