Electron Propagation in Orientationally Disordered Fullerides

We study the electronic spectrum for doped electronic states in the orientationally disordered M3C60 fullerides. Momentum-resolved Green's functions are calculated within a cluster-Bethe-lattice model, and compared with results from calculations on periodically repeated supercells containing quenched orientational disorder. Despite the relatively strong scattering from orientational fluctuations, the electronic states near the Fermi energy are well described by propagating states characterized by an effective Bloch wave vector, and a mean free path of approximately 20 Angstroms. The effective Fermi surface is calculated in this model. This differs from that previously calculated for the orientationally ordered crystal, but is relatively well described within a disorder-averaged virtual-crystal Hamiltonian, which we derive.