H. Yanagi

Publications (25)81.49 Total impact

• Source

  Available from: ac.jp

  Conference Proceeding: An analysis of a constant current controlled Bi-directional DC-DC converter with digital control

  S. Tachikawa, T. Hirose, H. Yanagi, Y. Ishizuka, H. Matsuo
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  ABSTRACT: This research aims at the development of switching power supply circuits that supply electric power from distributed power sources to electronic devices. A typical switching power supply uses constant voltage control, but in addition to constant voltage control, we propose constant current control enabled by means of digital PWM (pulse width modulation) control and a bidirectional DC-DC converter capable of converting energy in two directions using a single DC-DC converter. This paper supposes that the aforementioned technology will be applied to streetlight systems that use solar cells, LEDs and batteries. Experiments on the charging operation using batteries and LEDs, as well as of the discharging operation using solar cells and batteries, were conducted, and this technology was evaluated based on those results.
  Telecommunications Energy Conference, 2009. INTELEC 2009. 31st International; 11/2009
• Article: Pressure effects on Tc of iron-based layered superconductor LaTMPO (TM = Fe, Ni)

  K Igawa, K Arii, Y Takahashi, H Okada, H Takahashi, Y Kamihara, M Hirano, H Hiramatsu, T Watanabe, H Yanagi, T Kamiya, H Hosono, K Matsubayashi, Y Uwatoko
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  ABSTRACT: We report electrical and magnetic properties of LaTMPO (TM = Fe, Ni) under high pressures. From resistivity measurements, superconducting transition temperature Tc for LaFePO and LaNiPO increases steeply up to about 0.8 GPa, and the dTc/dP values are more than 4 K/GPa and 1 K/GPa, respectively. From the results of high-pressure x-ray diffraction measurements, the oxypnictide compounds show an anisotropic compression along a-axis and c-axis.
  Journal of Physics Conference Series 03/2009; 150(5):052075.
• Article: Multilayered solid-state organic laser for simultaneous multiwavelength oscillations

  K. Yamashita, A. Arimatsu, N. Takeuchi, M. Takayama, K. Oe, H. Yanagi
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  ABSTRACT: This paper describes an organic dye-doped polymeric laser with a multilayered structure of active waveguides. Using the technique of photonanoimprint lithography, organic active-waveguide layers with distributed-feedback cavities and a polymeric intermediate cladding layer were stacked on a silica substrate. Under optical pumping, lasing oscillations at 427 and 636 nm, which correspond to the Bragg reflection wavelengths in the respective active waveguides, were simultaneously observed. The fabrication scheme presented here is expected to be a promising technology for the development of compact multicolor laser sources.
  Applied Physics Letters 12/2008; 93(23):233303-233303-3. · 3.84 Impact Factor
• Article: Fabrication and transport properties of 12CaO·7Al2O3 (C12A7) electride nanowire

  Y. Nishio, K. Nomura, M. Miyakawa, K. Hayashi, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono
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  ABSTRACT: Electron-doped 12CaO·7Al2O3 (C12A7) nanowires (NWs) were fabricated by two approaches. First, spatially selective electron doping of C12A7 films was examined by using a chemical method specific to C12A7. However, the conductive NW was not formed due to diffusion of oxygen in the C12A7 films to the conductive area. Secondly, a combination of dry etching and electron doping was examined. NWs with conductivities ∼6 S/cm at 300 K were successfully fabricated by this method. The temperature dependence of the conductivity in the NWs exhibited almost degenerated conduction down to 5 K. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  Physica Status Solidi (A) Applications and Materials 07/2008; 205(8):2047 - 2051. · 1.46 Impact Factor
• Article: Simple fabrication technique of distributed-feedback polymer laser by direct photonanoimprint lithography

  K. Yamashita, M. Arimatsu, M. Takayama, K. Oe, H. Yanagi
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  ABSTRACT: This paper describes a fabrication technique of polymeric distributed-feedback lasers using the photonanoimprint lithography. In devices with a grating structure, which was imprinted directly onto the active layer surface in vacuum, single-mode laser oscillations were easily obtained. The emission color of the distributed-feedback laser can be controlled only by changing doping material and grating period of the master mold. It was demonstrated that the nanoimprint lithography would be a key technology for development of a multicolor light source.
  Applied Physics Letters 06/2008; 92(24):243306-243306-3. · 3.84 Impact Factor
• Article: Development and Electronic and Photonic Characteristics of Thiophene/Phenylene Co-Oligomers

