Carbon isotope effect in single-crystal Rb3C60

The sharp resistive transitions in superconducting single crystals of natur al abundance and 99% C-13-enriched Rb3C60 reveal the carbon isotope effect with unprecedented accuracy. The measured isotope exponent alpha(C) = 0.21 +/- 0.012 is outside the error bars of all previous reported values. Our pr ecise value for alpha(C), combined with T-c, alpha(Rb), and an ab initio ca lculation of the frequency distribution of the electron-phonon coupling, al lows a fully constrained determination of the coupling strength lambda = 0. 9(-0.1)(+0.15) and the Coulomb repulsion mu* = 0.22(-0.02)(0.03). These new measurements reveal a phonon-mediated superconductor with moderate couplin g to a wide range of phonons centered near similar to 1400 K.