The superconducting energy gap of Rb₃C₆₀

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 solids^2-13. Experimental studies of alkali-metal-doped C₆₀ have now provided insight into the structure^7,13 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 work^2,4,9-13, predicts (in the limit of weak coupling) that the reduced energy gap 2^Δ/kT_C 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.