Takashi Fujikawa

Chiba University, Tiba, Chiba, Japan

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Publications (218)316.17 Total impact

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    ABSTRACT: The photoluminescence of Gd-doped GaN multi-quantum wells (MQWs) is presented and discussed considering the formation of a Gd 3+ :Nitrogen-vacancy (N-vacancy) complex. A lower energy photolumi-nescence peak was observed for the Gd-doped GaN MQW sample with respect to the main peak assigned to a neutral donor bound exciton (D 0 X) of the undoped GaN MQW sample. The X-ray absorption near edge structure spectrum observed at Gd L III-edge indicates a nitrogen vacancy adjacent to the Gd substituting the Ga ion in Gd-doped GaN MQW sample. Local stresses around the Gd dopants in Gd-doped GaN matrix generated due to the larger diameter of the Gd 3+ ion with respect to the Ga 3+ ion can be relieved by the creation of vacancies. The lower formation energy of N-vacancies in GaN matrix introduce them as a preferred candidate to relieve the generated stresses. A Gd 3+ :N-vacancy complex consisting of a Gd 3+ ion and the created nitrogen vacancy adjacent to the Gd 3+ dopant is likely to form in GaN:Gd matrix. The lower photoluminescence peak energy observed in the Gd-doped GaN MQW sample is assigned to the recombination of an exciton captured at the Gd 3+ :N-vacancy complex forming a small polaron-like state. A model is presented considering the small exciton-polaron population in defect sites captured around the Gd 3+ ions in the Gd-doped GaN.
    Journal of Alloys and Compounds 04/2015; 628:401. DOI:10.1016/j.jallcom.2014.11.221 · 2.73 Impact Factor
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    ABSTRACT: Some important phonon effects observed in X-ray absorption and X-ray photoemission spectra are discussed on the basis of nonequilibrium Green's function theory. This theoretical framework allows us to incorporate phonon effects, such as Debye-Waller (DW) factors, Franck-Condon (FC) factors and electron-phonon interactions in a natural way. In the case of core level excitations, we can take into account the core-hole effects in lesser Green's function g< and photoelectron propagation in greater Green's function g>. For the core-hole propagation we derive some formulas to describe the thermally displaced core functions: we have p components even for deep core s orbital due to the thermal motion. We should notice that the thermal fluctuation is quite small but it is already in the order of the spread of the core functions. Applying Mermin's theorem, we can calculate the thermal average of the hole propagator g<: Here an important ingredient is the Debye-Waller factor used in X-ray and neutron diffraction. For the pre-edge structures, the intensity associated with forbidden electric dipole transition is sensitive to the temperature compared with allowed electric quadrupole transition. We also discuss the FC and their interference, which have negligible contribution to pre-edge intensity and energy shift. The quasi-particle energy is also influenced by the core displacement which can be responsible for the peak shift of the pre-edges. We also discuss the changes of the photoelectron angular distributions caused by the thermal atomic vibration.
    Journal of Electron Spectroscopy and Related Phenomena 12/2014; 198. DOI:10.1016/j.elspec.2014.12.002 · 1.55 Impact Factor
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    ABSTRACT: Nickel oxide is a p-type semiconductor with wide band gaps of 3–4 eV. The defects of O or Ni atoms are essential to explain some specific features of the semiconductors. The information of the local structure around Ni is important to clarify the conduction mechanism. We investigate the Ni K-edge X-ray absorption fine structure spectra of the nickel oxide thin films in order to obtain the structural information around Ni. For the X-ray absorption near edge structure (XANES) analyses, we apply a full multiple scattering theory. The experimental results suggest that the crystallinity of the thin films depends on the annealing temperature. The distances of Ni–O and Ni–Ni are 1.984 and 2.747 Å for the sample annealed at 100 °C and 2.084 and 2.947 Å for the sample annealed at 300 °C, respectively. Our multiple scattering calculations can satisfactorily explain the experimental result. Copyright © 2014 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 10/2014; 46(10-11). DOI:10.1002/sia.5455 · 1.39 Impact Factor
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    ABSTRACT: We have studied the surface and bulk plasmon losses associated with Li 1s photoemission. Here, full multiple scatterings of photoelectrons are taken into account before and after the plasmon losses within the quantum Landau formula that can describe overall features of the photoemission bands. We compared the depth profiles of the Li 1s single-loss spectra with full multiple scatterings and without elastic scatterings. They are strongly influenced by the elastic scatterings. The model accounting for the full multiple scatterings gives a rapidly decaying function of the depth because of the defocusing effects. Copyright © 2014 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 10/2014; 46(10-11). DOI:10.1002/sia.5504 · 1.39 Impact Factor
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    ABSTRACT: We have measured O 1s photoelectron angular distributions in coincidence with the O+-CO+ fragment ion pairs of CO2 molecules at the photoelectron energies of 90, 120 and 150 eV. The observed molecular frame photoelectron angular distributions are left-right asymmetric as a consequence of an enhancement in bond-breaking probability for the CO bond involving the O atom with the 1s hole, compared with the other CO bond. Comparison of the experimental data with the calculations due to our semi-empirical model enables us to deduce the value of 1.4 for this enhancement in the relative bond breaking probability.
