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 (δTc) upon isotopic substitution. A series of isotopically substituted Rb3C60 superconductors, where C60 corresponds to 13C60, (13C60)0.5(12C60) 0.5, or (13C0.5512C0.45)60, was investigated. The δTc determined for Rb313C60, 0.7 ± 0.1 K, provides an unambiguous value for the isotope shift in fully substituted fullerene superconductors. The δTc determined for Rb3(13C0.5512C0.45) 60, 0.5 K, is also consistent with simple mass scaling of the Rb313C60 result. However, an intermolecular effect not accounted for in existing theories is demonstrated by the unexpectedly large δTc, 0.9 K, that was observed for the Rb3(13C60)0.5(12C 60)0.5 materials. These results are used to critically assess proposed mechanisms of fullerene superconductivity.
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