Spin splitting of excitons in GaAs quantum wells at zero magnetic field

We have observed an energy splitting between +1 and -1 excitons in GaAs/AlAs quantum wells at zero magnetic field. The studies, which have been done by picosecond time-resolved photoluminescence spectroscopy, show that exciton spin splitting depends strongly on excitonic density and on the dynamics of spin relaxation: the spin splitting increases, raising the initial carrier density, and the time evolution of the spin splitting follows the exponential decay of the degree of polarization of the photoluminescence. For large carrier densities and short time delays the observed exciton spin splitting is as large as 4 meV. We present theoretical results which support the interpretation of experimental spin splittings in terms of inter-excitonic exchange interactions.