Photoemission study of superconductive and non-superconductive alkali-doped C₆₀ and C₇₀

Comparative photoemission measurements were performed on pristine C₆₀ and C₇₀ and their alkali metal (Rb, Cs) compounds to study the electronic structure. In particular, the change in structure upon alkali doping and the difference between the superconductive compound (RbC₆₀) and the non-superconductive compounds (Cs-C₆₀, RbC₇₀) were studied. The electrical conductivity of the sample films was simultaneously measured in situ to study the correlation between the conductivity and the photoemission spectrum. It was found that the density of states at the Fermi level gradually increases upon Rb doping until x reaches around 3 in AxC₆₀, where A is an alkali metal, then gradually decreases with further doping until x=6. However, the density of states at the Fermi level is considerably smaller in the non-superconductive compounds. A good correspondence between the electrical conductivity and the density of the states at the Fermi level measured by photoemission has been obtained. The observed lack of efficient density of states at the Fermi level in the non-superconductive CsC₆₀ and RbC₇₀ would account for the absence of superconductivity in these compounds.