  Dynamics of ionic-to-neutral and neutral-to-ionic phase transitions induced
by intrachain charge-transfer photoexcitations are studied in a
quasi-one-dimensional extended Hubbard model with alternating potentials and an
electron-lattice coupling for mixed-stack charge-transfer complexes. For
interchain couplings, we use electron-electron interactions previously
estimated for TTF-CA (TTF=tetrathiafulvalene, CA=chloranil). Photoexcitation is
introduced by a pulse of oscillating electric field. The time-dependent
Hartree-Fock approximation is used for the electronic part, and the classical
approximation for the lattice part. In the ionic-to-neutral transition, the
transferred charge density is a strongly nonlinear function of the
photoexcitation density, which is characterized by the presence of a threshold.
With substantial interchain couplings comparable to those in TTF-CA, the
interchain correlation is strong during the transition. Neutral domains in
nearby chains simultaneously grow even if their nucleation is delayed by
reducing the amplitude of the electric field. With weaker interchain couplings,
the growing processes are in phase only when the amplitude of the electric
field is large. Thus, the experimentally observed, coherent motion of a
macroscopic neutral-ionic domain boundary is allowed to emerge by such
substantial interchain couplings. In the neutral-to-ionic transition, by
contrast, the transferred charge density is almost a linear function of the
photoexcitation density. Interchain electron-electron interactions make the
function slightly nonlinear, but the uncooperative situation is almost
unchanged and consistent with the experimental findings.
