Photoinduced Phase Transitions and their Dynamics

本文主要研究了三个方面：1)蒙特卡罗模拟研究光诱导跃迁的畴形成：[Fe(2-pic)_3]Cl_3EtOH在光照射到LS相时表现出低自旋（LS）到高自旋（HS）的协同跃迁。利用蒙特卡罗和解析方法对这些现象进行了理论研究。结果表明，由光激发和去激发过程的重复引起的涨落是克服畴形核势垒的驱动力。为了解释宏观相分离现象，引入了包含体积伸缩模耦合的相分离模型。剧烈的阶梯状转换归因于体积收缩的去钉效应。2)磁性半导体：采用蒙特卡罗方法和解析方法研究了光学或化学方法掺杂的低密度载流子所产生的铁磁性。分析了磁极化子效应在磁性和光学过程中的作用。3) Kco[Fe(CN)_6][16]的电子能带结构：为了阐明光致铁磁态的电子结构，基于FLAPW方法进行了LDA能带计算。Co-d γ带的能量降低，位于费米能附近，电子从Fe离子位态转移到激发态的Co位态。

We studied mainly following three subjects :1) Monte-Carlo simulation Study on domain formation in photo-induced transition : [Fe(2-pic)_3]Cl_3EtOH shows cooperative low-spin (LS) to high-spin (HS) states transition when the light is irradiated to the LS phase. We studied theoretically these phenomena by Monte Carlo and the analytic methods. It was shown that the fluctuation due to the repetition of the excitation by light and de-excitation process acts as the driving force to surmount the barrier of domain nucleation. To explain the macroscopic phase separation, a model including the coupling with volume striction mode was introduced. The sharp step-like conversion was ascribed to the de-pinning effect of the volume striction.2) Magnetic semiconductors : The ferromagnetism induced by the low density carrier which is doped by optical or chemical methods was studied by both Monte Carlo and the analytic methods. The role of magnetic polaron effect in the magnetism and the optical process was analyzed.3) Electronic band structure of Kco[Fe(CN)_6][16] : To clarify the electronic structures of the photo-induced ferrimagnetic state, the LDA band calculations on the basis of the FLAPW method was carried out. The energy of Co-d γ band decreases and locates very near the Fermi energy, and thus electron transfers from Fe ion site state to Co site in the excited state.