    Chemical Physics Letters 07/2014; 608:152–156. DOI:10.1016/j.cplett.2014.05.086 · 1.99 Impact Factor
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    ABSTRACT: We have measured C 1s photoelectron angular distributions (PADs) in coincidence with the CO+–O+ fragment ion pairs of CO2 molecules at the photoelectron energies of 85, 120 and 150 eV. The observed left–right asymmetric PADs have been well reproduced by our theoretical model taking into account the two degenerate zero-point bending vibrations. This leads to a conclusive result on the interpretation of such PADs; although it has been believed so far that they are the molecular frame, in fact they are the recoil frame.
    Journal of Physics B Atomic Molecular and Optical Physics 03/2014; 47(7):071002. DOI:10.1088/0953-4075/47/7/071002 · 1.92 Impact Factor
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    ABSTRACT: We study the angular and energy dependence of surface and bulk plasmon losses accompanying deep core excitations in simple metals. Here full multiple scatterings of photoelectrons are taken into account before and after the plasmon losses within the quantum Landau formula, which can describe overall features of the photoemission bands. For example, multiple plasmon loss features can be calculated by use of the formula. Two simple metals, Al and Na, are studied here. The depth profiles of the plasmon losses are strongly influenced by the elastic scatterings. The model assuming single elastic scatterings overestimates the losses from deep emitters due to the forward focusing effects, whereas the model accounting for full multiple scatterings gives a much rapidly decaying function of the depth due to the defocusing effects and rich structures due to the photoelectron diffraction. The single elastic scattering approximation gives a poor result both for the depth profiles and for the loss spectra. The present multiple scattering calculations successfully explain the azimuthal dependence of the loss spectra, which reflect the local geometry around the emitters.
    Physical Review B 12/2013; 89(4). DOI:10.1103/PhysRevB.89.045110 · 3.66 Impact Factor
  • Source
    APPC12, Makuhari, Japan; 07/2013
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    ABSTRACT: We provide a molecular structure determination method, based on multiple-scattering x-ray photoelectron diffraction (XPD) calculations. This method is applied to our XPD data on several molecules having different equilibrium geometries. Then it is confirmed that, by our method, bond lengths and bond angles can be determined with a resolution of less than 0.1 Å and 10∘, respectively. Differently from any other scenario of ultrafast structure determination, we measure the two- or three-dimensional XPD of aligned or oriented molecules in the energy range from 100 to 200 eV with a 4π detection velocity map imaging spectrometer. Thanks to the intense and ultrashort pulse properties of x-ray free-electron lasers, our approach exhibits the most probable method for obtaining ultrafast real-time structural information on small to medium-sized molecules consisting of light elements, i.e., a “molecular movie.”
    Physical Review A 06/2013; 87(6). DOI:10.1103/PhysRevA.87.063417 · 2.99 Impact Factor
  • Ming-Hui Shang, Takashi Fujikawa, Nubuo Ueno
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    ABSTRACT: Recoil effects in valence band X-ray photoelectron spectroscopy (XPS) are studied for both abbtrifluorostyrene and styrene molecular crystal systems. The gradual changes of XPS spectra excited by several photon energies are theoretically investigated within the tight-binding approximation and harmonic approximation of lattice vibrations, and have been explained in terms of not only atomic mass but also atomic orbital (AO) population. The recoil effect of valence band photoemission strongly depends on the population and partial photoionization cross section (PICS) of AOs as well as the mass of composite ion. In abb-trifluorostyrene F 2p dominant bands show the recoil shift close to free F atom recoil shift, and C 2s dominant bands show that to free C atom recoil shift, whereas the mixed bands of C and F give rise to the peak asymmetries due to their different recoil shifts. For these systems hydrogen contribution is negligibly small which is contrast to our previous results for the crystals composed of small organic molecules. We also discuss some potential uses of the recoil shifts for these systems.
    Analytical Chemistry 02/2013; 85(7). DOI:10.1021/ac4000865 · 5.83 Impact Factor
  • Akihiro Koide, Takashi Fujikawa, Nobuyuki Ichikuni
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    ABSTRACT: We briefly review the basic theory and recent applications in the X-ray absorption fine structure (XAFS) spectroscopy. First we discuss a dressed one-electron XAFS formula starting from many-body scattering theory, where important many-body effects, intrinsic and extrinsic losses, and optical potential are naturally introduced. Next multiple scattering renormalization, and spherical wave effects are discussed. Phonon effects such as Debye-Waller factors are also discussed. Some interesting XAFS applications, in particular, ultrafast XAFS and XAFS applications to nano-particles are discussed in some detail.
    Journal of Electron Spectroscopy and Related Phenomena 01/2013; 195. DOI:10.1016/j.elspec.2013.11.007 · 1.55 Impact Factor
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    ABSTRACT: The spin-polarized electronic structures across the interface between single-layer graphene and a Ni(111) thin film are explored by employing depth-resolved X-ray absorption and magnetic circular dichroism spectroscopy with atomic layer resolution. The depth-resolved Ni L2,3-edge analysis clarifies that the Ni atomic layers adjacent to the interface show a transition of the spin orientation to the perpendicular one in contrast to the in-plane one in the bulk region. The C K-edge analysis reveals the intensification of the spin–orbit interactions induced by the π–d hybridization at the interface as well as out-of-plane spin polarization in the π band region of graphene. The present study indicates the importance of the interface design at the atomic layer level for graphene-based spintronics.
