Superconductivity in K3C60 and Rb3C60 surfaces

We present a detailed investigation of the surface electronic structure of solid K;CM, and Rb3C60 in connection with their electronic transport properties. The best conducting samples with the highest superconducting transition temperature T, have the highest density of states at the Fermi level as measured by photoemission, but the valence of the CM ions at the surface deviates appreciable from the 3- bulk value, namely 2.5- or even 1.5- depending on the preparation procedure. Despite the large discrepancy in the electronic structure, we find, using ultra-high resolution photoemission, that these surfaces have a BCS-like superconducting gap A which is not reduced from the bulk value, i.e. close to the weak coupling 2Delta/k(B)T(i) = 3.53 value.