Comparative photoemission study of Rb_xC₆₀, Rb_xC₇₀ and RbC₈. A pseudo-gap at the Fermi level in the fulleride

A comparative photoemission study on Rb_xC₆₀, Rb_xC₇₀ and RbC₈ (graphite intercalation compound) is reported. It was found that the Fermi level of Rb₃C₆₀ is located at the bottom of a valley of the density of states, while that of RbC₈ is at a maximum point of the occupied states. This experimental result is explained in terms of the existence of a pseudo-gap at the Fermi level in the fulleride. This indicates that a rigid-band picture is not a good starting point for understanding the change of the electronic structure upon the alkaline-doping. It was also found that the density of states at the Fermi level is much smaller in Rb_xC₇₀ than in Rb_xC₆₀. This accounts for the less conductive nature and the absence of superconductivity in Rb_xC₇₀.