Correlation effects and structural dynamics in the β-pyrochlore superconductor KOs₂O₆

Electronic, magnetic, and dynamical properties of superconducting β-pyrochlore KOs₂O₆ and related RbOs₂O₆ and CsOs₂O₆ 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 Å 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 KOs₂O₆.