Isotope effect on superconductivity in Rb3C60

To test whether the phonon that mediates electron pairing plays a part in controlling Tc for C60 superconductors, C60 was prepared containing various amounts of C-13, which was then doped with Rb to give Rb3C60. Measurements of diamagnetic shielding and Meissner effect show that Tc decreases as the C-13 content increases, as expected within the context of BCS-like phonon-mediated pairing; but the dependence on the mass is stronger than for most electron-phonon superconductors: the exponent alpha has the remarkably large value of 1.4 +/- 0.5. Regardless of the interpretation of this value, it is clear that phonons have an important role in th origin of superconductivity in doped C60.