Investigations of the Dynamical Response in Solids by Time-Dependent Density-Functional Theory

The present dissertation studies a joint theoretical-experimental investigation of the dynamical structure factor of wide-gap insulators, using lithium-fluoride as a prototype. Ground-state (energy bands and electron densities) was calculated using Linear Augmented Plane Wave (LAPW) method and local density approximation (LDA) of density functional theory (DFT). Ab-initio principal is applied to obtain a realistic description of the band structure, which is central to the current research in the condensed matter physics. Dynamical response function has been evaluated within time-dependent density functional theory (DFT) with an adiabatic approximation (TDLDA), for the exchange-correlation kernel. Our TDLDA spectra contain one adjustable parameter: a “scissors-operator” shift of conduction bands of LDA electronic structure. This parameter is determined in view of the line-shape of the Non-resonant Inelastic X-Ray Scattering (NIXS) cross section for q= 6Å-1 along (111) direction. All other spectra are calculated “ab-initio.” The important interplay between band structure and electron dynamics is emphasized within our results. The picture of excitations offers an alternative view to previous investigations involving an approximate solution of the Bethe-Salpeter equation coupled with a more limited range of NIXS data.