Abstract
Microwave penetration depth and surface resistance at 27 GHz are measured in high quality crystals of . Firm evidence for fully gapped superconductivity is provided from . Below the second transition at , the superfluid density shows a steplike change with a suppression of effective critical temperature . 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 .
- Received 26 February 2007
DOI:https://doi.org/10.1103/PhysRevLett.98.257004
©2007 American Physical Society