Theory for relaxations of optical excited states and photoinduced phase transitions

从基态的多重稳定性和光激发态的扩散性出发，提出了光致结构相变的一般概念。以有机电荷转移（CT）晶体TTF-CA中的离子（I）→中性（N）相变为例，首先从理论上证明了从I相中的CT激子开始，最终到达具有宏观尺寸的N畴的绝热路径。针对这种I-N跃迁，我们提出了初始条件敏感性的概念，从而阐明了实验上观察到的这种材料的非线性特性.其次，我们用一个更简化的多激子系统模型，从理论上研究了激子增殖的早期量子动力学，并最终导致了大量激子畴的形成.为此，我们导出了描述激子增殖的逐步迭代方程，阐明了初始条件敏感性的来源，并结合最近的有机金属配合物晶体光致相变实验，从一般和概念的角度阐明了高温下光致非平衡相和平衡相之间可能存在的区别。理论上已经证明，光致相可以使一个新的相互作用以对称性破缺的形式出现，即使这个相互作用几乎完全隐藏在所有的平衡相中，如基态和其他高温相。

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.