Electronic Excitations in Metals and Semiconductors: Ab Initio Studies of Realistic Many-Particle Systems

Electronic excitations in metals and semiconductors, measured through inelastic x-ray scattering, electronic energy-loss spectroscopy, and angle-resolved photoemission, provide valuable information about the intrinsic many-body interactions between electrons in the crystal environment. These dynamical interactions are usually analyzed with intuitive simple models. With the problems studied, which correspond to linear response and quasi-particle excitation, it will be shown that ab initio approaches not only can provide more realistic understanding of microscopic processes and mechanisms, but also result in new theoretical interpretations that resolve a number of remarkable "anomalies." Specifically, these problems include the dispersion of the plasmon lifetime in potassium, the electron-hole excitations near the d-band threshold of zinc, the occupied bandwidth of sodium, and the band gap of silicon and germanium. These analyses are based on newly developed all-electron, full potential implementations of the time-dependent density functional theory and the conserving finite temperature many-body perturbation theory. These studies also illustrate the limitation of current knowledge of many-body approaches, and demonstrate the importance of the interplay between experiment and theory.