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

TL;DR

This study investigates the electronic, magnetic, and dynamical properties of the superconducting $eta$-pyrochlore KOs$_2$O$_6$, revealing moderate Coulomb correlations and unstable K$^{+}$ ion modes that influence superconductivity and resistivity.

Contribution

It provides the first detailed comparison of the dynamical modes and Coulomb correlations in KOs$_2$O$_6$ and related compounds, linking structural dynamics to superconducting properties.

Findings

Moderate Coulomb correlations with susceptibility enhancement of 110%

Unstable K$^{+}$ optic mode causing large ionic excursions

Anharmonic K$^{+}$ mode linked to lower T$_c$ and anomalous resistivity

Abstract

Electronic, magnetic, and dynamical properties of the new superconducting $\beta$-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 $\lambda$ = 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 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$_6$.