Superfluidity of indirect excitons and biexcitons in coupled quantum wells and superlattices

The collective properties of indirect excitons in coupled quantum wells (CQWs) are considered. The energy of the ground state of the exciton liquid as a function of the density of electrons a and holes h at different separations D between a and h layers is analysed. The quantum gas-liquid transition as D decreases is studied. The superfluidity appearance temperatures in the system (Kosterlitz-Thouless transition temperatures) have been estimated at different separations D between a and h layers. For the anisotropic two-dimensional e-h system in CQWs the Mott metal-insulator quantum transition is considered. The instability of the ground state of the system of interacting two-dimensional indirect excitons in a slab of superlattice with alternating a and h layers is established. The stable system of indirect quasi-two-dimensional biexcitons, consisting of indirect excitons with opposite directed dipole moments, is considered. The radius and the binding energy of the indirect biexciton are calculated. The collective spectrum of a rare system of two-dimensional indirect biexcitons, interacting as electrical quadrupoles, is studied. The density of the superfluid component n(s) (T) and the Kosterlitz-Thouless phase transition to the superfluid state in this system are analysed.