Naohisa Happo

Hiroshima City University, Hirosima, Hiroshima, Japan

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Publications (48)71.77 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: X-ray fluorescence holography (XFH) is a method for investigating atomic order up to the medium ranges, and can provide 3D atomic images around specific elements within a radius of nm order. In addition to these characteristics, XFH is sensitive to positional fluctuations of atoms, and therefore it is useful for characterizing the local lattice distortions around specific elements. We have applied XFH to dopants and mixed crystals. We found interesting features in local lattice distortions, such as the displacements of first-neighbor atoms around dopants, far-sighted views of the atomistic fluctuations in mixed crystals, and the coexistence of distorted/undistorted sites in the same material.
    Journal of Electron Spectroscopy and Related Phenomena 08/2014; · 1.71 Impact Factor
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    ABSTRACT: To search the atomic sites of Mn impurities in Bi2Te3Mn0.1 topological insulator, a Mn Kα fluorescence x-ray holography experiment was performed. The reconstructed atomic image around the central Mn atom reveals a hexagonal configuration, where the local lattice constant is slightly larger than the original Bi2Te3 crystal. Thus, the most plausible atomic position of Mn impurities is the substitution with Bi or Te atom. The further-distant atomic images are hardly observed unlike usual crystals, indicating a formation of flat clusters around Mn in this functional crystal.
    Journal of Physics Conference Series 04/2014; 502(1):012024.
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    ABSTRACT: Nonparabolic nature of optical interband transitions of multi-quantum wells structure is important for related device designs. The optical interband transitions of In0.53Ga0.47As/In0.52Al0.48As MQWs including 20-nm-wide well clearly appeared on photocurrent spectra with polarized light source. The nonparabolic nature of conduction subbands was decided from experimental interband transitions. The electron effective mass smoothly increased from 0.041 m0 to 0.07 m0 in the conduction quantum well.
    Conference Proceedings - International Conference on Indium Phosphide and Related Materials 05/2013; 10(5):727-731.
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    ABSTRACT: To clarify lattice distortions induced by adding Ga atoms in the InSb crystal, Ga Kα x-ray fluorescence holography (XFH) experiments were carried out on an In0.995Ga0.005Sb diluted mixed single crystal, and three-dimensional atomic images around the Ga atoms were reconstructed. Although the atomic images are located almost at ideal positions of the InSb crystal, some differences can be observed for only the first- and second-neighboring atoms. By combining them with x-ray absorption fine structure data, large spatial fluctuations of the first-neighboring atoms appear in the angular direction, which can be clarified from the present XFH results. From the XFH results, it is concluded that lattice distortions are limited within the second neighbors in this diluted mixed crystal, in contrast to five chemical bonds in a heavily doped mixed crystal of Zn0.4Mn0.6Te reported previously.
    Physical review. B, Condensed matter 03/2013; 87(9). · 3.77 Impact Factor
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    ABSTRACT: X-ray fluorescence holography (XFH) is a method of atomic resolution holography which utilizes fluorescing atoms as a wave source or a monitor of the interference field within a crystal sample. It provides three-dimensional atomic images around a specified element and has a range of up to a few nm in real space. Because of this feature, XFH is expected to be used for medium-range local structural analysis, which cannot be performed by x-ray diffraction or x-ray absorption fine structure analysis. In this article, we explain the theory of XFH including solutions to the twin-image problem, an advanced measuring system, and data processing for the reconstruction of atomic images. Then, we briefly introduce our recent applications of this technique to the analysis of local lattice distortions in mixed crystals and nanometer-size clusters appearing in the low-temperature phase of a shape-memory alloy.
    Journal of Physics Condensed Matter 03/2012; 24(9):093201. · 2.22 Impact Factor
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    ABSTRACT: The local atomic structure around Mn atoms in Ge1-xMnxTe thin-film single crystal has been investigated by X-ray fluorescence holography (XFH) at room temperature. The obtained atomic image suggests that the Mn atoms replace the Ge atoms in the host GeTe, and the Mn position is stable in the exact positions of the anion fcc sublattice. The XFH result also suggests the fluctuation of the Ge positions or the cation vacancies.