  S. Hotta, T. Katagiri, T. Yamao, K. Shimizu, H. Yanagi, M. Ichikawa, Y. Taniguchi
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  ABSTRACT: A novel class of organic semiconducting materials, thiophene/phenylene co-oligomers (TPCO), has been developed and synthesized. The materials are formed into crystals of high quality in a gas or liquid phase. These crystals exhibit unique optoelectronic properties such as a high carrier mobility and high quantum yield of emission. The transport properties have been studied on a device configuration of the field-effect transistor. The laser oscillation has definitively been observed for single crystals of the TPCO materials. In this article these optoelectronic features are presented and summarized.
  International Journal of Polymeric Materials 01/2008; 57(5):515-531. · 1.20 Impact Factor
• Article: Work Function of a Room‐Temperature, Stable Electride [Ca24Al28O64]4+(e–)4

  Y. Toda, H. Yanagi, E. Ikenaga, J. J. Kim, M. Kobata, S. Ueda, T. Kamiya, M. Hirano, K. Kobayashi, H. Hosono
  Advanced Materials 11/2007; 19(21):3564 - 3569. · 13.88 Impact Factor
• Article: Low threshold amplified spontaneous emission from near-infrared dye-doped polymeric waveguide

  K. Yamashita, T. Kuro, K. Oe, H. Yanagi
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  ABSTRACT: We have investigated near-infrared light amplification in a dye-doped polymeric waveguide structure under optical pumping. Amplified spontaneous emission (ASE) at ∼800 nm was observed in poly(vinyl-pyrrolidone) thin film with organic dye LDS798. The ASE threshold energy was as low as ∼12 μ J / cm ² , which is about one-tenth of previously reported values for infrared-dye-doped polymeric waveguides. This low threshold ASE comes from a large Stokes shift of LDS798, in which the self-absorption of emission is suppressed.
  Applied Physics Letters 07/2006; · 3.84 Impact Factor
• Article: Comparative Carrier Transport Characteristics in Organic Field‐Effect Transistors with Vapor‐Deposited Thin Films and Epitaxially Grown Crystals of Biphenyl‐Capped Thiophene Oligomers

  H. Yanagi, Y. Araki, T. Ohara, S. Hotta, M. Ichikawa, Y. Taniguchi
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  ABSTRACT: Carrier transport characteristics in organic field-effect transistors were compared for vapor-deposited thin films and epitaxially grown needle crystals of biphenyl-capped thiophene oligomers with different lengths of the thiophene units. The hole mobility of the thin films deposited on Si/SiO2 substrate was improved up to 0.17 cm2 V–1 s–1 by formation of platelet crystallites with a domain size of a few micrometer. The hole transport in the epitaxial needle crystals grown on the KCl surface depended upon the molecular orientation with respect to the channel direction. The orientation of the needle axis bridging over the source–drain electrodes increased the mobility since π-electronic interaction through the parallel stack of the linear molecules enhanced the carrier transport along the needle. The deposition condition and electronic energy levels of the oligomers, depending on the length of the thiophene units, also affected their characteristics.
  Advanced Functional Materials 10/2003; 13(10):767 - 773. · 10.18 Impact Factor
• Article: Imaging of vanadylnaphthalocyanine aggregates by scanning tunneling microscopy

  A. Manivannan, L. A. Nagahara, K. Hashimoto, A. Fujishima, H. Yanagi, T. Kouzeki, M. Ashida
  04/2002;
• Article: Self‐Waveguided Gain‐Narrowing of Blue Light Emission from Epitaxially Oriented p‐Sexiphenyl Crystals

  H. Yanagi, T. Ohara, T. Morikawa
  Advanced Materials 09/2001; 13(19):1452 - 1455. · 13.88 Impact Factor
• Article: Epitaxial growth of transparent p-type conducting CuGaO2 thin films on sapphire (001) substrates by pulsed laser deposition

