Magnetic-field dependence of exciton spin relaxation in GaAs/AlxGa1-xAs quantum wells

The magnetic-field dependence of spin relaxation of heavy-hole excitons in GaAs/AlxGa1-xAs quantum wells is investigated at low temperatures. The variation of circular polarization of cw photoluminescence is modeled using steady-state solutions of rate equations describing the population dynamics of the exciton spin levels. Transitions between the levels are assumed to proceed via one-phonon ''direct'' processes, as well as zero-phonon processes when the levels are degenerate. Estimates of the exciton, hole, and electron spin-relaxation rates in units of the exciton population decay rate are obtained from least-squares fitting. We conclude that the exchange-driven exciton spin relaxation, involving simultaneous electron and hole spin flip, which dominates over hole or electron spin flips in zero field, is greatly reduced by a small applied field, which causes a Zeeman splitting and so suppresses the zero-phonon transitions. Level crossing signals in the data also show an enhancement of relaxation via zero-phonon hole, spin flip when the Zeeman energy cancels the exchange energy.