    Journal of Materials Chemistry 01/2013; 1:5533-5537. DOI:10.1039/c3tc30872c · 6.63 Impact Factor
  • Kei Takahashi, Takashi Fujikawa
    Shikizai Kyokaishi 01/2013; 86(2):62-66. DOI:10.4011/shikizai.86.62
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    ABSTRACT: 1s photoelectron angular distributions from fixed-in-space CO2, NO2, BF3 and CH3F molecules have been calculated by X-ray photoelectron diffraction (XPD) theory with muffin-tin-type molecular potential. For all the molecules, the calculated results show good agreements with those by density functional theory in the energy region ≳100 eV. Furthermore, for all the molecules experimental data on the angular distributions in such energy region are well reproduced by the XPD theory. These intensive studies lead to a rather general rule that the XPD theory is an adequate tool to describe high-energy photoelectron angular distributions for any single oriented molecules.
    Journal of Electron Spectroscopy and Related Phenomena 11/2012; 185(11):535–545. DOI:10.1016/j.elspec.2012.10.009 · 1.55 Impact Factor
  • Takashi Fujikawa, Katsumi Kaneko
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    ABSTRACT: We theoretically discuss X-ray absorption intensity in high-energy region far from the deepest core threshold to explain the morphology-dependent mass attenuation coefficient of some carbon systems, carbon nanotubes (CNTs), highly oriented pyrolytic graphite (HOPG) and fullerenes (C60). The present theoretical approach is based on the many-body X-ray absorption theory including the intrinsic losses (shake-up losses). In the high-energy region the absorption coefficient has correction term dependent on the solid state effects given in terms of the polarization part of the screened Coulomb interaction Wp. We also discuss the tail of the valence band X-ray absorption intensity. In the carbon systems C 2s contribution has some influence on the attenuation coefficient even in the high energy region at 20 keV.
    Journal of Electron Spectroscopy and Related Phenomena 11/2012; 185(11):509–511. DOI:10.1016/j.elspec.2012.10.005 · 1.55 Impact Factor
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    ABSTRACT: We measured molecular-frame photoelectron angular distributions (MFPADs) for a carbon 1s level of CO molecules in the kinetic energy range of up to 150 eV. X-ray photoelectron diffraction (XPD) theory was adopted to interpret the profiles of the MFPADs. Computational experiments within the framework of XPD theory were performed to demonstrate the emergence of interference effects between the direct photoelectron and scattered waves in the MFPAD profiles.
    Journal of Physics B Atomic Molecular and Optical Physics 09/2012; 45(19):194007. DOI:10.1088/0953-4075/45/19/194007 · 1.92 Impact Factor
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    ABSTRACT: Hard and soft x-ray photoelectron spectroscopy was performed for core and valence electrons in V3Si. The recoil effects on the photoelectron emission were observed not only in the case of the Si 2p core level, but also for the valence band, although such effects were not observed for the V 2p and V 3p core levels. The valence-band results are compared with the results of the theoretical model calculation based on the tight-binding cluster approximation and harmonic approximation for lattice vibrations. The Si 2p core-level results interpreted as due to the single nucleus recoil effects are consistent with the interpretation of those of the valence electrons calculated on these approximations. From the E-B dependence of the valence-band recoil shifts, useful information is obtained on the Si 3p partial density of states near the Fermi level.
    Physical review. B, Condensed matter 07/2012; 86(3). DOI:10.1103/PhysRevB.86.035146 · 3.66 Impact Factor
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    ABSTRACT: In this work we have measured Co K- and L2,3-edge X-ray absorption near edge structure (XANES) and Co L2,3-edge X-ray magnetic circular dichroism (XMCD) spectra, and also carried out their calculations for C60Cox compounds. The observed XANES and XMCD are sensitive to the Co concentration. In the low density region (x = 1.0 − 1.5), one Co is surrounded by three C60 balls. The Co L2,3-edge XMCD analyses gives the spin magnetic moment on Co in the range 0.5–0.9μB.In the high density region (x = 4.0 − 4.3) calculated XANES for one of the proposed model, 4C604Co model, by Nakajima and Kaya give rather good agreement with the observed XANES, where a Co4 cluster is surrounded by four C60 balls. Fourier transformed extended X-ray absorption fine structure (EXAFS) is well explained by this model. In contrary to these results, the calculated XMCD spectrum for that high-symmetric model shows rather poor agreement with the observed spectrum. Lowering the symmetry, we can improve the agreement. The cluster density functional calculations for the 4C604Co model show that spin quintet (4μB) is in the lowest energy. Each Co has 1μB in ferromagnetic like order. The results also indicate that the band gap for the minority spin is smaller than that for the majority spin. This result provides us with fundamental information to understand the tunnel magnetoresistance (TMR) mechanism in those systems.
    Journal of Electron Spectroscopy and Related Phenomena 03/2012; 185(s 1–2):32–38. DOI:10.1016/j.elspec.2011.11.001 · 1.55 Impact Factor
  • e-Journal of Surface Science and Nanotechnology 01/2012; 10:565-568. DOI:10.1380/ejssnt.2012.565
  • e-Journal of Surface Science and Nanotechnology 01/2012; 10:145-148. DOI:10.1380/ejssnt.2012.145

