Dephasing dynamics of photoexcited state in correlated electron system

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Photo-response of correlated electron (CE) system has attracted much attention because of their ultrafast dynamics aspects. In this research project, I) We have investigated dephasing dynamics of photoexcited state in one-dimesional Mott insulator [Ni(chxn)_2Br]Br_2 by using 2-pulse degenerative four-wave mixing (DFWM), II) ultrafast photoinduced insulator-to metal transition (PIMT) was detected in two-dimensional organic conductor (BEDT-TTF)_2X by mid-IR femtosecond pump-probe spectroscopy for the first time.I)In the DFWM study of [Ni(chxn)_2Br]Br_2, i) for strong excitation condition, the slower decay component (6 ps) of DFWM signal appears, while only sub-ps component is observable for the weak excitation. ii) A temperature dependence of the slow decay for the strong excitation can not be understood in terms of the holon-doublon (h-d) cluster-optical phonon interaction. The temperature dependence of the dynamics can be summarized as follows; a) sub-ps component, which is detected fo … More r T>30 K and becomes shorter for T<100 K, b) several ps component which becomes shorter for T >100 K. The relaxation process of b) can be explained by using the interaction between the carrier and the optical phonon. Nevertheless, the a) is suggested to be related to the magnon excitation since the temperature dependence of the decay constants and their fractions resemble the temperature dependence of the magnetic susceptibility. Moreover, for b), although, the frequency of the interacting optical phonon is evaluated to be same as that of for weak excitation limit, coupling constant is smaller for one order of magnitude. These results indicate that 1) h-d cluster, which is generated by the strong excitation, is scattered by phonon and magnon, 2) the coupling constant between the h-d cluster and optical phonon is markedly suppressed, 3) resultantly, decay reflecting the magnon scattering is considered to appear.II)In the research of the two-dimensional organic conductor (BEDT-TTF)_2X, ultrafast and efficient (>100 molecules /photon) photoinduced insulator-to metal transition was observed. The critical slowing down of the photoinduced metallic state and the coherent phonon reflecting the inhomogeneous structure of the photoinduced domain.Our results will provide the useful information for designing the material for ultarfast CE switching devise whose mechanisms are third order optical nonlinearity or photoinduced phase transition, respectively. Less