[en] The impurity (Cr³⁺)-doping effect on the stability of charge and orbital ordering has been systematically investigated for Nd_1/2Ca_1/2Mn_1-yCr_yO₃ crystals by measurements of magnetotransport and x-ray diffraction. The random field in terms of e_g orbital deficiencies on the Cr sites drives the charge and orbital correlations to dynamical and short range, which is most relevant to the high-resistive state exhibiting colossal magnetoresistance. In the Cr-doped manganite, we can observe the coexistence of ferromagnetic - metallic and charge - orbital ordered phases, their spatial distributions, diffuse x-ray scattering, magnetic-field annealing, and the aging effect on the magnetic and electric properties, etc. These phenomena are reminiscent of those of relaxor ferroelectrics composed of ferroelectric clusters embedded in a paraelectric matrix. We propose that the mixed-valent manganite can be viewed as a 'magneto- and electrorelaxor.' [copyright] 2001 American Institute of Physics

[en] Optical conductivity spectra and their variation with temperature and doping level x have been investigated for single crystals of La_1-xSr_xMnO₃ (0≤x≤0.3). For the low-doped insulating crystal (x=0.1) which shows a ferromagnetic insulating state at low temperature, the spectral weight of the optical conductivity increases only in the inner-gap region around 0.5 eV, but no Drude part emerges due to carrier localization effect. For x≥0.17, where the low-temperature ferromagnetic metallic state shows up, the optical conductivity spectrum above T_c is characterized by interband transitions between the exchange-split conduction bands, and it gradually changes into that of intraband excitations below T_c. The energy scale (up to ∼2 eV) of the spectral weight transfer is determined by the effective Hund close-quote s-rule coupling energy. In the metallic phase, low-energy spectra arising from intraband excitations can be sorted into two parts: One is a nearly ω-independent broad structure (incoherent part), and the other a sharp coherent Drude peak with anomalously low spectral weight. This can hardly be reconciled with the simple double-exchange theory, but indicates that another degree of freedom (e.g., the orbital ordering and/or electron-lattice interactions) should be taken into account. copyright 1997 The American Physical Society

[en] In JOYO, the measurement of control rod worths have been carried out in the beginning of the each cycle, using both period method and neutron source multiplication method. In this paper, the calculational method of control rod worths in the design stage and the comparison with the design values and measured ones are shown. The reasons that the control rod worths change slightly in each cycle, are also investigated. (author). 13 figs, 12 tabs

[en] Photoirradiation effect in a ferroelectric cocrystal Hdppz-Hca is investigated in terms of time-resolved second harmonic (SH) generation measurements. By the photoexcitation with a visible pulse (530 nm) causing the intramolecular transition of Hca molecule, the SH intensity was suddenly suppressed by ≈30% on the time scale of picosecond, while the reflectivity scarcely changed. This result is discussed based on reversal of the polarized domain by the photoexcitation. (paper)

[en] The dynamics of the photoinduced effect in the low temperature phase of (C₂H₅)₂(CH₃)₂Sb[Pd(dmit)₂]₂ (Et₂Me₂Sb[Pd(dmit)₂]₂) salt was studied by using pump-probe time resolved reflection spectroscopy with the time resolution of about 120 fs. The observed photoinduced reflectivity change was highly efficient and spectral shape of the photoinduced state was similar to that of dimer Mott insulator. In addition, oscillatory structure in photoinduced signal due to coherent phonon was observed reflecting the structural modulation coupling with valence change of the dimer state ([Pd(dmit)₂]₂). We discuss the primary dynamics of the photoinduced phenomena in this salt at low temperature phase.

[en] The temperature and polarization dependence of the optical reflectivity spectra of a quasi-one-dimensional 1/4-filled band system, (DMEDO-EBDT)₂PF₆, have been investigated. We observed clear anisotropy in the electronic structures corresponding to the anisotropic transport properties. The appearance of a charge gap (E_g > 0.1 eV) and transfer of the spectral weight accompanied by the metal-insulator phase transition were clearly observed. In addition, a split of the intramolecular vibrational modes was observed, which strongly suggested the existence of charge disproportionation in the low temperature phase. We also observed a photoinduced reflectivity change, which implied the occurrence of a photoinduced phase transition from the low temperature insulating phase to the high temperature metallic phase. (paper)

[en] Structural and electronic properties have been investigated for a series of Re-based ordered double perovskites, A₂MReO₆ (A=Sr,Ca; M=Mg,Sc,Cr,Mn,Fe,Co,Ni,Zn), which contain some prospective candidates of the high-temperature half-metals for future spin-electronic use. Neutron diffraction measurements have revealed the variation of the Re-O bond length with the change of its effective valence states depending on the counter M ion, namely, Re⁶⁺- or Re⁵⁺-based insulating states, or otherwise mixed-valent metallic (or barely Mott-insulating) states. Magnetotransport and specific heat studies have clarified metal-insulator phenomena for M=Cr and Fe compounds with the change of A site as well as the intergrain tunneling magnetoresistance characteristic of the ceramics of half-metal oxides

[en] The conducting charge transfer complex (EDO-TTF)₂PF₆ has two types of charge transfer bands in the low temperature insulator phase: CT1 at 0.56 eV and CT2 at 1.38 eV. We excited these bands independently with a tunable ultrashort pulse laser and studied the difference of the photo-induced phases by measuring change in reflectivity spectrum over a wide photon energy range. As a result, we found that both the photo-induced phases by CT1 and CT2 excitation are the same except for their photo-conversion efficiencies

[en] We examined the ultrafast response of the metallic high-temperature phase in the conducting charge transfer complex (EDO-TTF)₂PF₆. A large reflectivity change of approximately 10% was observed when the intra-molecular band was excited by a weak 3.1 eV ultraviolet light pulse. The lifetimes of the photo-induced states were 0.2 and 0.7 ps in the Drude-like band and the intra-molecular band, respectively. Measurement of the photo-induced spectrum just after photo-excitation and simulation using the Drude model revealed that the electronically excited EDO-TTF molecules shortened the relaxation time of conduction electrons and reduced the reflectivity of the Drude-like band.

[en] We report on synthesis and electronic characterization of single-crystalline film of Sr₂CoO₄ with square-lattice CoO₂ sheets (K₂NiF₄-type structure). In this compound, we have found both ferromagnetic and metallic behaviors with fairly high Curie temperature T _C of ∼250 K, as contrasted by the superconductivity recently found in a triangular-lattice CoO₂-sheet compound. The electronic structure of Sr₂CoO₄ was also investigated in terms of in situ photoemission spectroscopy