Tunneling Spectroscopy of M3C60 Superconductors: The Energy Gap, Strong Coupling, and Superconductivity

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Science  13 Dec 1991:
Vol. 254, Issue 5038, pp. 1619-1621
DOI: 10.1126/science.254.5038.1619


Tunneling spectroscopy has been used to characterize the magnitude and temperature dependence of the superconducting energy gap (▵) for K3C60 and Rb3C60. At low temperature the reduced energy gap, 2▵κTc (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 M3C60 superconductors. In addition, measurements of ▵(T) for both K3C60 and Rb3C60 exhibit a similar mean-field temperature dependence. The characterization of ▵ and ▵(T) for K3C60 and Rb3C60 provides essential constraints for theories evolving to describe superconductivity in the M3C60 materials.