    Japanese Journal of Applied Physics 05/2011; 50(5):05FC11. · 1.07 Impact Factor
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    ABSTRACT: Three-dimensional (3D) atomic images around the Tl atoms in single-crystal TlInSe2 thermoelectric material in the incommensurate phase were reconstructed by Tl LIII X-ray fluorescence holography measured at room temperature. In the obtained 3D image, the In atoms are clearly visible at the proper positions in the normal phase. The Tl atomic images are also observed but very weakly. By comparing the experimental image with a theoretically calculated one, it is concluded that only the Tl atoms have large spatial fluctuations, which are related to the incommensurate phase of this material.
    Japanese Journal of Applied Physics 05/2011; 50. · 1.07 Impact Factor
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    ABSTRACT: X-ray fluorescence holographic study on a room-temperature ferromagnetic semiconductor film of ZnSnAs2:Mn was performed using a strong X-ray beam of third generation synchrotron radiation of SPring-8. The real space reconstructions of the environments around Mn atoms were successfully visualized from the observed holograms despite the very small amount of Mn atoms. The reconstructions revealed that the Mn atoms occupy the cation (Zn or Sn) site.
    Japanese Journal of Applied Physics 01/2011; 50(1):01BF05. · 1.07 Impact Factor
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    ABSTRACT: X-ray fluorescence holography (XFH) is a powerful technique for determining three-dimensional local atomic arrangements around a specific fluorescing element. However, the raw experimental hologram is predominantly a mixed hologram, i.e., a mixture of hologram generated in both normal and inverse modes, which produces unreliable atomic images. In this paper, we propose a practical subtraction method of the normal component from the inverse XFH data by a Fourier transform for the calculated hologram of a model ZnTe cluster. Many spots originating from the normal components could be properly removed using a mask function, and clear atomic images were reconstructed at adequate positions of the model cluster. This method was successtully applied to the analysis of experimental ZnTe single crystal XFH data.
    Japanese Journal of Applied Physics 01/2010; 49. · 1.07 Impact Factor
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    ABSTRACT: In order to investigate lattice distortions in CdTe crystal when smaller ZnTe4 tetrahedra are added as impurities, we measured Zn Kα X-ray fluorescence holography (XFH) on Cd0.96Zn0.04Te mixed crystal, and obtained atomic images around the Zn atoms in a wide spatial range. The image intensity of the first-neighbor anion Te atoms is remarkably weak compared to those of the distant atoms. Thus, the lattice of this mixed crystal is distorted only in the first nearest neighbor range. This result is different from those of diluted magnetic semiconductors Zn1−xMnxTe and Cd1−xMnxTe, or In1−xGaxSb mixed crystal, whose lattice distortions continue to the intermediate range up to the five chemical bonds.
    Journal of Electron Spectroscopy and Related Phenomena - J ELECTRON SPECTROSC RELAT PH. 01/2010; 181(2):154-158.
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    ABSTRACT: Core-level photoemission spectroscopy and anomalous x-ray scattering (AXS) measurements were performed for the Pd42.5Ni7.5Cu30P20 (PNCP) excellent metallic glass to investigate the chemical nature and local atomic structure, and the results were compared to those in Pd40Ni40P20 and Pd40Cu40P20. The P 2p core levels clearly separate into two states, indicating that the P atoms have two different chemical sites, which is a strong experimental proof for the existence of an elastic inhomogeneity. From the AXS close to the Pd K edge, a specific Pd-P-Pd atomic configuration was observed, which is related to the stable state in the P 2p core levels. All of the core levels measured in PNCP have the deepest binding energies among these glasses, indicating the most stable electronic states. Local structure around the P atoms is discussed by the AXS data and a metastable crystal appeared in a supercooled metallic alloy close to PNCP.
