INTERCALATION of C60 (buckminsterfullerene1,2) by alkali metals3 leads to superconducting compounds of stoichiometry A3C60 (refs 4-6) with transition temperatures Tc 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 (Tc = 19.3 K). We find significant changes in the peak positions and intensities principally of the intramolecular Hg 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.
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