Language：English   Publisher：AMER PHYSICAL SOC  

A large photoinduced change in reflectivity has been observed in the low-temperature charge separated (CS) phase of a dimeric radical anion salt, Et2Me2Sb[Pd(dmit)(2)](2) (dmit = 1,3-dithiol-2-thione-4,5-dithiolate). Just after the photoexcitation, the reflectivity abruptly changed reflecting the appearance of a photoinduced metastable state, indicating occurrence of recrystallizing of the CS phase by intradimer photoexcitation within a picosecond. Quantitative analysis considering the linear combination of the dielectric functions of the CS and the dimer-Mott state suggests a rather high efficiency of the photoinduced phase transition. One photon can change the valence of about five dimers. This photoinduced metastable state relaxed to the initial CS state via two successive types of relaxation processes, a fast and a slow one. The relaxation time (tau) and the reflectivity of the fast process showed a clear excitation intensity and temperature dependence. In particular, tau and the estimated domain size were enhanced up to the transition temperature (T-c) with increasing temperature. This phenomenon, a sort of critical slowing down around T-c, suggests that the density of the photoinduced state as well as the external temperature plays an important role in determining the relaxation dynamics of the photoinduced state. The results obtained indicate that this photoinduced phenomenon can be classified as a tuning of the charge in crystals via cooperative interaction between the degrees of freedom of "charge" and "molecular orbital" of the constituents, i.e., as a type of photoinduced phase transition.

DOI： 10.1103/PhysRevB.80.115108

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Language：English   Publisher：PHYSICAL SOC JAPAN  

Femtosecond reflection spectroscopy was performed on a perovskite-type manganite, Gd0.55Sr0.45MnO3, with the short-range charge and orbital order (CO/OO). Immediately after the photoirradiation, a large increase of the reflectivity was detected in the mid-infrared region. The optical conductivity spectrum under photoirradiation obtained from the Kramers-Kronig analyses of the reflectivity changes demonstrates a formation of a metallic state. This suggests that ferromagnetic spin arrangements occur within the time resolution (similar to 200fs) through the double exchange interaction, resulting in an ultrafast CO/OO to FM switching.

DOI： 10.1143/JPSJ.76.043702

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Language：English   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

In this paper, a competition between a ferromagnetic metal and a charge/orbital-ordered insulator has been investigated for RE(1-x)AE(x)MnO(3) (x = 0.45) with RE and AE being rare earth elements, ranging from La to Gd, and alkaline earth elements, Ca, Sr and Ba, respectively. A phase diagram is deduced in the plane of the effective one-electron bandwidth versus the magnitude of quenched disorder, which can be controlled by the averaged value and the variance of the RE/AE ionic radii, respectively. In the vicinity of the phase boundary, the ferromagnetic transition temperature is critically suppressed, and the colossal magnetoresistance (CMR) is enhanced. (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2006.06.005

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Neutral-ionic (NI) phase transition is a reversible switching of organic charge-transfer complexes between distinct valence states by external stimuli. This phase transformation in the low-dimensional system is demonstrated to provide a variety of novel dielectric, structural, and electronic properties. Importantly, ionization of the electron donor-acceptor pairs is usually accompanied by a ferroelectric or antiferroelectric order of the molecular lattice, leading to huge dielectric response near the transition point. Although these characteristics are potentially useful for future electronic and optical applications, the thermally accessible NI transition (TINIT) is still an extremely rare case. The TINIT compounds including some new materials are overviewed in order to provide convenient guides to their design and experimental identifications. The phase transition and dielectric properties can be closely controlled in various ways depending on chemical and physical modifications of the crystals. Among them, a quantum phase transition and relaxor ferroelectricity, both of which are currently attracting subjects from both scientific and practical perspectives, are highlighted as the first achievements in organic charge-transfer complexes. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.chemphys.2005.09.025

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Language：English   Publisher：AMER INST PHYSICS  

The magnetic-field effect on electroluminescence (EL) has been investigated for the tris-(8-hydroxyquinolino) aluminum (Alq(3))-based organic light emitting diode. The EL intensity sharply increases up to 8% with increasing magnetic field to 500 Oe at room temperature. The magnetic field effect on EL depends on the interface structure between a hole transporting and a light emitting layers, indicating the importance of the spin-state dynamics of the electron-hole pairs at the interface.

DOI： 10.1063/1.2185256

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Presentation type：Poster presentation  

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