Stimulated scattering of indirect excitons in coupled quantum wells: Signature of a degenerate Bose-gas of excitons

We observe and analyze strongly nonlinear photoluminescence kinetics of ind irect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperature s, greater than or equal to 50 mK. The long recombination lifetime of indir ect excitons promotes accumulation of these Bose particles in the lowest en ergy states and allows the photoexcited excitons to cool down to temperatur es when the: dilute 2D gas of indirect excitons becomes statistically degen erate. Our main result-a strong enhancement of tile exciton scattering rate to the low-energy states with increasing concentration of the indirect exc itons-reveals bosonic stimulation of exciton scattering, which is a signatu re of a degenerate Bose-gas of excitons.