Superfluidity of indirect biexcitons in superlattices

Instability in a system of interacting quasi-two-dimensional excitons in a type II superlattice of a finite thickness due to attraction between oppositely-directed excitonic dipoles in neighboring layers has been discovered. A stable system is that of indirect quasi-two-dimensional biexcitons formed by indirect excitons with dipole moments oriented in opposite directions. The radius and binding energy of indirect biexcitons has been calculated. A collective spectrum of a system of such biexcitons with a weak quadrupole interaction between them has been studied. Feasibility of Bose condensation, the density n _s(T) of the superfluid component, and a phase transition to the superfliud state in a low-density system of indirect biexcitons have been analyzed. Zh. éksp. Teor. Fiz. 115, 1786–1798 (May 1999)