Superconductivity and Unusual Lattice Dynamics in the $\beta $-Pyrochlore Oxides

Recently two families of pyrochlore oxide superconductors were found: one is $\alpha $ -pyrochlore oxide Cd$_{2}$Re$_{2}$O$_{7}$ with $T_{c}$ = 1.0 K$^{1}$ and the other is $\beta $ -pyrochlore oxides AOs$_{2}$O$_{6}$, where A = Cs, Rb and K, with $T_{c}$ = 3.3 K, 6.3 K and 9.6 K, respectively.$^{2}$ The superconductivity of the former compound is of weak-coupling BCS type, while, in the latter compouds, the superconductivity changes from conventional weak-coupling to extremely strong-coupling from Cs to K. In particular, KOs$_{2}$O$_{6}$ with the highest $T_{c}$ exhibits various unconventional features, which may be ascribed to anomalous electron-phonon couplings arising from the heavy rattling of the K ions.$^{3}$ Possibly related to this, a first-order phase transition at $T_{p}$ = 7.6 K below $T_{c}$ has been found only for KOs$_{2}$O$_{6}$. I will discuss on what is the rattling and how it affects the surrounding conduction electrons in the $\beta $-pyrochlores. \newline \newline $^{1}$M. Hanawa\textit{ et al.}, Phys. Rev. Lett. \textbf{87}, 187001 (2001). \newline $^{2}$S. Yonezawa, Y. Muraoka, Y. Matsushita, and Z. Hiroi, J. Phys.: Condens. Matter \textbf{16}, L9 (2004). \newline $^{3}$Z. Hiroi, S. Yonezawa, Y. Nagao, and J. Yamaura, submitted to PRB.