The Instabilities of Excitons in Deformable Lattice and the Photo-Induced Phase Transition

Abstract

Possible instabilities of excitons in deformable lattice are explored in the increasing order of lattice relaxation. Particular attention is paid to the effect of dimensionality on self-trapping, the modes of symmetry breaking and the radiative versus nonradiative decay channels of an exciton. As the ultimate instability, we consider a catastrophic situation in which the self-trapped excitons are destabilized against spontaneous generation and condensation through the attractive interactions, resulting in the electronic-structural phase transition of the entire lattice. Such a view-point is shown to be useful for some quasi-one dimensional materials which are subject to instability against charge density wave (Wolffram’s red) or to the neutral-ionic phase transition (organic charge transfer compounds). We apply this view-point to the latter system to interrelate the photo-induced phase transition and the phase transition under thermal equilibrium, with the use of a microscopic model with dipole-dipole interaction between the parity-broken self-trapped excitons. It is emphasized that the study of photo-induced phase transition will reveal the dynamical aspect of phase transition, thus shedding a new light on it.