Electronic structure of alkali-metal doped C60: dependence of the Fermi-level densities of states on the lattice parameter

Since the discovery of superconductivity in the alkali metal doped Fullerenes A3C60 (where A is an alkali atom), it is known that the critical temperature Tc depends strongly on the size of the alkali atom. Tc increases with the increasing size of the atom. There is simultaneously an increase in the lattice parameter. It has generally been assumed that this increase in the lattice parameter results in an increase in the densities of states at the Fermi level which in turn leads to an increase in Tc. We present the results of our electronic structure calculations for A3C60 as a function of lattice parameter. Our calculations show that the densities of states in this compound are highly structured in the vicinity of the Fermi level, and vary strongly as a function of lattice parameter. As a result the value of N(EF) cannot be scaled in a simple manner with the lattice parameter. This result casts a serious doubt on the assumed direct relationship between Tc and the densities of states in these compounds. (orig.)