Carbon isotope effect in single-crystal Rb₃C₆₀

We have synthesized single crystals of isotopically enriched Rb₃C₆₀. The very sharp superconducting transitions in single crystal superconducting fullerides allow us to determine the isotope effect on the superconducting transition temperature, T_c, with unprecedented accuracy. We find that the carbon isotope shift exponent αcarbon for 99% ¹³C substitution is 0.21±0.012, significantly smaller than other values reported in the literature^1-6. This, coupled with the near-zero rubidium isotope effect reported by B. Burk et. al. ⁷, should place considerable constraints on any theoretical model of superconductivity in the alkali fullerides.