Ab initio calculation of optical absorption in semiconductors: A density-matrix description

Coulomb renormalization effects and dielectric screening in optically excited semiconductors and semiconductor nanostructures are described within a first-principles density-matrix description. Those dynamic variables and approximation schemes that are required for a proper description of dielectric screening are identified, It is shown that within the random-phase approximation the direct Coulomb interactions become screened, with static screening being a good approximation, whereas the electron-hole exchange interactions remain unscreened. Differences and similarities between results obtained in the present study and those obtained from a corresponding GW approximation and Bethe-Salpeter equation Green's function analysis are discussed.