DENSITY-MATRIX METHODS FOR SEMICONDUCTOR COULOMB DYNAMICS

Current experiments on semiconductor devices using femtosecond lasers provide new theoretical challenges for the description of charge carri er dynamics. Among the new features of such experiments are states dri ven very far from equilibrium and probes on time scales short compared to scattering and other characteristic material relaxation times. Sta ndard many-body methods must be modified and extended to accommodate t hese features. We propose that the quantum hierarchy for reduced densi ty operators is an ideal formulation of such initial value problems an d describe how the dominant effects of exchange and charge correlation s can be accounted for in a simple and physically transparent closure of the hierarchy of equations. The transformations, approximations, an d interpretation can be accomplished independent of any particular mat rix representation. Decomposition into kinetic equations for band occu pation densities and polarization densities follows in a straightforwa rd way after the many-body problem has been brought under control. (C) 1997 John Wiley & Sons, Inc.