ENERGY-GAP IN SUPERCONDUCTING FULLERIDES - OPTICAL AND TUNNELING STUDIES

Tunneling and optical transmission studies have been performed on supe rconducting samples of Rb3C60. At temperatures much below the supercon ducting transition temperature T-c, the energy gap is 2 Delta = 5.2 +/ - 0.2 meV, corresponding to 2 Delta/k(B)T(c) = 4.2. The low temperatur e density of states and the temperature dependence of the optical cond uctivity resemble BCS behavior, although there is an enhanced ''normal state'' contribution. The results indicate that this fulleride materi al is an s-wave superconductor, but the superconductivity cannot be de scribed in the weak coupling limit.