ELECTRONIC-STRUCTURE, ELECTRON PHONON INTERACTION AND SUPERCONDUCTIVITY IN K3C60, RB3C60 AND CS3C60

Results of electronic structure calculations of C60 and M3C60 (M = K, Rb, Cs) fcc crystals carried out by the full-potential linear muffin-tin orbital (LMTO) method are reported. Undoped C60 is found to be semiconducting with a direct gap between the fully occupied valence band and empty conduction band of 0.75 eV at the X-point. The Fermi level in M3C60 falls on the half-occupied peak formed mainly by the 2p states of the closest carbon atoms with neighboring C60 molecules. Estimates of electron-phonon coupling using the crude rigid iron approximation give lambda values of 0.51, 0.61 and 0.72 for K, Rb, and Cs doping, respectively. Using an average phonon frequency of 1100 K the McMillan formula gives T(c) values of 16 K for K3C60, 30 K for Rb3C60 and 47 K for CS3C60. The calculated T(c) values for K and Rb doping are in very good agreement with experiment and the predicted T(c) value of CS3C60 is close to the value obtained by linear extrapolations based on the lattice constant. These are the first calculated results for lambda and T(c) and they strongly support the idea that BCS superconductivity is caused by the electron-phonon interaction and that T(c) changes are caused by negative chemical pressures effects resulting in the increase of lattice constants in the K, Rb, and Cs series.