Effects of Rattling Phonons on the Quasiparticle Excitation and Dynamics in the Superconducting $\beta$-Pyrochlore KOs$_2$O$_6$

TL;DR

This study investigates how rattling phonons influence quasiparticle behavior and superconductivity in KOs$_2$O$_6$, revealing a strong coupling that enhances superconductivity and causes a transition to an ordered state at low temperatures.

Contribution

It provides experimental evidence linking rattling phonons to quasiparticle dynamics and superconductivity enhancement in KOs$_2$O$_6$, highlighting a novel interaction mechanism.

Findings

Superconductivity in KOs$_2$O$_6$ is fully gapped.

A step change in superfluid density occurs at 8 K.

Quasiparticle scattering time increases sharply below 8 K.

Abstract

Microwave penetration depth $\lambda$ and surface resistance at 27 GHz are measured in high quality crystals of KOs$_2$O$_6$. Firm evidence for fully-gapped superconductivity is provided from $\lambda(T)$. Below the second transition at $T_{\rm p}\sim 8$ K, the superfluid density shows a step-like change with a suppression of effective critical temperature $T_{\rm c}$. Concurrently, the extracted quasiparticle scattering time shows a steep enhancement, indicating a strong coupling between the anomalous rattling motion of K ions and quasiparticles. The results imply that the rattling phonons help to enhance superconductivity, and that K sites freeze to an ordered state with long quasiparticle mean free path below $T_{\rm p}$.