CHANGE OF ELECTRONIC-STRUCTURES WITH VARYING MOLECULAR VALENCE IN AXC60

In the past few years, a variety of intercalation compounds of C60 have been synthesized. Among them, alkali metal fullerides (AxC60) are a subject of considerable interest, because of the occurrence of superconductivity with high transition temperatures, T(c) surpassed only by cuprate superconductors. In these compounds, both the molecular valence of C60 and crystal structures are changed by the alkali metal composition x. The molecular valence corresponds to the number of electrons which occupy the triply degenerate t1u LUMO. The electronic properties of AxC60 strongly depend on the filling of the t1u LUMO and the crystal structures. In this article we report the filling and lattice constant dependence of the electronic structures of AxC60 by means of optical reflection spectroscopy. We find that the optical spectra for AxC60 are drastically changed as a function of x. The electronic structures of A3C60 and A6C60 can be understood in the framework of the band picture, but this fails in the case of A3C60.