Correlation Effects and Structural Dynamics in the $β$-Pyrochlore Superconductor KOs$_{2}$O$_{6}$

Abstract

Electronic, magnetic, and dynamical properties of the new superconducting $β$-pyrochlore KOs$_2$O$_6$ and related RbOs$_2$O$_6$ and CsOs$_2$O$_6$ compounds are calculated and compared with experiment and contrasted with structurally related spinel pyrochlores. The calculated susceptibility Stoner enhancement (110%) and thermal mass enhancement $λ$ = 2.5-3 reflect moderate but perhaps important Coulomb correlations. The K$^{+}$ ion optic mode is found to be unstable, allowing large excursions of 0.5-0.6 à from its ideal site of the K ion along $<111>$ directions. This dynamical mode is much less anharmonic in the isostructural Rb and Cs compounds (with larger cations), perhaps accounting for their progressively lower values of T$_c$. Electron scattering from this very anharmonic mode may be the cause of the anomalous concave-downward resistivity that is seen only in KOs$_2$O$_6$.

6 pages, 8 figures