THEORY OF SPIN BEATINGS IN THE FARADAY-ROTATION OF SEMICONDUCTORS

We show from first principles that the coherent excitation of the Zeeman-split exciton spin states, leading to quantum beatings, is entirely due to correlation between two excitons of opposite spin, and is, thus, beyond the phase-space filling and the mean-field effects between excitons. This explains the ultrafast terahertz oscillations in the delay time between the pump and probe pulses in the presence of a magnetic field observed by the recent time-integrated measurements of the optically induced Faraday rotation in diluted magnetic semiconductor quantum wells.