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

The presence of strong electron correlations in the fullerites has bee n suggested, in part due to the underestimation of the band pp by loca l density functional calculations in solid C60, and due to the apparen t lack of dispersion seen in angle-resolved photoemission data. By usi ng a first-principles quasiparticle approach to include correlation co ntributions to electron excitation energies, we find a significant imp rovement in agreement with experiment for the band gap in undoped, sol id 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 im provement in other spectroscopic quantities. Meanwhile, we find a comb ination of factors which, together, nearly completely eliminate the si gnatures of dispersion seen in angle-resolved photoemission measuremen ts. These factors include orientational disorder, the lack of definiti on of crystal momentum normal to the surface, and the multi-band natur e of this system. Our results suggest that, although correlation effec ts are sizable, solid C60 may nonetheless be viewed as a standard band insulator with molecular orientational disorder. Since screening effe cts should change drastically with the addition of electrons in the al kali 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.