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/kTc (where Tc 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.
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