We have measured the optical reflectivity of single phase ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{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.

• Received 17 August 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.2987