Resonant coherent pairing of excitons in molecular crystals

Abstract

A self-consistent approach is used to study the coherent pairing of Frenkel excitons produced by a resonant electromagnetic field in molecular crystals. In the presence of the resonant electromagnetic field the effective energy gap is equal to the difference between the transition frequency and the frequency of the field and vanishes at resonance. A dielectric energy gap is induced by the electromagnetic field that depends on the oscillator strength of the electronic transition in question. It is shown that under resonance or near resonance conditions and at temperatures below some value ${T}_{c}$, the existence of a bound biexciton state with zero total wave vector is possible, provided that the dielectric gap is less than that for the biexciton state. The excitation spectrum is formally analogous to that of a superconductor but the new state is not expected to carry current because of the neutrality of the electron-hole pairs. A splitting of the excitation spectrum occurred arising from the existence of the dielectric gap. The contribution to the ground-state energy of the crystal arising from the coherent pairing of excitons is calculated and the possibility of observing the biexciton spectrum is discussed.