Gap kinematic Frenkel biexciton

Abstract

A general dispersion equation is derived determining the wave-vector dependence of the energy of a bound kinematic Frenkel biexciton, previously discovered by the author, in crystals of arbitrary symmetry. A solution of this equation shows that, in crystals of a certain symmetry, bound gap Frenkel biexcitons can exist, the isolated terms of which lie in the gap between the components of the Davydov multiplet of unbound two-exciton states. In crystals with two monomers in the unit cell, these terms lie in the gap between two low-frequency components of the Davydov multiplet and are not in resonance with the central component, in contrast to what was observed in previously investigated crystals of higher symmetry. Besides, the biexcitons discovered in this work have a finite, rather than zero, dispersion, although their bandwidth is extremely small. The bound biexcitons also have some other specific features followed from the dispersion relation.