Coherent pairing of excitons in insulators and semiconductors

Abstract

A microscopic theory is developed concerning the coherent pairing of Wannier-Mott type excitons in insulators and semiconductors. The coherent pairing of excitons occurs in the presence of a resonant electromagnetic field and the resulting energy gap is in competition with that induced by the electromagnetic field. Due to the electrical neutrality of the electron-hole pairs, the excitation spectrum is of the excitonic insulator type and the presence of the electromagnetic field results in the splitting of the energies of excitation. The expression for the coherent gap function depends on the exciton density, the effective exciton mass, the exciton-exciton interaction, and the effective energy band gap, which is equal to the difference between the transition frequency and the frequency of the field and the binding energy of the exciton. Under resonance conditions and at temperatures below some valueT c , the biexciton state can exist provided that the dielectric gap is less than that for the biexciton state. The ground-state energy resulting from the coherent pairing of excitons is calculated and is found to be lower in energy than the normal exciton state. This indicates the existence of a bound biexciton state as long as there is a net attractive exciton-exciton interaction.