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.
@article{arxiv.cond-mat/0610760,
title = {NMR Observation of Rattling Phonons in the Pyrochlore Superconductor KOs2O6},
author = {M. Yoshida and K. Arai and R. Kaido and M. Takigawa and S. Yonezawa and Y. Muraoka and Z. Hiroi},
journal= {arXiv preprint arXiv:cond-mat/0610760},
year = {2009}
}