Publication Stats

1k Citations
316.17 Total Impact Points

Institutions

  • 1996–2015
    • Chiba University
      • • Graduate School of Advanced Integration Science
      • • Graduate School of Science
      • • Department of Image and Materials Science
      • • Faculty of Science
      • • Department of Chemistry
      Tiba, Chiba, Japan
  • 2012
    • High Energy Accelerator Research Organization
      • Photon Factory
      Tsukuba, Ibaraki, Japan
  • 2010
    • Nagoya University
      • Graduate School of Science
      Nagoya-shi, Aichi-ken, Japan
  • 2007
    • Hokkaido University
      • Catalysis Research Center
      Sapporo, Hokkaidō, Japan
  • 1993–2006
    • Hirosaki University
      • Department of Advanced Physics
      Khirosaki, Aomori, Japan
  • 1980–2006
    • Yokohama National University
      Yokohama, Kanagawa, Japan
  • 1999–2002
    • University of Washington Seattle
      • Department of Physics
      Seattle, Washington, United States
  • 1995
    • Tosoh Corp.
      Kawasaki Si, Kanagawa, Japan
  • 1990–1993
    • Lund University
      • Department of Theoretical Physics
      Lund, Skåne, Sweden
  • 1989
    • Kyoto University of Education
      Kioto, Kyōto, Japan
  • 1974–1986
    • The University of Tokyo
      • Department of Chemistry
      Tōkyō, Japan