We have measured the low-energy inelastic neutron scattering spectra of superconducting ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and insulating ${\mathrm{Rb}}_6$${\mathrm{C}}_{60}$. 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 ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ as in ${\mathrm{C}}_{60}$. Substantial disorder persists for ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ in the superconducting state.

• Received 19 June 1992

DOI:https://doi.org/10.1103/PhysRevB.46.12088