Tunneling spectroscopy of M₃C₆₀ superconductors: The energy gap, strong coupling, and superconductivity

Tunneling spectroscopy has been used to characterize the magnitude and temperature dependence of the superconducting energy gap (Δ) for K ₃C₆₀ and Rb₃C₆₀. At low temperature the reduced energy gap, 2Δ/kT_c (where T_c is the transition temperature) has a value of 5.3±0.2 and 5.2±0.3 for K₃C₆₀ and Rb₃C₆₀, respectively. The magnitude of the reduced gap for these materials is significantly larger than the value of 3.53 predicted by Bardeen-Cooper-Schrieffer theory. Hence, these results show that the pair-coupling interaction is strong in the M ₃C₆₀ superconductors. In addition, measurements of Δ(T) for both K₃C₆₀ and Rb₃C₆₀ exhibit a similar mean-field temperature dependence. The characterization of Δ and Δ(T) for K₃C₆₀ and Rb₃C ₆₀ provides essential constraints for theories evolving to describe superconductivity in the M₃C₆₀ materials.