Photoinduced structural phase transitions and their dynamics

A general concept for photoinduced structural phase transitions is developed, in terms of the hidden multi-stability of the ground state and the proliferations of optically excited states. Taking the ionic (I) to neutral (N) phase transition in the organic charge transfer (CT) crystal, tetrathiafuluvalene-p-chloranil, as a typical example for this type of transition, we, at first theoretically show an adiabatic path which starts from CT excitons in the I-phase, but finally reaches to a N-domain with a macroscopic size. In connection with this I-N transition, the concept of the initial condition sensitivity is also developed so as to clarify experimentally observed nonlinear characteristics of this material. Then, using a simplified model for the many-exciton system, we theoretically study the early-time quantum dynamics of the exciton proliferation, which finally results in the formation of a domain with a large number of excitons. For this purpose, we derive a stepwise iterative equation to describe the exciton proliferation, and clarify the origin of the initial condition sensitivity. Possible differences between a photoinduced non-equilibrium phase and an equilibrium phase at high temperatures are also clarified from general and conceptional points of view, in connection with recent experiments on the photoinduced phase transition in an organo-metallic complex crystal. It will be shown that the photoinduced phase can make a new interaction appear as a broken symmetry only in this phase, even when this interaction is almost completely hidden in all the equilibrium phases, such as the ground state and other high-temperature phases.