THE discovery of superconductivity in potassium-doped C60(ref. 1) has been followed by an intense effort to understand the physics and chemistry of metal-doped fullerene solids2-13. Experimental studies of alkali-metal-doped C60 have now provided insight into the structure7,13 and the coherence length and penetration depth4 of the superconducting phase. No measurements of the superconducting energy gap (Δ) have, however, been reported. The BCS theory of superconductivity15, which has been used to interpret much of this experimental work2,4,9-13, predicts (in the limit of weak coupling) that the reduced energy gap 2Δ/kTC has a material-independent value of 3.53. Values in excess of 3.5 define strong coupling, and thus provide insight into the nature of the pairing mechanism. Here we describe the measurement of Δ for single-phase superconducting Rb3C60 by tunnelling spectroscopy using a scanning tunnelling microscope. We obtain a value of Δ at 4.2 K of 6.6±0.4 meV, corresponding to a reduced energy gap of 5.3. This is significantly larger than predicted by BCS theory, but similar in magnitude to values found for high-temperature copper oxide superconductors14. Our finding of strong coupling in Rb3C60 suggests the need for caution in using standard BCS theory to interpret superconductivity in metal-doped C60.
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