Explorations of new optical switching phenomena in strongly correlated electron systems

在本研究项目中，我们的目的是探索新型的光致相变（光开关现象），其中传输，光学和磁性在很大程度上被光改变，并阐明其物理机制。主要成就概述如下。我们在一种有机电荷转移化合物（M_2 P-F_4 TCNQ）的二维Mott绝缘体中发现了超快的光诱导绝缘体-金属转变。从瞬态反射率谱及其时间依赖性的详细分析，我们发现，光致金属态的衰减时间是非常快的，为0.4 ps，和金属态是二维的性质。这种快速衰减被认为是二维相关电子系统的特征。我们在一种有机电荷转移化合物（BEDTTTF-F_2 TCNQ）的一维Mott绝缘体中发现了超快的光致绝缘体-金属转变。该跃迁的重要特征是即使在非常弱的激发光子密度（<0.003光子/位点）下也观察到金属行为。我们的结论是，这种由弱激发形成金属态是由于一维关联电子系统的自旋-电荷分离特性。我们在一个典型的一维关联电子过渡金属化合物碘桥铂化合物中发现了超快光致电荷密度波（CDW）-Mott Hubbard跃迁。我们发现，过渡的效率是非常高的（约。70个Pt位点/光子），并且通过强激发发生CDW到金属的跃迁。也就是说，在该化合物中，三个相（CDW，Mott-Hubbard，金属）之间的光致相变是可能的，因此，在本项目中，我们成功地探索了新的光致相变，并实现了其物理机制的澄清。

In this research project, we aimed to explore new kinds of photoinduced phase transitions (optical switching phenomena) in which transport, optical and magnetic properties are largely changed by lights and to clarify their physical mechanisms. The main achievements are summarized below.?Z We found an ultrafast photoinduced insulator-metal transition in a two-dimensional Mott insulator of an organic charge-transfer compound,(M_2P-F_4TCNQ). From the detailed analyses of the transient reflectivity spectra and their time dependences, we revealed that the decay time of the photoinduced metallic state is very fast, being 0.4 ps, and the metallic state is two-dimensional in nature. Such a fast decay is considered characteristic of two-dimensional correlated electron systems.?Z We found an ultrafast photoinduced insulator-metal transition in a one-dimensional Mott insulator of an organic charge-transfer compound,(BEDTTTF-F_2TCNQ). The important feature of this transition is that a metallic behavior was observed even for a very weak excitation photon density (<0.003 photon/site). We concluded that such a formation of a metallic state by a weak excitation is attributable to the spin-charge separation characteristic of 1D correlated electron systems.?Z We found an ultrafast photoinduced charge density wave (CDW)-Mott Hubbard transition in a typical transition metal compound with a one-dimensional correlated electron system, an iodine-bridged platinum compound. Werevealed that the efficiency of the transition is very high (ca. 70 Pt sites per photon) and that a CDW to metal transition occurs by a strong excitation. Namely, photoinduced phase transitions among three phases (CDW, Mott-Hubbard, metal) are possible in this compound.Thus, in this project, we have succeeded in exploring new photoinduced phase transitions and achieved the clarifications of their physical mechanisms.