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 (δT_c) upon isotopic substitution. A series of isotopically substituted Rb₃C₆₀ superconductors, where C₆₀ corresponds to ¹³C₆₀, (¹³C₆₀)_0.5(¹²C₆₀) _0.5, or (¹³C_0.55¹²C_0.45)₆₀, was investigated. The δT_c determined for Rb₃¹³C₆₀, 0.7 ± 0.1 K, provides an unambiguous value for the isotope shift in fully substituted fullerene superconductors. The δT_c determined for Rb₃(¹³C_0.55¹²C_0.45) 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 δT_c, 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.