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

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.