In this paper we study the phonon-induced exciton-exciton interaction. It is found that the interaction can be attractive under certain conditions. Taking into account this attractive interaction, the pairing of excitons with opposite momenta is studied and the excitation spectrum determined. The results are similar to a system of bosons. There appears to be some possibility of superfluid behaviour.
Dr. V . M. Nandakumaran and Sinha, K. P., “Possibility of a condensed phase of excitons because of phonon induced exciton-exciton pairing”, Zeitschrift für Physik B Condensed Matter, vol. 22, pp. 173–176, 1975.