ELECTRON-EXCITATION ENERGIES IN FULLERITES - MANY-ELECTRON AND MOLECULAR ORIENTATIONAL EFFECTS

The presence of strong electron correlations in the fullerites has been suggested, in part due to the underestimation of the band pp by local density functional calculations in solid C60, and due to the apparent lack of dispersion seen in angle-resolved photoemission data. By using a first-principles quasiparticle approach to include correlation contributions to electron excitation energies, we find a significant improvement in agreement with experiment for the band gap in undoped, solid C60 (2.15 eV vs 1.9-2.6 eV in experiment, as compared to 1.0-1.2 eV obtained in local density functional theory). We also find similar improvement in other spectroscopic quantities. Meanwhile, we find a combination of factors which, together, nearly completely eliminate the signatures of dispersion seen in angle-resolved photoemission measurements. These factors include orientational disorder, the lack of definition of crystal momentum normal to the surface, and the multi-band nature of this system. Our results suggest that, although correlation effects are sizable, solid C60 may nonetheless be viewed as a standard band insulator with molecular orientational disorder. Since screening effects should change drastically with the addition of electrons in the alkali fullerides, the results of the present quasiparticle calculations would not carry over directly to such cases. However, our results may help guide the development of model Hamiltonians which are to be used in studying those systems.