Effects of Rattling Phonons on the Quasiparticle Excitation and Dynamics in the Superconducting ${\hat{I}}^2$-Pyrochlore KOs${}_2$O${}_6$

Abstract

Microwave penetration depth $\hat{I}»$ 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 $\hat{I}»(T)$. Below the second transition at $T_{\mathrm{p}}\sim 8$ K, the superfluid density shows a step-like change with a suppression of effective critical temperature $T_{\mathrm{c}}$. 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 $T_{\mathrm{p}}$.

5 pages, 5 figures, to be published in Phys. Rev. Lett