Exciton correlations in coupled quantum wells and their luminescence blue shift

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 multiparticle correlation so that the system cannot be considered as a gas. At high concentration the dipole-dipole repulsion suppresses the quantum degeneracy. As a result there exists a temperature region where the system behaves a classical liquid; such a region does not exist in case of contact interaction. We calculate the blue shift of the exciton luminescence line which is a sensitive tool to observe the exciton-exciton correlations.