    Physical review. B, Condensed matter 11/2009; 80(17). · 3.77 Impact Factor
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    ABSTRACT: In order to bridge experimental results for the atomic structure in mixed crystals between x-ray diffraction showing a Vegard’s law and x-ray absorption fine structure spectroscopy indicating preservations of each Pauling’s bond length, an x-ray fluorescence holography (XFH) experiment was carried out on Zn0.4Mn0.6Te single crystal for drawing a three-dimensional atomic image around the Zn atom. The XFH image intensities of Te atoms in Zn0.4Mn0.6Te with respect to the reference ZnTe are almost constant beyond the five chemical bonds from the central Zn atom, while the nearer Te atomic images are much weaker, indicating that the Pauling’s bond length interconnects with Vegard’s law at about five chemical bonds. The randomness of the atomic positions is well explained by a locomotive wheel atomic configuration model.
    Physical review. B, Condensed matter 10/2009; 80(13). · 3.77 Impact Factor
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    ABSTRACT: Nonparabolic band structure of InGaAs/InAlAs multi-quantum wells was studied theoretically and experimentally. The electron effective mass was derived even from eigen-states and this nonparabolicity was explicitly determined as a function of energy. Electron eigen-state energies applying Kane’s bulk band theory fitted very well with our experiments in the multi-quantum wells.
    Optical and Quantum Electronics 01/2009; 41(11):903-912. · 1.08 Impact Factor
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    ABSTRACT: The powder X-ray diffraction (PXRD) and X-ray absorption fine structure (XAFS) measurements were performed on In1-xGaxSb mixed polycrystals in the whole x range, and the former indicates Vegard's law, i.e., the lattice constant linearly changes with varying the concentration x, while the latter reveals a Pauling's bond length, i.e., an almost constant In–Sb or Ga–Sb interatomic distance is preserved all over the concentration range. In order to clarify the lattice distortions by adding the Ga atoms, Ga Kα X-ray fluorescence holography (XFH) experiment was carried out on In0.995Ga0.005Sb mixed single crystal, and a three-dimensional atomic image around the Ga atoms was constructed. Although the atomic images locate almost on the atomic positions of the InSb crystal, some deviations (distortions from the lattice) can be observed, in particular for the first-, and second-nearest-neighboring atoms. We found that the lattice distortion by the Ga atoms is induced within the fifth neighbor atoms. From the XFH result, we discuss how the small GaSb4 tetrahedra can be stably located in the InSb lattice with the larger lattice constant.
    Journal of Crystal Growth 01/2009; 311(3):978-981. · 1.55 Impact Factor
  • N. Happo, K. Hayashi, S. Hosokawa
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    ABSTRACT: The lattice constant of diluted magnetic semiconductors Cd1−xMnxTe obtained from X-ray diffraction linearly changes with the Mn concentration, while extended X-ray absorption fine structure (EXAFS) studies showed that the bond lengths are almost kept as their own values. In order to investigate how the MnTe4 tetrahedra can be stretched in the large Cd–Te lattice, we have measured Mn Kα X-ray fluorescence holography on Cd0.6Mn0.4Te single crystal and obtained the wider atomic images around the Mn atoms than EXAFS. The atomic image intensities of the first-, third- and fifth-neighbor anion Te became weak, and the fourth- and sixth-neighbor cation Cd or Mn intensities almost unchanged. The second-neighbor Cd or Mn atomic images look highly shifted from the appropriate positions. Thus, it is concluded that the lattice of this mixed crystal is highly distorted in the intermediate range, and the distortion in the anion sublattice is induced to the larger distance from the central Mn atom than that in the cation sublattice.
    Journal of Crystal Growth 01/2009; 311(3):990-993. · 1.55 Impact Factor
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    ABSTRACT: The phase transition behavior of the local structure around Fe in a shape-memory-alloy-related material, Ti50Ni44Fe6, was evaluated by x-ray fluorescence holography. The Fe Kα holograms were recorded at 225 and 100 K, which correspond to the parent and commensurate phases, respectively. The atomic images at both the phases show that the first neighbor Ti atoms around Fe, fluctuating in the parent phase, is strongly stabilized in the commensurate phase and that a clusterlike structure with a radius of 8 Å is formed in this lower temperature phase. These dynamically structural changes offer important keys to understanding the precursors to the martensite phase transition of the TiNi series.
