EXCITON SPIN RELAXATION IN THE 2D DENSE EXCITONIC PHASE - THE ROLE OFEXCHANGE INTERACTION

We have investigated the exciton luminescence and polarization dynamic s in GaAs/AlGaAs quantum wells under resonant excitation at low temper ature. Fast initial decays of both the luminescence intensity and the circular luminescence polarization are observed at high photogeneratio n density, typically for exciton areal densities above 2 x 10(10) cm(- 2). These fast decays, which are not observed in a dense excitonic sys tem with well defined angular momentum J(z) = +1 (or J(z) = -1), are s imultaneously initiated by the increase of the ellipticity of the phot ogenerated excitons. We interpret these fast decays in terms of a very efficient angular momentum relaxation mechanism associated with excit on-exciton exchange interaction.