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The possibility of realizing collective phases in an optically excited two-dimensional semiconductor system has attracted much attention over the past few decades. When the average inter-particle distance is larger than the exciton Bohr diameter, excitons form a dilute Bose gas
and are expected to undergo Bose-Einstein condensation (BEC) at standard cryogenic
temperatures. Indirect excitons (IX) in coupled quantum wells (CQW) are particularly appropriate prospects in this regard due to their long lifetime, which enables establishing quasi-equilibrium.
In this project, we study the formation dynamics of a dark IX condensate in a GaAs/Al0.28GaAs CQW system at low temperatures, spreading over hundreds of microns. We find that the condensate density is determined by spin flipping collisions among the condensate excitons and between them and the thermal bath. We construct a rate equations model that accounts for these interactions and quantitatively reproduces the behavior of the system. Next, by spatial imaging of
the PL intensity, the evolution of the condensate is studied as a function of reducing bath temperature. The results imply gradual formation of a smooth and delocalized condensate from a fragmented Bose glass phase. Resonant Rayleigh scattering measurements show that the static disorder in the sample is effectively screened in the presence of the condensate, providing support
for this interpretation.
Finally, in a system where the phase transition occurs abruptly at relatively high excitation powers, we find that the condensate exhibits a set of properties of a high-density liquid. Hence, the
transition can be described as being between a gas and a liquid. We determine its phase diagram in the density-temperature plane. We find that the latent heat of the transition exhibits a linear
dependence on the temperature (T) at T less than 1 K, a behavior expected for a BEC transition. This behavior serves as evidence for a quantum liquid-like behavior of the long-lived IXs in the
system at low temperatures. |