Microwave penetration depth λ and surface resistance at 27 GHz are measured in high quality crystals of KOs2O6. Firm evidence for fully gapped superconductivity is provided from λ(T). Below the second transition at Tp∼8K, the superfluid density shows a steplike change with a suppression of effective critical temperature Tc. 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 Tp.
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