SUPERCONDUCTIVITY IN ALKALI-DOPED C-60 FULLERIDES

The superconductivity in alkali-doped C-60 fullerides is analyzed theoretically on the basis of band structure results derived for potassium-doped C-60 solids. Theoretical tool is an INDO CO (intermediate neglect of differential overlap crystal orbital) formalism on the basis of Bloch orbitals. For KxC60 fullerides (x = 1, 2, 3) a quasi-degeneracy between a metallic configuration and a Mott-like state with a maximum number of singly occupied k-points is observed. With the molecular variant of the INDO Hamiltonian we derive the Hubbard on-site (intramolecular) two-electron Coulomb parameter U and for the dimer (= supermolecule) the nearest-neighbour V integral. Both numbers are compared with accessible experimental and theoretical estimates. Direct carrier attraction in M(3)C(60) (M = K, Rb, Cs) at certain parts of the Fermi surface seems to be possible. The ungerade symmetry of the conduction band leads to a localized hole-pocket in the vicinity of the center of the Brillouin zone (Gamma-point). It is suggested that this pocket is decisive for a direct charge carrier attraction which leads to an enhancement of the conventional electron-phonon superconducting pairing. Cooper-pair delocalization over two bands renders possible an additional enhancement in the strength of the pairing interaction.