  K. Ueda, T. Hase, H. Yanagi, H. Kawazoe, H. Hosono, H. Ohta, M. Orita, M. Hirano
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  ABSTRACT: Transparent p-type conducting CuGaO2 thin films were prepared on α-Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60° with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (∼80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3×10−2 S cm−1, 1.7×1018 cm−3, and 0.23 cm2 V−1 s−1, respectively. © 2001 American Institute of Physics.
  Journal of Applied Physics 01/2001; 89(3):1790-1793. · 2.17 Impact Factor
• Article: Electronic structure of polycrystalline AMoO3 (A=Sr or Ba)

  H. Mizoguchi, K. Fukumi, N. Kitamura, T. Takeuchi, J. Hayakawa, H. Yamanaka, H. Yanagi, H. Hosono, H. Kawazoe
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  ABSTRACT: Two strong peaks were observed in the electron energy loss spectra of polycrystalline AMoO3 (A=Sr and Ba) perovskites with metallic conductivities. One, observed at 1.7 eV, was attributed to plasmon loss originating from conduction electrons. This explains the reddish color observed. The other, located at ∼5 eV, was attributed to the charge transfer transition from the Mo4d–O2p bonding band to an unoccupied t2g band through photoemission, ultraviolet inverse photoemission, and x-ray absorption near edge structure spectra. No intense peaks were detected in the energy region between the two absorption bands (2.5<hν<3.5 ev), indicating the possibility of application in transparent conductors for the ultraviolet region. © 1999 American Institute of Physics.
  Journal of Applied Physics 04/1999; 85(9):6502-6505. · 2.17 Impact Factor
• Article: Vacuum ultraviolet reflectance and electron energy loss spectra of

  K Ueda, H Yanagi, R Noshiro, H Hosono, H Kawazoe
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  ABSTRACT: Normal incidence light reflection measurements have been performed for single crystals over the wide energy region from 0.4 to 35 eV. The dielectric functions and optical constants are evaluated from the reflectance spectrum by the Kramers-Krönig transformation technique. The experimental electron energy loss spectrum is in reasonable agreement with the energy loss function calculated from the dielectric functions. In comparison of the dielectric functions with joint density of states, the peaks in the energy region from 3 to 8 eV are mainly attributed to the transition from O 2p to Ti 3d orbitals, and those from 8 to 15 eV are assigned to the transitions from O 2p to Ti 3d, Ca 4s and Ti 4s orbitals.
  Journal of Physics Condensed Matter 12/1998; 10(16):3669. · 2.55 Impact Factor
• Article: Study on electronic structure of CaTiO3 by spectroscopic measurements and energy band calculations

  K Ueda, H Yanagi, H Hosono, H Kawazoe
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  ABSTRACT: The electronic structure of CaTiO3 was experimentally examined by x-ray and ultraviolet photoemission spectroscopy, bremsstrahlung isochromat spectroscopy (BIS) and x-ray absorption spectroscopy. The photoemission spectra and BIS spectra were compared with the partial density of states (PDOS) estimated by an energy band calculation for a fundamental interpretation of the experimental spectra. The calculated PDOS were in reasonable agreement with the experimental energy spectra and the features in the spectra were interpreted by a comparison of the spectra with the PDOS. It was concluded that the top of the valence band mainly consists of O 2p orbitals in non-bonding states and the bottom of the conduction band is composed of Ti 3d orbitals in d-p* anti-bonding states.
  Journal of Physics Condensed Matter 12/1998; 11(17):3535. · 2.55 Impact Factor
• Article: Spectroscopic measurements of thin organic films using near-field scanning optical microscopy

  L A Nagahara, H Yanagi, H Tokumoto
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  ABSTRACT: We describe the modification of a commercially available near-field scanning optical microscope, which uses an optical shear-force feedback technique to regulate probe/sample distance, for the purpose of investigating thin organic films. The commercially installed 670 nm diode laser used for feedback was replaced with a 980 nm diode laser. The replacement of the 670 nm diode laser with a diode laser which emitted at a much longer wavelength allowed for unobstructed acquisition of the fluorescence spectra of a thin layer of poly (phenylene vinylene) and a dye-doped oxide film.
  Nanotechnology 12/1998; 8(3A):A50. · 3.98 Impact Factor
• Article: Carrier generation and compensation in Y-and Nb-doped CaTiO_ {3} single crystals

