| Research Abstract |
A general concept for photoinduced structural phase transitions is theoretically developed, in terms of the hidden multi-stability of the ground state and the proliferations of optically excited states. Taking the ionic(I)→neutral(N) phase transition in the organic charge transfer(CT) crystal, TTF-CA, as a typical example for this type transition, we have at first, theoretically shown 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.
In the next, using a more simplified model for the many-exciton system, we have theoretically studied 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 have derived a stepwise iterative equation to describe the excition proliferation, and clarified the origin of the initial condition sensitivity.
Possible differences between a photoinduced nonequilibrium 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 has theoretically shown that the photoinduce 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.
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