    Phys. Rev. B. 01/2009; 80(6).
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    ABSTRACT: X-ray fluorescence holography is a relatively new experimental method for the local three-dimensional atomic structure in an element-specific way. We measured the holograms of Ti50Ni44Fe6 single crystal at 100 and 225 K, which correspond to its commensurate and parent phases, respectively. The atomic images of both phases were successfully reconstructed. The difference of both images shows a possibility that a cluster structure around the Fe atoms forms with a radius of 8 Å in the commensurate phase.
    Journal of Crystal Growth 01/2009; · 1.55 Impact Factor
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    ABSTRACT: In-house photoemission and inverse-photoemission spectra (PES and IPES) were mea- sured on Pd42.5Ni7.5Cu30P20 and Pd40Ni40P20 bulk metallic glasses together with incident photon- energy dependent PES spectra using synchrotron radiation. Minima are observed at slightly higher energy than the Fermi level. Pd 4d, Ni 3d and Cu 3d partial density of states were estimated from the PES and soft X-ray emission spectra, and the feature of the electronic structure in these glasses were discussed in detail in connection to the excellent glass-forming ability.
    Reviews on Advanced Materials Science. 01/2008; 18:180-183.
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    ABSTRACT: In-house photoemission and inverse-photoemission spectra (PES and IPES) were measured on Pd42.5Ni7.5Cu30P20 and Pd40Ni40P20 bulk metallic glasses in order to clarify the origin of excellent glass-forming ability from the viewpoint of electronic structure. Minima are observed for both the metallic glasses at a slightly higher energy than the Fermi level. Incident photon-energy dependent PES spectra were obtained using synchrotron radiation and the Pd 4d partial density of states (DOS) was estimated from the PES data. Soft X-ray emission spectra were also measured near the Ni and Cu 2p3/2 absorption edges to evaluate, respectively, the Ni and Cu 3d partial DOS in the valence band. The Pd 4d and the Ni and Cu 3d partials in the conduction band were obtained from X-ray absorption spectra around the Pd 3p3/2 and Ni and Cu 2p3/2 absorption edges, respectively. It was found that the Pd 4d partial DOS near the Fermi energy largely decreases and becomes localized by replacing the Ni atoms with the Cu atoms, which may be closely related to the excellent glass-forming ability of the Pd42.5Ni7.5Cu30P20 bulk metallic glass due to a selective formation of Pd–P covalent bonds.
    Acta Materialia 06/2007; · 3.94 Impact Factor
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    ABSTRACT: The authors discuss x-ray fluorescence holography (XFH) measurements taken from an epitaxial layer of the digital versatile disk random access memory (DVD-RAM) material Ge2Sb2Te5 grown on a single-crystal GaSb(100) substrate. By using fluorescent photons from the Ge atoms in the matrix, a three-dimensional atomic image was obtained around the Ge atoms in a Ge2Sb2Te5 film; details of the three-dimensional atomic arrangement will aim at clarification of the high-speed writing and erasing mechanism of the laser-induced crystal-amorphous phase transition in this DVD-RAM material. Analysis of the XFH images revealed that the epitaxial layer did not possess a hexagonal structure as in the equilibrium phase of Ge2Sb2Te5, but a cubic structure with tetrahedral site symmetry about Ge atoms, different from the previous powder diffraction result. The present structure may support the umbrella-flip model of the Ge atoms between the octahedral site in the distorted rocksalt crystal and the tetrahedral site in the amorphous phase on the laser-induced phase transition.
    Applied Physics Letters 03/2007; 90(13):131913-131913-3. · 3.52 Impact Factor

Publication Stats

94 Citations
71.77 Total Impact Points

Institutions

  • 1998–2014
    • Hiroshima City University
      • Graduate School of Information Sciences
      Hirosima, Hiroshima, Japan
  • 1994–1998
    • Hiroshima University
      • Department of Biomaterials Science
      Hiroshima-shi, Hiroshima-ken, Japan
  • 1993–1994
    • Tokuyama College of Technology
      Kakuyama, Yamaguchi, Japan