Isotope effect on superconductivity in Rb₃C₆₀

Abstract

THE surprisingly high transition temperatures (T_c) for superconductivity in alkali-meta-doped C₆₀ has spurred wide interest in understanding its mechanism^1–6. Recently the increase in T_c 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 T_c 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 T_c 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 T_cm^−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.