Electronic, magnetic, and dynamical properties of the new 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 \AA 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 Tc. Electron scattering from this very anharmonic mode may be the cause of the anomalous concave-downward resistivity that is seen only in KOs2O6.
@article{arxiv.cond-mat/0412031,
title = {Correlation Effects and Structural Dynamics in the $\beta$-Pyrochlore Superconductor KOs$_{2}$O$_{6}$},
author = {J. Kunes and T. Jeong and W. E. Pickett},
journal= {arXiv preprint arXiv:cond-mat/0412031},
year = {2009}
}