Neutron-scattering study of C60n- (n=3,6) librations in alkali-metal fullerides

We have measured the low-energy inelastic neutron scattering spectra of superconducting K3C60 and insulating Rb6C60. Well-defined peaks are observed at room temperature at 3.59(4) meV [full width at half maximum (FWHM) =1.11(15) meV] and 5.49(38) meV [FWHM = 3.41(44) meV], respectively. They harden with decreasing temperature and the dependence of their intensities on the scattering vector shows that they are due to small-amplitude librational motion. No anomalous behavior of the librational peak was observed on cooling through the superconducting transition temperature, indicating that electron-librational coupling is weak. The energy barrier for reorientation is estimated to be at least twice as large in K3C60 as in C60. Substantial disorder persists for K3C60 in the superconducting state.