Electronic excitations in boron-doped graphite

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Abstract
Electronic excitation spectra of graphite substitutionally doped with boron to a level of 0.5 at.% were measured by inelastic electron scattering spectroscopy. In comparison with similar spectra obtained from pristine graphite, new absorption appears in the valence region from 0.2 to 1.5 eV and in the core region near 284 eV due to the lowering of the graphite Fermi level. A tight-binding representation of the three-dimensional graphite band structure is constructed and used to compute the longitudinal dielectric function in the random-phase approximation, including local-field effects. This calculation identifies the new valence absorption as the predecessor of the strong "intraband" plasmon which is observed in acceptor intercalation compounds when more charge is depleted from the graphite planes. The analysis of both core and valence excitations indicates that the Fermi level is lowered by 0.2 eV in borondoped graphite and that the fractional ionization of the boron is 0.8.