Phonon-induced exciton spin relaxation in semimagnetic quantum wells

Theoretical results are given for spin relaxation in semimagnetic semiconductor quantum wells due to longitudinal optical (LO) phonon-induced flips of exciton spins at zero temperature and modest magnetic fields. Relaxation in this scenario is due to spin-flip transitions within the heavy-hole exciton subbands which are mediated by the coupling of excitonic spin states via the electron-hole exchange interaction. Relaxation rates are found to depend strongly on a magnetic field, exciton momentum, and size of the quantum well. Results are illustrated by evaluations for the ZnSe-based semimagnetic quantum wells. In longitudinal magnetic fields (Faraday geometry) a maximum in the relaxation rate is found for zero-momentum excitons at a Zeeman splitting of similar to 60 meV. In transverse magnetic fields (Voigt geometry) the LO-induced spin relaxation is strongly suppressed.