Tunneling Spectroscopy of M 3 C 60 Superconductors: The Energy Gap, Strong Coupling, and Superconductivity

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Abstract
Tunneling spectroscopy has been used to characterize the magnitude and temperature dependence of the superconducting energy gap (Δ) for K 3 C 60 and Rb 3 C 60 . At low temperature the reduced energy gap, 2Δ/ k T c (where T c is the transition temperature) 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 Bardeen-Cooper-Schrieffer theory. Hence, these results show that the pair-coupling interaction is strong in the M 3 C 60 superconductors. In addition, measurements of Δ( T ) for both K 3 C 60 and Rb 3 C 60 exhibit a similar mean-field temperature dependence. The characterization of Δ and Δ( T ) for K 3 C 60 and Rb 3 C 60 provides essential constraints for theories evolving to describe superconductivity in the M 3 C 60 materials.