Abstract

We review the theory of coherent pairing of excitons and biexcitons in two-dimensional and three-dimensional semiconductor structures.

Introduction

The coherent pairing of bosons, formally analogous to Cooper pairing of electrons in superconductors, and the coexistence of particle and pair Bose–Einstein condensation (BEC) was first studied in Refs. [1–4]. BEC of excitons in semiconductors was investigated using the approximation of coherent pairing of the electrons and holes [5]. Later on, the idea of coherent pairing of bosons was applied to excitons [6–8]. In Ref. [6], it was assumed that the boson pairs consist of two electron–hole pairs instead of two excitons, and it was suggested that the new collective state is a BEC even though the two-pair bound state does not exist. It was proved independently [7, 8] that in a system of bosons and excitons with an attractive pair interaction sufficient for biexciton formation, the coherent pairing of excitons with momenta k and −k coincides with the BEC of biexcitons with zero translational momentum.

The possibility of particle and pair BEC in a system of four species of excitons with different values of spin projections of the electron and hole was studied in Ref. [9]. It is known [10, 11] that the interaction of an exciton of either type with other exciton species is repulsive on the average, but that pairs of excitons with antiparallel spins of electrons and holes interact attractively, and biexcitons can be formed.