Structural and electronic factors influencing the superconductivity in alkali-metal-doped superconductors A3C60

We have examined the relationships between the shortest inter-ball C···C distances, the n(ρf) values, the electron-phonon coupling constants λ, and Tc values of the alkali-metal-doped fullerene superconductors A3C60 (i.e., Na2CsC60, K3C60, Rb3C60 and ammoniated Na2CsC60). The n(ρf) values are calculated by using the extended Hückel tight-binding band electronic band structure method, and the λ values are estimated from the observed Tc values. The n(ρf) value and lattice softness influence the Tc values of these superconductors, both working to reinforce each other. The Tc values of the A3C60 superconductors increase steadily with increasing inter-ball C ··· C distance, reaching a plateau near the van der Waals C···C distance. This plateau suggests that inter-ball interactions within the van der Waals C···C contacts are necessary for the superconductivity.