  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.
