Spin relaxation and spin-dependent exciton interactions in ZnSe quantum wells

Abstract

We demonstrate the mutual influence of the dynamics in the real-space and the spin parts of the exciton wavefunction evoked by the exchange interaction. In the regime of high exciton density, the exciton–exciton interaction in ZnSe/ZnMgSSe quantum wells is found to be strongly affected by the exciton spin orientation and the longitudinal spin relaxation via single-fermion spin flips. The exciton's homogeneous linewidth as deduced from degenerate four-wave mixing with prepulse is significantly enhanced in spin-pure exciton populations by scattering including fermionic exchange. The transverse spin relaxation in the low-density limit is deduced from the decay of spin quantum-beats in photoluminescence experiments. We find an anti-proportionality between the spin relaxation and the optical dephasing times. The temperature dependence of both relaxation times is determined by the exciton–phonon interaction. The latter gradually reduces the electron–hole exchange coupling which is the dominating term in the exciton's transverse spin relaxation.