Superconductivity appearing from C60 doped with rare-earth metals

The superconductivity observed in rare-earth C60 fullerides RExC60 (RE=Yb and Sm) and La-C60 compounds are described. It is shown that RE2.75C60's do not superconduct and the true superconducting phase in Yb-C60 fulleride resides in the t1g 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 SmxC60, and suggests that the true superconducting phase is most likely YbxC60 (x>3) with the primitive cubic lattice. In addition, the superconductivity observed in C60 with La is shown to result from La2C3 carbides with two transition temperatures.