THE surprisingly high transition temperatures (Tc) for superconductivity in alkali-meta-doped C60 has spurred wide interest in understanding its mechanism1-6. Recently the increase in Tc with lattice constant was demonstrated for these materials6, and was interpreted as resulting from the corresponding increase in the density of states at the Fermi level. According to the standard (BCS) theory of superconductivity, the other important factor controlling Tc is the phonon that mediates electron pairing. To test whether this factor plays a part for the C60 superconductors, we prepared C60 containing various amounts of 13C, which we then doped with rubidium to give Rb3C60. Measurements of diamagnetic shielding and Meissner effect show that Tc decreases as the 13C 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 where Tcm-α with α ≤0.5. Instead, the exponent a 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 the origin of superconductivity in doped C60.
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