Carbon Isotope Effect in Single-Crystal Rb3C60

The sharp resistive transitions in superconducting single crystals of natural abundance and 99% 13C-enriched Rb3C60 reveal the carbon isotope effect with unprecedented accuracy. The measured isotope exponent aC 0.21 6 0.012 is outside the error bars of all previous reported values. Our precise value for aC, combined with Tc, aRb , and an ab initio calculation of the frequency distribution of the electron-phonon coupling, allows a fully constrained determination of the coupling strength l 0.9 20.1 and the Coulomb repulsion m 0.22 10.03 20.02. These new measurements reveal a phononmediated superconductor with moderate coupling to a wide range of phonons centered near 1400 K.