  K. Ueda, H. Yanagi, H. Hosono, H. Kawazoe
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  ABSTRACT: Electrical and optical properties of Ca1-xYxTiO3 and CaTi1-xNbxO3 (x=0, 10-4, 10-3, and 10-2) single crystals were studied to understand the mechanisms of carrier generation and compensation by donor doping. Single crystals were grown by the floating-zone method in O2 flow using an infrared radiation furnace. In both Y- and Nb-doped cases, the crystals of x=0, 10-4, and 10-3 are almost insulating, and show a pale yellowish color. On the other hand, the crystals of x=10-2 are conductive and show a blue color. The conductivity measurement under air at high temperatures, from 500 to 1700 K, showed that the conductivity first decreases with an increase in dopant concentration from x=0 to 10-3, and remarkably increases with a further increase in the concentration to x=10-2. Moreover, the Seebeck measurement at high temperature showed p-type conduction in the samples with a dopant concentration up to x=10-3, which indicates the existence of unintentional acceptors. The origins of acceptors are suggested to be Al impurity and oxygen excess. All of the doped samples (x=10-4, 10-3, and 10-2) become conductive after H2 reduction at 1000 °C, and metallic behavior is observed at low temperature. Simultaneously with the remarkable increase in the conductivity after H2 reduction treatments, an optical-absorption band due to carriers appears in the near-infrared region. Carrier concentrations in the metallic samples are 1013–1015cm-3 for x=10-4, (5–6)×1018cm-3 for x=10-3, and (2–3)×1019cm-3 for x=10-2, and Hall mobilities are found to be 2–4cm2/V s for all conductive samples. The change in the electronic state of carriers from small polarons to delocalized electrons is observed in the samples of x=10-3 with increasing temperature during H2 reduction.
  Phys. Rev. B. 11/1997; 56(20).
• Article: Electroluminescence of epitaxial perylene films

  Y. Toda, H. Yanagi
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  ABSTRACT: A multilayered electroluminescent device was constructed with ordered perylene thin films as an emission layer. The epitaxial perylene film was prepared by vapor deposition onto the KCl (001) surface, then removed from the KCl substrate and sandwiched between the hole‐transport layer coated on an indium tin oxide electrode and Al top electrode. In this epitaxial layer, the perylene molecules oriented their molecular planes perpendicular to the electrode surfaces. This epitaxial device emitted an electroluminescence in the longer wavelengths of 560–580 nm due to the ordered dimeric structure. On the other hand, the device with polycrystalline perylene films exhibited an electroluminescence mainly below 500 nm. © 1996 American Institute of Physics.
  Applied Physics Letters 10/1996; 69(16):2315-2317. · 3.84 Impact Factor
• Article: Polymerization to graphite‐like nitride clusters from tetracyanoethylene vapor

  H. Yanagi, H. Itoh, Y. Ueda, M. Ashida
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  ABSTRACT: Various types of graphite‐like nitride clusters were synthesized by chemical vapor deposition of tetracyanoethylene (TCE) onto a substrate of cleaved KCl crystals at 230–300 °C. At a low reaction temperature of 230 °C, TCE was polymerized to ribbon‐like clusters, which resembled curved graphitic sheets. Upon increasing the reaction temperature to 250–300 °C, polymerized TCE condensed on KCl to form spheric clusters. The most regular structure of the clusters was obtained at 300 °C in a nanoscale needle‐like morphology. High‐resolution electron microscopy suggested that these clusters were composed of polymerized TCE layers with a interlayer distance of 0.34–0.35 nm. © 1994 American Institue of Physics.
  Applied Physics Letters 11/1994; · 3.84 Impact Factor
• Article: Epitaxial growth of C60 and KI(001) surface

  H. Yanagi, T. Sasaki
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  ABSTRACT: Epitaxial growth of C 60 is observed for vacuum‐deposited crystals on the KI(001) cleavage surface. When the KI surface is kept at 240 °C and the deposition rate is controlled to be less than a monolayer min^-1 (∼0.5 nm min^-1), rectangular platelike crystals of C 60 grow epitaxially. Transmission electron micrographs and electron diffraction patterns of the crystals exhibit that the (002) planes of the face‐centered cubic C 60 crystal orient in parallel to the KI(001) surface. This epitaxial orientation is explained in terms of the c(2×2) commensurate lattice matching.
  Applied Physics Letters 10/1994; · 3.84 Impact Factor