Photoinduced structural phase transitions and theirdynamics

Abstract

A general concept for photoinduced structural phase transitions is developed,in terms of the hidden multi-stability of the ground state and theproliferations of optically excited states. Taking the ionic (I) to neutral (N)phase transition in the organic charge transfer (CT) crystal, tetrathiafuluvalene-p-chloranil, as atypical example for this type of transition, we, at first theoretically show anadiabatic path which starts from CT excitons in the I-phase, but finallyreaches to a N-domain with a macroscopic size. In connection with this I-Ntransition, the concept of the initial condition sensitivity is also developedso as to clarify experimentally observed nonlinear characteristics of thismaterial.Then, using a simplified model for the many-exciton system, wetheoretically study the early-time quantum dynamics of the excitonproliferation, which finally results in the formation of a domain with a largenumber of excitons. For this purpose, we derive a stepwise iterative equationto describe the exciton proliferation, and clarify the origin of the initialcondition sensitivity.Possible differences between a photoinduced non-equilibrium phase and anequilibrium phase at high temperatures are also clarified from general andconceptional points of view, in connection with recent experiments on thephotoinduced phase transition in an organo-metallic complex crystal. It willbe shown that the photoinduced phase can make a new interaction appear as abroken symmetry only in this phase, even when this interaction is almostcompletely hidden in all the equilibrium phases, such as the ground state andother high-temperature phases.