NMR Observation of Rattling Phonons in the Pyrochlore Superconductor   KOs2O6
  We report nuclear magnetic resonance studies on the beta-pyrochlore oxide superconductor KOs2O6. The nuclear relaxation at the K sites is entirely caused by fluctuations of electric field gradient, which we ascribe to highly anharmonic low frequency oscillation (rattling) of K ions. A phenomenological analysis shows a crossover from overdamped to underdamped behavior of the rattling phonons with decreasing temperature and its sudden sharpening below the superconducting transition temperature Tc. Absence of the Hebel-Slichter peak in the relaxation rate at the O sites below Tc also indicates strong electron-phonon coupling. 
M. Yoshida, K. Arai, R. Kaido, M. Takigawa, S. Yonezawa, Y. Muraoka,   and Z. Hiroi