Explorations of new optical switching phenomena in strongly correlated electron systems

在这个研究项目中，我们的目的是探索新型光诱导相变（光开关现象），其中传输、光学和磁特性被光极大地改变，并阐明其物理机制。主要成果总结如下。 Z 我们在有机电荷转移化合物(M_2P-F_4TCNQ)的二维莫特绝缘体中发现了超快光致绝缘体-金属转变。通过对瞬态反射率光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非常快，为0.4 ps，并且金属态本质上是二维的。如此快速的衰减被认为是二维相关电子系统的特征。Z我们在有机电荷转移化合物（BEDTTTF-F_2TCNQ）的一维莫特绝缘体中发现了超快光致绝缘体-金属转变。这种转变的重要特征是，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。我们得出的结论是，这种通过弱激发形成金属态的现象归因于一维相关电子系统的自旋电荷分离特性。我们在具有一维相关电子系统的典型过渡金属化合物（碘桥铂化合物）中发现了超快光致电荷密度波（CDW）-莫特哈伯德跃迁。我们发现跃迁的效率非常高（每个光子约 70 个 Pt 位点），并且 CDW 到金属的跃迁是通过强激发发生的。也就是说，该化合物可以实现三相（CDW、莫特-哈伯德、金属）之间的光致相变。因此，在本项目中，我们成功地探索了新的光致相变并阐明了其物理机制。

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.