Anomalous Dynamics of K Ion in $\beta$-Pyrochlore

Among the recently discovered $\beta$-pyrochlore superconductors AOs$_2$O$_6$ (A=K, Rb, and Cs) KOs$_2$O$_6$ exhibits several anomalous features (electrical resistivity, 1/T$_1$ NMR decay). We have studied electronic structure of AOs$_2$O$_6$ using the density functional theory based methods. We have found a moderate Stoner enhancement of the Pauli susceptibility of 2.15 and a sizable thermal mass enhancement of 2.9 in KOs$_2$O$_6$. While the electronic structure of the three systems is very similar the structural stability of the alkali atom site is rather different. In particular the K ion potential well has a flat bottom, beyond small anharmonic corrections, allowing for large excursions from the symmetric position. We suggest that this feature is behind the striking differences in behavior of otherwise similar AOs$_2$O$_6$ compounds.