Origin of the difference between the high and low-Tcphases in the yttrium sesquicarbide system

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.