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Science 13 December 1991:
Vol. 254. no. 5038, pp. 1619 - 1621
DOI: 10.1126/science.254.5038.1619
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Articles

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

ZHE ZHANG 1, CHIA-CHUN CHEN 1, and CHARLES M. LIEBER 1

1 Department of Chemistry and Division of Applied Sciences, Harvard University, Cambridge, MA 02138

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

Submitted on October 13, 1991
Accepted on November 1, 1991

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Science. ISSN 0036-8075 (print), 1095-9203 (online)