Microscopic theory of coherent carrier dynamics and phase breaking in semiconductors

Markovian Boitzmann-Bloch (BB) equations are derived within the Keldysh approach using a gradient expansion in time, a generalized Kadanoff-Baym ansatz, the screened Hartree-Fock approximation to self-energies for particle-particle interactions, as well as a careful account of fast frequency components from the free-particle Hamilton. The BE equations, which account for both coherence and phase breaking in the system, are applied to the situation of photoexcited carriers in semiconductor structures where the electron-electron and electron-phonon interaction provide phase breaking mechanisms. Free-carrier screening within the random-phase approximation leads to a hybridization of these two interactions. It is shown that, in nonequilibrium situations, a consistent account of screening goes beyond a simple replacement of matrix elements of the unscreend interaction by those of the screened interaction. The application of this approach to the single-band case with and without spin polarization and homogeneous bulk semiconductors within an N-band model is discussed. A numerical study of the subpicosecond dynamics of photogenerated electron-hole pairs in asymmetric GaAs-AlxGa1-xAs double wells based on this approach provides a nonphenomenological microscopic picture of the role played by the carrier-carrier Coulomb interaction in the destruction of phase coherence in mesoscopic semiconductor systems. The carrier-carrier Coulomb interaction is shown to provide time-dependent detuning of electron subbands and renormalization of the interwell tunneling frequency, the formation of excitons, as well as damping of interwell charge oscillations.