While we understand well how two classical dipoles interact with each other, the problem becomes much more complex and interesting when we put many dipoles together and form a dipolar fluid, especially when collective quantum effects become important. The possible phases of two-dimensional (2D) interacting dipolar particles is a long-sought problem in many-body physics.
In parallel to real cold dipolar atoms and molecules, indirect excitons in GaAs semiconductor bilayers also form a system of 2D atomic-like dipolar quasiparticles with four internal spin degrees of freedom, two of which are dark. Not only such dipolar excitons exhibit strong manybody interactions and correlations, but also they are a unique dissipative system where the thermodynamics of their center-of-mass degrees of freedom could be intimately related to their internal spin degrees of freedom. This link can lead to new and interesting effects.
In this talk I will review some of our most recent experimental results on dipolar exciton fluids in GaAs bilayer systems. In particular, I will present our recent observation that as a fluid of dipolar excitons is cooled down, it exhibits strong particle correlations not seen in weakly interacting gases, followed by a spontaneous condensation into a high density dipolar liquid state that strikingly seems to be made of mostly dark dipoles. These results will be discussed in context of a dynamical quantum condensation into the dark spin states of the excitons. I will also present the concept of vertical coupling of dipolar exciton fluids and the expected bound complexes and dipolar drag in such vertically coupled systems.