Effects of Rattling Phonons on the Dynamics of Quasiparticle Excitation in the β-Pyrochlore KOs_2O_6 Superconductor

摘要

Microwave penetration depth λ and surface resistance at 27 GHz are measured in high quality crystals of KOs_2O_6. Firm evidence for fully gapped superconductivity is provided from λ(T). Below the second transition at T_p ～ 8 K, the superfluid density shows a steplike change with a suppression of effective critical temperature T_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_p.