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

doi:10.1126/science.254.5038.1619

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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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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 magnitudeand temperature dependence of the superconducting energy gap( $utri$ ) for K₃C₆₀ and Rb₃C₆₀. At low temperature the reduced energygap, 2 $utri$ $kgr$ T_c (where T_c is the transition temperature) has a valueof 5.3 ± 0.2 and 5.2 ± 0.3 for K₃C₆₀ and Rb₃C₆₀, respectively.The magnitude of the reduced gap for these materials is significantlylarger than the value of 3.53 predicted by Bardeen-Cooper-Schrieffertheory. Hence, these results show that the pair-coupling interactionis strong in the M₃C₆₀ superconductors. In addition, measurementsof $utri$ (T) for both K₃C₆₀ and Rb₃C₆₀ exhibit a similar mean-fieldtemperature dependence. The characterization of $utri$ and $utri$ (T) forK₃C₆₀ and Rb₃C₆₀ provides essential constraints for theoriesevolving to describe superconductivity in the M₃C₆₀ materials.