Hiroyuki Oyanagi

X-ray Detection of the Period-Four Cycling of the Manganese Cluster in Photosynthetic Water Oxidizing Enzyme.

X-ray absorption near-edge structure spectra of the manganese (Mn) cluster in physiologically native intermediate states of photosynthetic water oxidation induced by short laser flash were measured with a compact heat-insulated chamber equipped with an x-ray detector near the sample surface. The half-height energy of the Mn Kedge showed a period-four oscillation dependent on cycling of the Joliot-Koks oxygen clock. The flash number-dependent shift in the Mn K-edge suggests that the Mn cluster is oxidized by one electron upon the S0-to-S1, S1-to-S2, and S2-to-S3 transitions and then reduced upon the S3-to-S0 transition that releases molecular oxygen.

Evidence of Critical Oxygen Concentration at y=6.7∼6.8 for 90 K Superconductivity in Ba2YCu3Oy

A study of precise oxygen concentration dependence of the superconducting volume fraction in Ba2YCu3Oy (6.22≤y≤6.96) revealed the phase diagram of this superconducting system. Three characteristic phases are found based on the oxygen concentration: (1) a nonsuperconducting tetragonal phase for 6.2<y<6.4, (2) a low-Tc (Tc= 55~70 K) orthorhombic phase for 6.4≤y<6.8 and (3) a high-Tc (Tc=90 K) orthorhombic phase for 6.8≤y<7.0. Evidence for a distinct critical concentration at y=6.7~6.8, necessary to sustain the bulk high-Tc (90 K) superconductivity, is presented.

Effects of the surface Cu2−xSe phase on the growth and properties of CuInSe2 films

Drastic changes in average molecularities (m=Cu/In) from m≫1 to m=0.92–0.93 and in hole concentrations from p≫1019 cm−3 to as low as p=7.5×1016 cm−3 have been observed in molecular beam epitaxy grown CuInSe2 after selective etching of the Cu–Se phase by a KCN aqueous solution; high hole concentrations and Cu-excess compositions of the as-grown films were attributed to the Cu–Se phase. On the other hand, well-defined photoluminescence emissions were found characteristic of intrinsic CuInSe2. The presence of the Cu–Se phase made possible the growth of high-quality CuInSe2 epitaxial films at a temperature well below the melting point of any Cu–Se compound. Surface topology measurements showed that the surface of the as-grown films was not fully covered by Cu–Se grains, leaving holes with depths of 200–300 nm after KCN etching. The enhanced two-dimensional growth and the reduced defect concentration imply that a very thin Cu-excess surface layer controls the growth of CuInSe2 when grown under Cu-excess condit...

Valence Study of Orthorhombic and Tetragonal Ba2YCu3Oy: The Role of Oxygen Vacancies in High-Tc Superconductivity

The valency of Cu ions in the orthorhombic and tetragonal phases of high-Tc superconductor Ba2YCu3Oy (6.22<y<6.96) has been studied by near-edge X-ray absorption spectroscopy. The results indicate that the Cu valency at Cu1 sites on the z=1/2 plane is strongly correlated with the high-Tc (90 K) superconductivity, while the Cu valency of Cu2 sites on the basal planes affects the low-Tc (50-60 K) superconductivity of the orthorhombic phase.

Heteroepitaxy and characterization of CuInSe2 on GaAs(001)

Abstract CuInSe 2 (CIS) films with Cu/In ratios of γ = 0.81–1.81 have been grown on (001)-oriented GaAs substrates by molecular beam epitaxy at substrate temperatures of T s = 350–550° C . Film properties were found to be substantially different for Cu- and In-rich regions. Cu-rich films were p-type, and streaky reflection high-energy electron diffraction (RHEED) patterns and sharp photoluminescence (PL) emissions were observed, suggesting high quality epitaxial films. In-rich films were highly resistive, and contained a large number of twins formed on {112} planes. A broad and strong PL emission, a donor-acceptor pair emission, was observed, which blue-shifted with increasing excitation power, indicating heavy compensation.

The stripe critical point for cuprates

The experimental determination of the quantum critical point (QCP) that triggers the self-organization of charged striped domains in cuprate perovskites is reported. The phase diagram of doped cuprate superconductors is determined by a first variable, the hole doping ?, and a second variable, the micro-strain ? of the Cu-O bond length, obtained from the Cu K-edge extended x-ray absorption fine structure. For a fixed optimum doping, ?c = 0.16, we show the presence of the QCP for the onset of local lattice distortions and stripe formation at the critical micro-strain ?c. The critical temperature Tc(?,?) reaches its maximum at the quantum critical point (?c,?c) for the formation of bubbles of superconducting stripes. The critical charge, orbital and spin fluctuations near this strain QCP provide the interaction for the pairing.

