ISOTOPE EFFECT AND SUPERCONDUCTIVITY IN METAL-DOPED C60

The mechanism of superconductivity in alkali metal-doped fullerenes remains a fascinating and controversial issue. One powerful probe of this mechanism involves the determination of the shift in transition temperature (deltaT(c)) upon isotopic substitution. A series of isotopically substituted Rb3C60 superconductors, where C60 corresponds to C-13(60), (C-13(60), (C60(0.5)C60)0.5)-C-13-C-12 (C0.55C0.45)60)-C-13-C-12 investigated. The deltaT(c) determined for (Rb3C60)-C-13 0.7 +/- 0.1 K provides an unambiguous value for the isotope shift in fully substituted fullerene superconductors. The deltaT(c) determined for (Rb3(C0.55C0.45)60)-C-13-C-12 K is also consistent with simple mass scaling of the (Rb3)(C60)-C-13 result. However, an intermolecular effect not accounted for in existing theories is demonstrated by the unexpectedly large deltaT(c), 0.9 K that was observed for the Rb3(C60)0.5(C60)0.5(C60)0.5)-C-13-C-12 materials. These results are used to critically assess proposed mechanisms of fullerene superconductivity.