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We study the photoluminescence (PL) of a two-dimensional liquid of oriented dipolar excitons in InxGa1−xAs coupled double quantum wells confined to a microtrap. Generating excitons outside the trap and transferring them at lattice temperatures down to T = 240 mK into the trap we create cold quasi-equilibrium bosonic ensembles of some 1000 excitons with thermal de Broglie wavelengths exceeding the excitonic separation. With decreasing temperature and increasing density n < ∼ 5 × 10 10 1 cm 2 we find an increasingly asymmetric PL lineshape with a sharpening blue edge and a broad red tail which we interpret to reflect correlated behavior mediated by dipolar interactions. From the PL intensity I(E) below the PL maximum at E0 we extract at T < 5 K a distinct power law I(E) ∼ (E0 − E)−|α| with -|α| ≈ -0.8 in the range E0 − E of 1.5-4 meV, comparable to the dipolar interaction energy.Weakly interacting bosons confined in an external potential and cooled to very low temperatures such that the thermal de Broglie wavelength becomes comparable to the inter-particle distance can form a Bose-Einstein condensate (BEC). In this new state of matter, a large fraction of the bosons condense into the energetically lowest quantum state of the external potential, and form a correlated state with a macroscopic wavefunction. BEC has been observed in different systems such as ultra cold diluted atomic gases [1, 2] or superfluids [3]. More recently, condensation of non-equilibrium quasi-particles in solids, namely cavity exciton polaritons [4][5][6] and magnons [7], have been reported as other examples of BEC. In an effort to realize BEC of excitons in quasi-equilibrium, predicted already in the 1960s [8, 9], coupled double quantum wells containing two-dimensional systems of long-living and spatially indirect excitons (IX) have been intensively explored for more than a decade [see, e. g. [2, 8,[10][11][12][13][14][15][16][18][19][20]], but fully convincing signatures of a BEC ground state are still missing. Such IX consist of an electron and a hole, spatially separated in the adjacent wells of coupled double quantum well (CDQW) heterostructures and bound by their Coulomb attraction. With their oriented dipolar nature they form a complex and rather strongly interacting bosonic systems with internal structure and not yet fully understood ground state properties. Only recently the influence of dipolar interactions on the ground state properties of such excitonic ensembles has been more intensely theoretically investigated [5].To achieve a quantum phase transition, an efficient trapping of suitable IX densities thermalized to cryogenic temperatures is needed [15,23]. Going beyond previous experiments, we made special efforts to avoid a thermal imbalance between the temperature of the trapped IX and the lattice. In choosing the InGaAs material systems and resonantly exciting direct excitons in the CDQW at an energy below the band gap of the GaAs substrate we assure that most absorption of laser light occurs only in CDQW under the tight focu...

Voltage-tunable quantum traps confining individual spatially indirect and long-living excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wigner-like molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization.

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