[en] The recent development of the techniques for dynamical structural analysis based on pulsed x-ray from SR is encouraging the materials science in the field of cooperative phenomena induced by external stimulations. We introduce even protein system shows the cooperative structural deformation triggered by light stimulation based on laser-SR combined method. (author)

[en] The search for materials which exhibit phase transitions triggered by weak external excitation by light is an important and attractive target for photonic and materials science. We review recent developments in ultra-fast X-ray diffraction techniques which are a result of a synergistic collaboration between researchers in the fields of photo-induced phase transition and synchrotron radiation. We review a study of electron (spin)-lattice coupled changes in the organic charge transfer complex TTF-CA. (author)

[en] Watching with an atomic resolution at structural changes as fast or ultra-fast photoinduced physical processes take place benefits from recent progresses in time-resolved X-ray diffraction. Molecular materials where electronic and structural changes are strongly coupled are model systems to perform such time-resolved crystallography studies. We report the structural investigation of photoinduced phase transformations between ionic and neutral states in an organic charge-transfer molecular material, using 100 picosecond (ps) synchrotron pulses. This light-induced phenomenon, triggered by an ultra-short optical pulse from a femtosecond laser, occurs by virtue of intrinsic cooperativity. Since electronic and structural changes are strongly coupled, it is of fundamental interest to perform time-resolved X-ray diffraction to obtain information at the atomic scale. We also discuss the problem of coexistence of phases and the interest of future investigations in faster timescale

[en] It is an attractive target for materials science to find a system which shows the phase transition triggered by external stimulation of light. The purpose of our study is to review experimental evidences indicating that the photo-injected local excitation can really trigger the cooperative phenomena in solids. In this sense, this unique photo-induced effect can be named as photo-induced phase transition (PIPT). Here, I will also make brief review on the experimental research on PIPT combining with a development of ultra-fast quantum electronics technology

[en] The multi-instability of the electronic structure of (EDO-TTF)₂PF₆, where EDO-TTF means ethylene-dioxytetrathiafulvalene, is reviewed. This complex showed the metal-insulator transition at 280 K associated with distinct molecular deformations. The mechanism is interpreted as the cooperation of Peierls transition, charge ordering, and the order-disorder transition of the countercomponent. The charge ordering pattern in the low-temperature phase is of the novel [0, 0, 1, 1] type. The sensitivity of the electronic state to external perturbations is demonstrated applying not only static but also instantaneous stimuli. In the latter case, the photo-induced phase transition is ultrafast and highly efficient. One photon causes the transition of several hundreds of donor molecules in the low-temperature phase to relax into a highly conducting metastable state within about 1.5 ps. In the early stage of the transient state, the charge ordering of the [1, 0, 1, 0] type occurs. As for the chemical modifications of this material, the partial deuteration of this complex increases the metal-insulator transition temperature. The introduction of a methyl group greatly modulates the electronic structure of the complex, i.e. (methyl-EDO-TTF)₂X (X=BF₄, ClO₄) shows a two-dimensional electronic structure. The working hypotheses for developing the systems with multi-instability are described. (topical review)

[en] In this study, we report the first direct observation of the transient spin-state in the disordered system by time-resolved XAFS (X-ray absorption fine structure) using a pulsed hard X-ray beam. By detecting the evolution of the inner-atomic transition of the divalent iron ion, the transient spin-crossover transition from ¹A₁ low spin state to ⁵T₂ high spin state has been directly observed on the picosecond timescale. Furthermore, the time-resolved investigation in the X-ray absorption near-edge structure and the extended X-ray absorption fine structure provided dynamic information on the altered electronic states of the iron as well as information on the structural change in the Fe-N bond elongation of 0.17 A resulting from the formation of the high-spin excited state. This unique experimental technique probes the excited states and provides a new level of understanding of ultrafast reactions in spin crossover transitions as well as in magnetic photocatalysts, nanomagnets, biological magnetic systems, and photo-induced phase transition. (author)

[en] Experimental strategy and setup for the 100 ps time-resolved X-ray absorption spectroscopy (TR-XAS) is presented. X-ray positional active feedback combined with a figure-of-merit scan of the laser beam position was employed as a key technique. It is shown that the pump-probe TR-XAS technique using the X-ray pulse structure of synchrotron radiation sources is a powerful tool to investigate dynamics of the electronic state and molecular structure of non-crystalline samples. A TR-XAS measurement of a photo-induced spin crossover reaction of the tris (1,10-phenanthroline)iron(II) complex in water is presented. (author)

[en] Much effort has been devoted to the development of techniques to probe carrier dynamics, which govern many semiconductor device characteristics. We report direct imaging of electron dynamics on semiconductor surfaces by time-resolved photoemission electron microscopy using femtosecond laser pulses. The experiments utilized a variable-repetition-rate femtosecond laser system to suppress sample charging problems. The recombination of photogenerated electrons and the lateral motion of the electrons driven by an external electric field on a GaAs surface were visualized. The mobility was estimated from a linear relationship between the drift velocity and the potential gradient

[en] Picosecond time-resolved Laser pump/X-ray probe experiments are becoming general and powerful tools to explore structural dynamics in materials and biological sciences. In order to perform time-resolved experiments, we constructed the beam line NW14A at the Photon Factory Advanced Ring, KEK. This beam line is designed to conduct a wide variety of time-resolved X-ray measurements, such as time-resolved X-ray diffraction, scattering, spectroscopy and imaging. Experimental protocols for time-resolved fluorescence XAFS and X-ray solution scattering measurements are presented. (author)

[en] Lattice distortions under an electric field in a mono-domain of BaTiO₃ ferroelectric crystal have been detected with synchrotron x-ray radiation. The variation of the lattice constant with an electric field observed with high angle diffraction shows a linear response nature of the piezoelectric effect. When an electric field is applied along the spontaneous polarization direction, the c-axis of the lattice elongates and the a-axis of the lattice shrinks at a rate of d₃₃ = 149 ± 54 pm V^-1 and d₃₁ = -82 ± 61 pm V^-1; these represent the longitudinal and transverse piezoelectric coefficients of BaTiO₃ crystal, respectively. These results give an insight into the intrinsic piezoelectric response on the lattice scale in BaTiO₃ that has been widely used to explore high performance lead-free piezoelectric alloys.