The Measurement of the Polaron Size in the Metallic Phase of Cuprate Superconductors

The statistical distribution of the Cu site conformations in the CuO2 plane of Bi2Sr2CaCu2O8+y (Bi 2212) has been measured by a fast and local probe Cu K-edge extended x-ray absorption fine structure (EXAFS) as a function of temperature. The Cu site conformations are frozen at T < T* = 120 K ~ 1.4 Tc in two well-defined conformations characterized by two Cu-O(apical) bond lengths separated by 0.17 ± 0.02 A. The Cu site conformations with short bond 2.38 ± 0.02 A have a probability of 41 ± 2% at T < T*. The diffraction experiments show that the Cu site conformations exhibit a one-dimensional anharmonic modulation, with period λp = 25 A, that has been measured by electron diffraction. These data show that the modulation is anharmonic and the plane is decorated with stripes of distorted lattice characterized by the short Cu-O(apical) bonds of width W = 11 ± 1 A at T < T* that form domain walls for the stripes of width L ~ 14 ± 1 A, characterized by the long Cu-O(apical) bond. The stripes of distorted lattice run below the lines of interstitial oxygen ions in the BiO plane, playing the role of acceptors. The lattice modulation is therefore assigned to polaronic impurity states ordered in linear arrays. The measure of the width W of the stripes provides the measure of the area of the polarons Sp ~ 4a2 (a = 5.4 A) where each hole dressed by the lattice distortion spans a domain of 8 Cu sites.

Sagittally focusing double-crystal monochromator with constant exit beam height at the photon factory

Abstract A fixed-exit, sagittally focusing double crystal monochromator was constructed and tested at the Photon Factory in Tsukuba, Japan. This monochromator is tunable in an angle range from 4.5° to 70° without a significant movement of the exit-beam height by simply rotating the two crystals about a single rotation axis. One order of magnitude higher intensity was obtained at the focal point by the sagittal focusing. This monochromator has been already used for EXAFS measurements in a fluorescence detection mode above the Pt-L, Ga-K and As-K edges.

The optically active center and its activation process in Er-doped Si thin film produced by laser ablation

The local structure of erbium-doped silicon produced by the laser ablation technique is investigated by Er LIII-edge x-ray absorption fine structure analysis. The combined analysis of extended x-ray absorption fine structure analysis and an x-ray absorption near-edge structure simulation based on multiple-scattering theory reveals the most probable atomic coordination of the optically active center; Er bonded with six oxygen atoms has a C4v symmetry. The optical activation process of this system is also discussed. The Si target with 10 wt% Er2O3 has two kinds of local structures, C-rare-earth Er2O3 grain and another Er phase incorporated in Si. The laser ablation homogenizes these phases, and deposits a new single-phase structure of the octahedron (Oh point group) on the substrates. In this phase, the optical transition probability is low due to the forbidden 4f transition of Er in the crystal field originating from the higher-order symmetry of O. After annealing, degradation of the symmetry from Oh to C4...

Local structure of Ge nanoislands on Si(111) surfaces with a SiO2 coverage

We have investigated the local structure and photoluminescence properties of ultrasmall Ge islands grown on Si(111) covered with SiO2. Scanning electron microscopy and transmission electron microscopy measurements show that the islands have a hemispherical shape, and depending on the growth temperature, can be either epitaxial or nonepitaxial. X-ray absorption near-edge structure measurements demonstrate that the nonepitaxial islands have the local structure of bulk diamond Ge and are very stable towards oxidation. The epitaxial islands are found to be partly oxidized, but no alloying with the Si substrate is observed. The nonepitaxial islands exhibit a photoluminescence peaked at 2.3 eV, which is typical of Ge nanocrystals embedded in SiO2. Possible mechanisms for the stability of the nonepitaxial Ge islands towards oxidation are discussed.We have investigated the local structure and photoluminescence properties of ultrasmall Ge islands grown on Si(111) covered with SiO2. Scanning electron microscopy and transmission electron microscopy measurements show that the islands have a hemispherical shape, and depending on the growth temperature, can be either epitaxial or nonepitaxial. X-ray absorption near-edge structure measurements demonstrate that the nonepitaxial islands have the local structure of bulk diamond Ge and are very stable towards oxidation. The epitaxial islands are found to be partly oxidized, but no alloying with the Si substrate is observed. The nonepitaxial islands exhibit a photoluminescence peaked at 2.3 eV, which is typical of Ge nanocrystals embedded in SiO2. Possible mechanisms for the stability of the nonepitaxial Ge islands towards oxidation are discussed.