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Search for the Photo-Induced Hidden Phase in Inorganic and Organic Systems

100%
Koshihara S., Onda K., Okimoto Y., Ishikawa T.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
328-331
EN
We review the dynamical behavior of strongly electron-lattice (orbital) coupled system induced by photo-excitation of fs laser pulse. By virtue of the femtosecond (fs) spectroscopy and picosecond (ps) time-resolved X-ray diffraction measurements, we demonstrate that the photo-irradiation creates new ordered state so called as "hidden phase" which can never be realized under thermo-equilibrium condition. This exotic phase commonly plays the key role for realizing the gigantic photo-induced changes in optical properties of inorganic and organic electron-lattice strongly coupled systems.
2
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Photoinduced Ionic to Neutral Phase Transition in TTF-CA Studied by Time-Resolved Infrared Vibrational Spectroscopy

76%
Matsubara Y., Yoshida T., Ishikawa T., Okimoto Y., Koshihara S., Onda K.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
340-342
EN
We studied the photoinduced ionic to neutral phase transition in tetrathiafulvalene-p-chloranil (TTF-CA) using time-resolved infrared vibrational spectroscopy. By monitoring the charge sensitive b_{1u} ν_{10} band of CA after photoexcitation by a 1.55 eV pulse, we found that the photoinduced N-phase has the same charge of the N-phase in thermal equilibrium and there is no charge variation during the photoinduced phase transition. We also found that the photoinduced N-phase grows through at least two steps over 300 ps.
3
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Photo-Induced Structural Changes at a Surface οf Organic Single Crystals Observed by Vibrational Sum Frequency Generation Spectroscopy

76%
Fukumoto K., Fukazawa N., Ishikawa T., Koshihara S., Yamamoto H., Kato R., Onda K.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
313-315
EN
Phase transitions from insulator to metal induced by thermal excitation and by photo-irradiation at the surface of β'-(BEDT-TTF)(TCNQ) are investigated using vibrational sum frequency generation spectroscopy. The results are compared with linear reflectivity measurements which detect the information mainly from bulk. IR absorption spectra of a_{g} vibrational modes of TCNQ induced by the dimerization are observed using vibrational sum frequency generation spectroscopy, and its intensity is found to become smaller by thermal excitation indicating the transition to the metal phase. Time-dependent degree of dimerization is also observed by pump-probe experiments with almost the same statistics as the static measurements. On the other hand, in the reflectivity experiments, the reduction of dimerization is also observed by elevating the temperature, however, the spectral shapes are deformed for the time-resolved measurements. These results imply that the different dynamics occur at the surface and in the bulk.
4
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PIPT from the Beginning to Future

76%
Cailleau H., Luty T., Koshihara S., Servol M., Lorenc M., Buron-Le Cointe M., Collet E.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
297-306
EN
The birth of the field of photoinduced phase transitions was strongly influenced by the conceptual viewpoint expressed by Professor Y. Toyozawa on the condensation of relaxed excitons. Since this first period, twenty years ago, this young field has been expanding rapidly along a diversity of directions. Nowadays, it goes hand in hand with the challenges of today's science: emergence, nonlinearity, coherence, far away from equilibrium, for example. The control of the functionality of a material via photoexcited states poses many new fundamental questions. Some of them will be overviewed: (i) the nature of the control parameters and the nature of the relevant collective variables, especially the order parameters, which characterize the evolution of the system, (ii) the difference between photoinduced transformations under continuous light irradiation and those resulting from an ultrashort laser pulse, (iii) the physical mechanisms of ultrafast photoinduced phase transitions from the formation and proliferation of phototransformed entities to the softening of a collective mode.
5
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Photoinduced Phase Transition in Pr_{0.5}Ca_{0.5}CoO_3 Studied by Sequential Pulse Excitations

64%
Seko K., Okimoto Y., Kurashima M., Fukaya R., Egawa T., Ishikawa T., Onda K., Koshihara S., Kyomen T., Itoh M.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
369-371
EN
A perovskite-type cobalt oxide, Pr_{0.5}Ca_{0.5}CoO_3 (PCCO), shows photoinduced phase transition. In this study, we successively irradiated two laser pulses with different intensities to PCCO and probed the transient change of the reflection at 2.0 eV. Assuming propagation of the two different photoinduced metallic states, we could reproduce the time profiles as well as the magnitude in the reflectance change, indicating the fabrication of the photoinduced multilayered thin film in Pr_{0.5}Ca_{0.5}CoO_3.
6
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Femtosecond Reflection Spectroscopy in La_{1.5}Sr_{0.5}CoO_4

64%
Egawa T., Okimoto Y., Fukaya R., Onda K., Ishikawa T., Koshihara S., Isayama A., Sasagawa T.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
307-309
EN
Ultrafast optical response has been investigated using fs laser system on a perovskite-type cobalt oxide, La_{1.5}Sr_{0.5}CoO_4. After the photoirradiation at room temperature, the time profile of relative change of reflectance (Δ R/R) shows a sudden change within the pulse duration ( ≈ 150 fs) and decays with a lifetime of ≈ 330 fs. The sign of Δ R/R after the photoexcitation is positive in the mid-infrared region (at 0.50 eV) while negative in the visible energy region (at 2.0 eV), implying photoinduced change of the electronic structure after the photoexcitation.
7
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Ultrafast Dynamics of Photoinduced Electronic Phase Modulation in Ladder Cuprate of Sr_{14 - x}Ca_{x}Cu_{24}O_{41}

64%
Fukaya R., Kunitomo M., Okimoto Y., Ishikawa T., Onda K., Koshihara S., Yui H., Sasagawa T.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
310-312
EN
Ultrafast carrier dynamics caused by photoexcitation in quasi-one-dimensional two-leg ladder cuprate Sr_4Ca_{10}Cu_{24}O_{41} was investigated by the femtosecond reflection spectroscopy. After the photoexcitation along the leg direction, the transient reflectivity changes (Δ R/R) in the mid-infrared region showed instant decrease within 150 fs. The suppressed Δ R/R increased so rapidly on the picosecond time scale that the reflectivity value finally became larger than that in the initial state. Such a successive response in Δ R/R, which was also observed in other photon energy regions, is discussed in terms of ultrafast variation of the Drude weight in the ladder system by the photoirradiation.
8
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Slow Dynamics of the Photoinduced Phase Transition in Pd(dmit)_2 Salts (dmit = 1,3-dithiol-2-thione-4,5-dithiolate)

64%
Ishikawa T., Tanaka T., Fukazawa N., Matsubara Y., Okimoto Y., Onda K., Koshihara S., Tamura M., Kato R.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2B
316-318
EN
Slow dynamics of the initially photoinduced state has been observed by the pump-probe type time-resolved reflection spectroscopy in the charge separated phase of the half-filled strong dimer system, Et_2Me_2Sb[Pd(dmit)_2]_2 (dmit = 1,3-dithiol-2-thione-4,5-dithiolate). We have succeeded to reproduce the probe photon energy dependence of the time profile qualitatively in the time delay range from 10 ps to 1 ns assuming the dynamical expansion of the domain of the photo-induced dimer-Mott insulating phase in the host charge-separated one.
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