BCS-like density of states in superconducting A3C60 surfaces

We report on an ultrahigh resolution photoemission study on the topmost molecular layer of K3C60 and Rb3C60 films below and above the superconducting transition temperature T(C). We observed not only clear evidence for the opening of the superconducting gap, but also a modification in the photoemission line shape consistent with a change from a normal-metallic to a BCS-like density of states, including the formation of a condensation peak. The data can be accurately modeled by a BCS-type function with a gap Delta derived from T(C) in the weak-coupling limit ( 2Delta/k(B)T(C) = 3.53).