Superconductivity in K3C60 and Rb3C60 surfaces

We present a detailed investigation of the surface electronic structure of solid K₃C₆₀ and Rb₃C₆₀ in connection with their electronic transport properties. The best conducting samples with the highest superconducting transition temperature T_{c have the highest density of states at the Fermi level as measured by photoemission, but the valence of the C60 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 Δ which is not reduced from the bulk value, i.e. close to the weak coupling 2Δ/kBTc}=3.53 value.