Vibrational spectroscopy of superconducting K₃C₆₀ by inelastic neutron scattering

INTERCALATION of C₆₀ (buckminsterfullerene^1,2) by alkali metals³ leads to superconducting compounds of stoichiometry A₃C₆₀ (refs ^4-6) with transition temperatures T_c as high as 33 K (ref. 7). These transition temperatures are considerably higher than those for alkali-metal-intercalated graphite (<0.6 K)⁸ and scale with the size of the face-centred-cubic unit cell⁹. Here we present the results of an inelastic neutron scattering study of the vibrational spectrum of the superconducting fulleride K₃C₆₀ (T_c = 19.3 K). We find significant changes in the peak positions and intensities principally of the intramolecular H_g vibrational modes, both in the high-energy tangential (130-200 meV) and the low-energy radial (∼50 meV) regions, compared with the vibrational spectrum of C₆₀ (refs ^10,11). Our results provide strong evidence for the importance of these modes in the pairing mechanism for superconductivity.