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

由于在1,3,5-三硫-2,4,6-三氮杂二烯基（TTTA）晶体中发现了在宽温度范围内具有宽热滞回线的大一阶磁相变，因此有机自由基晶体有望成为一种适合于光致相变（PIPT）现象研究的新型强相关材料。在本研究中，我们首先从实验和理论上研究了TTTA的PIPT现象以及电子和磁性结构。我们发现电荷转移和分子内π-π跃迁带的光激发（即单光子激发）诱导低温（LT）反磁向高温（HT）顺磁相变，且激发密度阈值明确。这种非线性行为表明，光激发产生了HT相团簇作为前驱体，随后，如果团簇的总尺寸超过临界值，则在LT相晶体中形成宏观HT秩序。我们还发现双光子激发在光学上诱导了具有较大非线性响应的抗磁向顺磁相变。我们采用基于局部密度近似的从头算分子动力学方法，从理论上研究了晶体结构和电子结构。我们发现LT相是一个佩尔斯绝缘子，而HT相是一个莫特绝缘子。我们还开发了一种飞秒实时泵浦探针成像光谱，适用于不可逆光反应，如固体中的PIPT，它允许在宽光谱和时间区域内实时绘制频率和时间分辨的瞬态吸光度。

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.