Superfluidity of indirect excitons and biexcitons in coupled quantum wells andsuperlattices

Abstract

The collective properties of indirect excitons in coupled quantum wells (CQWs)are considered. The energy of the ground state of the exciton liquid as afunction of the density of electrons e and holes h at different separationsDbetween e and h layers is analysed. The quantum gas–liquid transition asDdecreases is studied. The superfluidity appearance temperatures in the system(Kosterlitz–Thouless transition temperatures) have been estimated at differentseparations Dbetween e and h layers. For the anisotropic two-dimensional e–h system in CQWsthe Mott metal–insulator quantum transition is considered. The instability of theground state of the system of interacting two-dimensional indirect excitons in aslab of superlattice with alternating e and h layers is established. The stablesystem of indirect quasi-two-dimensional biexcitons, consisting of indirect excitonswith opposite directed dipole moments, is considered. The radius and the bindingenergy of the indirect biexciton are calculated. The collective spectrumof a rare system of two-dimensional indirect biexcitons, interacting aselectrical quadrupoles, is studied. The density of the superfluid componentns (T)and the Kosterlitz–Thouless phase transition to the superfluid state in this systemare analysed.