Electronic States and Superconductivity in Alkali-Intercalated Fullerides:13C-NMR Study in Na2RbC60, Na2CsC60, K3C60, K2RbC60, K2CsC60, KRbCsC60, Rb2CsC60and RbCs2C60

A systematic 13 C NMR experiment is reported in alkali-fullerides A 3 C 60 (where A 3 is alkali-metals) between superconducting transition temperature T c and ∼100 K. Careful 13 C NMR spin-lattice relaxation time study clarified the properties of the conduction electron wave functions at Fermi surface. An upper limit of Stoner exchange parameter was estimated to be ∼3 eV/(spin-carbon), which is far from spin-wave instability criterion. Based on these studies, a variation of the density of states at Fermi level, N ( E f ), with T c was deduced. The results were analyzed in terms of McMillan formula, leading to weak or medium electron-phonon coupling constants and relevant phonon energy larger than 600 K. It was strongly suggested that the A 3 C 60 superconductor is described within a framework of the conventional phonon-mediated BCS superconductivity.