DIELECTRIC-MATRIX CALCULATION OF THE VOLUME-PLASMON DISPERSION-RELATION FOR SILICON

Plasmon-band splitting in silicon has been the subject of theoretical interest recently, since reports of its experimental observation using coherent inelastic x-ray scattering. The theoretical plasmon-dispersi on relation can be obtained from dielectric-response theory and knowle dge of the electronic structure of the solid. In this work the frequen cy- and wave-vector-dependent dielectric matrix (DM) has been calculat ed in the random-phase approximation, with a nonlocal empirical pseudo potential (EPM) electronic band structure. We present results for the volume-plasmon dispersion relation along high-symmetry directions in t he first Brillouin zone. Contrary to previous local EPM-based DM calcu lations, this nonlocal EPM band structure results in excellent agreeme nt with the experimental plasmon-dispersion anisotropy. In addition, t hese dispersion results are in better agreement with experiment than p revious local-density-approximation calculations. Evidence for a weak approximate to 2-eV band gap is found at the L point. Unlike previous calculations, the presence of two plasmon resonances is directly obser vable from the calculated loss function in the Lambda direction.