Study of Photoinduced Phase Transition in an Organic Radical Crystal with Strong Correlated Electronic System

由于在1，3，5-三硫杂-2，4，6-三氮杂并环戊二烯基（TTTA）晶体中发现了一个大的一级磁相变，并在很宽的温度范围内具有很宽的热滞回线，有机自由基晶体有望成为研究光诱导相变（PIPT）现象的新型强关联材料。本研究首先从实验和理论上研究了TTTA的PIPT现象及其电子和磁性结构，发现TTTA的电荷转移和分子内π-π跃迁带的光激发（即单光子激发）诱导了低温（LT）抗磁到高温（HT）顺磁的相变，并具有明显的阈值激发密度。这种非线性行为表明，光激发创建HT相团簇作为前体，随后，如果团簇的总尺寸超过临界值，宏观HT秩序在LT相晶体中形成。我们还发现双光子激发光诱导了具有较大非线性响应的反磁-顺磁相变，并利用基于局域密度近似的从头算分子动力学方法对晶体结构和电子结构进行了理论研究。我们发现LT相是Peierls绝缘体，而HT相是Mott绝缘体。我们还开发了一种飞秒实时泵浦探测成像光谱，适用于固体中的不可逆光反应，如PIPT，它允许在宽光谱和时间区域内同时实时映射频率和时间分辨的瞬态吸收。

Since a large first order magnetic phase transition with a wide thermal hysteresis loop over wide temperature range was found in 1,3,5-trithia-2,4,6-triazapentalenyl(TTTA) crystal, an organic radical crystal is expected to be a new type of strong correlated materials suitable for studying photoinduced phase transition(PIPT) phenomena. In this research, we first experimentally and theoretically investigated the PIPT phenomena as well as electronic and magnetic structures of TTTA.We found that the photoexcitations of the charge-transfer and intramolecular π-πtransition bands (i.e.one-photon excitations) induce a low-temperature(LT) diamagnetic to high-temperature(HT) paramagnetic phase transition with a clear threshold excitation density. This nonlinear behavior shows that the photoexcitations create the HT phase clusters as precursors, and subsequently, if the total size of the clusters is beyond a critical value, the macroscopic HT order is formed in the LT phase crystal. We also found that two-photon excitation optically induces the diamagnetic to paramagnetic phase transition with large nonlinear response.We investigated theoretically the crystal structures as well as electronic structures by means of the ab-initio molecular dynamics method based on the local density approximation. We found that the LT phase is a Peierls insulator while the HT phase is a Mott insulator.We also developed a femtosecond real-time pump-probe imaging spectroscopy applicable to irreversible photoreactions such as the PIPT in solids, which allows real-time mapping of frequency-and time-resolved transient absorbance in a wide spectral and temporal region at once.