Exciton Spin Relaxation in Semiconductor Quantum Wells: The Role of Disorder

Ultrafast pulse measurements on high-quality $(\mathrm{Zn},\mathrm{Cd})\mathrm{Se}/\mathrm{ZnSe}$ quantum wells yield the very surprising result that the decay of the exciton spin transients after resonant excitation with circularly polarized light is even faster than the exciton dephasing time. We demonstrate that this fact is a direct consequence of (alloy) disorder which gives rise to a kind of inhomogeneous broadening and associated interference effects for spin dynamics across the band of localized exciton states.