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 indirect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperatures, greater than or equal to 50 mK. The long recombination lifetime of indirect excitons promotes accumulation of these Bose particles in the lowest energy states and allows the photoexcited excitons to cool down to temperatures when the: dilute 2D gas of indirect excitons becomes statistically degenerate. Our main result-a strong enhancement of the exciton scattering rate to the low-energy states with increasing concentration of the indirect excitons-reveals bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons.