Superconductivity in the Osmium-based Beta-Pyrochlore Oxides

Superconductivity is reported in recently discovered \textit{$\beta $}${\rm t}$pyrochlore oxides AOs$_{2}$O$_{6}$. The $T_{c}$ is 3.3 K, 6.3 K, and 9.6 K for A = Cs, Rb, and K, respectively. The highest $T_{c}$ of KOs$_{2}$O$_{6}$ is almost one order higher than the $T_{c}$ = 1.0 K of previously reported \textit{$\alpha $}-pyrochlore oxide superconductor Cd$_{2}$Re$_{2}$O$_{7}$ which is believed to be a conventional $s$-wave superconductor. Moreover, the upper critical field $H_{c2}$ of KOs$_{2}$O$_{6}$ is estimated to be 38 T, which seems to exceed Pauli's limit expected for conventional superconductivity. This is again in contrast to the case of Cd$_{2}$Re$_{2}$O$_{7}$, in which the $H_{c2}$ is 0.29 T, much smaller than the corresponding Pauli's limit. These distinct contrasts strongly suggest that the mechanism of superconductivity is essentially different between the two pyrochlore oxides. It is to be noted that the $T_{c}$ of these \textit{$\beta $}${\rm t}$pyrochlore oxides decreases with increasing the ionic radius of the alkaline metal ions, imposing negative chemical pressure upon the Os pyrochlore lattice. I believe that interesting physics is involved on the basis of strong electron correlations on the highly frustrated pyrochlore lattice.