Spin relaxation time of CdZnSe/ZnSe self-assembled quantum dots in a magnetic field

We have performed polarization-resolved magneto-photoluminescence (PL) experiments on a self-assembled CdZnSe quantum dot (QD) system to investigate the influence of an external magnetic field on the spin relaxation time of excitons in QDs. When the QDs are excited with circularly polarized light, the PL emissions at phonon resonance energies exhibit a noticeable degree of circular polarization (33% and 6% for 1-LO and 2-LO resonances, respectively) even at zero magnetic field while the PL at non-resonant positions shows zero circular polarization. The degree of polarization increases significantly when an external magnetic field is applied. Based on a simple two-level rate equation model, including a spin-flip time τ _s and an exciton recombination time τ _r, we are able to extract the magnetic field dependence of the ratio τ _s/τ _r. This ratio increases monotonically with the field and eventually becomes larger than 1, indicating that at high magnetic fields, spin relaxation times can exceed exciton recombination times.