Explorations of new optical switching phenomena in strongly correlated electron systems

在本研究项目中，我们旨在探索新型的光诱导相变(光开关现象)，在这种相变中，光的输运、光和磁性质发生了很大的变化，并阐明了它们的物理机制。我们在有机电荷转移化合物(M_2P-F_4TCNQ)的二维Mott绝缘子中发现了超快光致绝缘体-金属相变。从详细的瞬时反射率谱及其时间依赖关系的分析中，我们发现光生金属态的衰减时间非常快，为0.4ps，并且金属态本质上是二维的。这种快速衰减被认为是二维关联电子系统的特征。我们在有机电荷转移化合物(BEDTTTF-F_2TCNQ)的一维Mott绝缘体中发现了超快光致绝缘体-金属相变。这一转变的重要特征是，即使在非常弱的激发光子密度(&lt；0.003光子/位)下也能观察到金属行为。我们的结论是，这种由弱激发形成的金属态是由于一维关联电子系统的自旋-电荷分离特性所致。我们在具有一维关联电子系统的典型过渡金属化合物中发现了超快光致电荷密度波(CDW)-Mott Hubbard转变。我们发现这种跃迁的效率很高(每个光子大约有70个铂位置)，而且CDW到金属的跃迁是由强激发引起的。也就是说，在这种化合物中，三个相(CDW，Mott-Hubbard，Metals)之间的光诱导相变是可能的。因此，在本项目中，我们成功地探索了新的光诱导相变，并对其物理机制进行了阐明。

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.