THE ENERGY GAP OF THE M₃C₆₀ SUPERCONDUCTORS

We briefly review tunneling spectroscopy studies of the new fullerene-based superconductors, K 3 C 60 and Rb 30 C 60 from our laboratory. At low temperature the reduced energy gap, 2∆/ k T c , has a value of 5.3 ± 0.2 and 5.2 ± 0.3 for K 3 C 60 and Rb 3 C 60 , respectively. The magnitude of the reduced gap for these materials is significantly larger than the value of 3.53 predicted by BCS theory. These results demonstrate that the energy gap scales with T c and show that the pair coupling interaction is strong in the M 3 C 60 superconductors, and thus they provide information essential to understanding superconductivity in the M 3 C 60 materials.