Phonon Dynamics and Multipolar Isomorphic Transition in beta-pyrochlore KOs2O6

TL;DR

This paper models anharmonic K-cation oscillations in KOs2O6, explaining a first-order structural transition as a multipolar octupolar transition without symmetry change, aligning with neutron and electron-phonon data.

Contribution

It introduces a microscopic model that reproduces experimental observations and explains the multipolar nature of the structural transition in KOs2O6.

Findings

Successful reproduction of temperature dependence and Tc

Identification of octupolar transition as the cause of the first-order transition

Explanation of experimental neutron and electron-phonon data

Abstract

We investigate with a microscopic model anharmonic K-cation oscillation observed by neutron experiments in beta-pyrochlore superconductor KOs2O6, which also shows a mysterious first-order structural transition at Tp=7.5 K. We have identified a set of microscopic model parameters that successfully reproduce the observed temperature dependence and the superconducting transition temperature. Considering changes in the parameters at Tp, we can explain puzzling experimental results about electron-phonon coupling and neutron data. Our analysis demonstrates that the first-order transition is multipolar transition driven by the octupolar component of K-cation oscillations. The octupole moment does not change the symmetry and is characteristic to noncentrosymmetric K-cation potential.