In this paper we present a study of an exciton system where electrons and holes are confined in double quantum well structures. The dominating interaction between excitons in such systems is a dipole -dipole repulsion. We show that the tail of this interaction leads to a strong correlation between excitons and substantially affects the behavior of the system. Making use of qualitative arguments and estimates we develop a picture of the exciton -exciton correlations in the whole region of temperature and concentration where excitons exist. It appears that at low concentration degeneracy of the excitons is accompanied with strong multi-particle correlation so that the system cannot be considered as a gas. At high concentration the repulsion suppresses the quantum degeneracy down to temperatures that could be much lower than in a Bose gas with contact interaction.We calculate the blue shift of the exciton luminescence line which is a sensitive tool to observe the exciton -exciton correlations. d and the average separation between excitons are assumed to be larger than the exciton radius a X .

Dipolar excitons are long-lived quasi-particle excitations in semiconductor heterostructure that carry an electric dipole. Cold dipolar excitons are expected to have new quantum and classical multi-particle correlation regimes, as well as several collective phases, resulting from the intricate interplay between the many-body interactions and their quantum nature. Here we show experimental evidence of a few correlation regimes of a cold dipolar exciton fluid, created optically in a semiconductor bilayer heterostructure. In the higher temperature regime, the average interaction energy between the particles shows a surprising temperature dependence, which is evidence for correlations beyond the mean field model. At a lower temperature, there is a sharp increase in the interaction energy of optically active excitons, accompanied by a strong reduction in their apparent population. This is evidence for a sharp macroscopic transition to a dark state, as has been suggested theoretically.

The gas of interacting excitons in quantum wells is studied. We obtain the Hamiltonian of this gas by the projection of the electron-hole plasma Hamiltonian to exciton states and an expansion in a small density. Matrix elements of the exciton Hamiltonian are rather sensitive to the geometry of the heterostructure. The mean field approximation of the exciton Hamiltonian gives the blue shift and spin splitting of the exciton luminescence lines. We also write down the Boltzmann equation for excitons and estimate the energy and spin relaxation time resulting from the exciton-exciton scattering. Making use of these calculations we succeeded to explain some recent experimental results which have not been explained so far.

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A novel semiconductor switching device is proposed. It is based on unique control over the two-dimensional band structure of an AlSb-GaSb-InAs-AlSb heterostructure. By applying small electric fields, virtually any value can be achieved for such parameters as the energy gaps, effective masses, and carrier types and densities in the material. The proposed heterostructure can be readily fabricated with existing epitaxial techniques.

We present an InAs-GaSb-based system in which the electric-field tunability of its 2D energy gap implies a transition towards a thermodynamically stable excitonic condensed phase. Detailed calculations show a 3 meV BCSlike gap appearing in a second-order phase transition with electric field. We find this transition to be very sharp, solely due to exchange interaction, and so, the exciton binding energy is greatly renormalized even at small condensate densities. This density gradually increases with external field, thus enabling the direct probe of the Bose-Einstein to BCS crossover.The long search for a condensed phase of excitons has greatly expanded in recent years Two major obstacles interfere in forming an exciton condensate. One is the short recombination lifetime (∼ 1 ns) of photo-excited direct excitons. This time can be shorter than the time free excitons need to thermalize and condense. The recombination rate also produces enough heat to possibly destroy the condensate. In Cu 2 O, its unique crystal structure results in a dipole-forbidden recombination time of ∼ 10 µs. A usual way to increase the 1

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