Excitonic anomalous currents in semiconductor quantum wells

Anomalous photocurrents following optical interband excitation and the interaction with a Terahertz field are investigated in GaAs quantum wells. Our approach is based on the multiband semiconductor Bloch equations in length gauge, which are formulated in the basis of eigenfunctions of a 14-band k center dot p model. Excitonic effects are included by treating the Coulomb interaction on the level of the time-dependent Hartree-Fock approximation and, furthermore, carrier longitudinal-optical and carrier longitudinal-acoustic phonon-scattering processes are included on the second-order Born-Markov level. The random k-dependent phase factors that originate from the numerical diagonalization of the eigenstates are treated by implementing a smooth gauge transformation. The flexible numerically implemented gauging procedure allows us to include many-body effects when integrating the semiconductor Bloch equations. Our results demonstrate that anomalous currents appear after resonant optical excitation of excitonic resonances. These currents are even stronger than those obtained when neglecting excitonic effects and exciting above the band gap. In the excitonic anomalous current transients, we identify temporal oscillations which can be attributed to the dynamics of excitonic wave packets since their frequency corresponds to the separation of the 1s and 2s exciton resonances. The numerical results are in very good agreement with experiments.