Phase diagram of the Bose condensation of interwell excitons in GaAs/AlGaAs double quantum wells

The luminescence of interwell excitons in GaAs/AlGaAs double quantum wells ($n{-}i{-}n$ heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied at low temperatures down to 0.5 K. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells by a tunneling barrier were studied as functions of density and temperature. The studies were performed within domains about one micron in size, which played the role of macroscopic traps for interwell excitons. For this purpose, the sample surface was coated with a metal mask containing special openings (windows) of a micron size or smaller. Both photoexcitation and observation of luminescence were performed through these windows by the fiber optic technique. At low pumping powers, the interwell excitons were strongly localized because of the residual charged impurities, and the corresponding photoluminescence line was nonuniformly broadened. As the laser excitation power increased, a narrow line due to delocalized excitons arose in a threshold-like manner, after which its intensity rapidly increased with growing pumping and the line itself narrowed (to a linewidth less than 1 meV) and shifted toward lower energies (by about 0.5 meV) in accordance with the filling of the lowest exciton state in the domain. An increase in temperature was accompanied by the disappearance of the line from the spectrum in a nonactivation manner. The phenomenon observed in the experiment was attributed to Bose-Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature interval studied (0.5-3.6) K, the critical exciton density and temperature were determined and a phase diagram outlining the exciton condensate region was constructed.