Superconductivity appearing from C₆₀ doped with rare-earth metals

The superconductivity observed in rare-earth C₆₀ fullerides RE_x C₆₀ (RE = Yb and Sm) and La - C₆₀ compounds are described. It is shown that RE_2.75 C₆₀'s do not superconduct and the true superconducting phase in Yb - C₆₀ fulleride resides in the t_{1 g} band. No superconductivity is observed in Sm - C₆₀ fullerides. The phase sequence between Yb - C₆₀ fullerides and Sm - C₆₀ ones as a function of the intercalant concentration is shown to be greatly different, although Yb and Sm are the same family of divalent rare-earth elements. This seems to explain the loss of superconductivity in Sm_x C₆₀, and suggests that the true superconducting phase is most likely Yb_x C₆₀(x > 3) with the primitive cubic lattice. In addition, the superconductivity observed in C₆₀ with La is shown to result from La₂ C₃ carbides with two transition temperatures.