Optical response of the superconducting state of K3C60 and Rb3C60.

We have measured the optical reflectivity of single phase K 3 C 60 and Rb 3 C 60 compounds, and have evaluated the optical conductivity both below and above the superconducting transition temperature. Our results are in full agreement with a BCS singlet ground state, with the measured gap values consistent with the weak-coupling limit