Stability of indirect biexcitons in superlattices

The unstability of the ground state of the system of interacting two-dimensional indirect excitons in a slab of superlattice with alternating electron and hole layers is established. The stable system of indirect quasi-two-dimensional biexcitons, consisting from indirect excitons with opposite directed dipole moments is considered. The radius and the binding energy of indirect biexciton are calculated. Collective spectrum of low-density system of two-dimensional indirect biexcitons, interacting as electrical quadrupoles, is derived. The density of superfluid component ns(T) and the temperature of the Kosterlitz-Thouless phase transition to the superfluid state in this system are obtained. We assume there are no external fields applied to the slab of superlattice under consideration.