Theory for photoinduced structural phase transitions

A general concept for photoinduced structural phase transitions is develope d in terms of the hidden multistability of the ground state and the prolife rations of optically excited states. Taking the ionic-neutral (I-N) phase t ransition in the organic charge transfer crystal, TTF-CA, as a typical exam ple for this type of transition, we, at first, theoretically show an adiaba tic path of this transition, which starts from a single charge transfer exc iton in the ionic phase, but finally reaches a neutral domain with a macros copic size. In connection with this I-N transition, the concept of the init ial condition sensitivity is also developed so as to clarify experimentally observed nonlinear characteristics of this material. In the next, using a more simplified model for the many-exciton system, we theoretically study t he early time quantum dynamics of the exciton proliferation, which finally results in the domain formation of a large number of excitons. For this pur pose, we derive a stepwise iterative equation to describe the exciton proli feration, and clarify the origin of the initial condition sensitivity.