K Kodama

Japan Atomic Energy Agency, Muramatsu, Niigata, Japan

Are you K Kodama?

Claim your profile

Publications (194)497.18 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The effects of lithium absorption on the crystal structure and electronic properties of IrSi3, a binary silicide with a noncentrosymmetric crystal structure, were studied. X-ray and neutron diffraction experiments revealed that hexagonal IrSi3 (space group P6_3mc) transforms into trigonal Li2IrSi3 (space group P31c) upon lithium absorption. The structure of Li2IrSi3 is found to consist of a planar Kagome network of silicon atoms with Li and Ir spaced at unequal distances between the Kagome layers, resulting in a polar structure along the c-axis. Li2IrSi3 exhibited type-II superconductivity with a transition temperature of Tc = 3.8 K, displaying a structure type that no previous superconductors have been reported to have.
    07/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have performed neutron diffraction measurement on a single crystal of parent compound of iron-based superconductor, BaFe$_2$As$_2$ at 12~K. In order to investigate in-plane anisotropy of magnetic form factor in the antiferromagnetic phase, the detwinned single crystal is used in the measurement. The magnetic structure factor and magnetic form factor are well explained by the spin densities consisting of $3d_{yz}$ electrons with a fraction of about 40~\% and the electrons in the other four $3d$ orbitals with each fraction of about 15~\%. Such anisotropic magnetic form factor is qualitatively consistent with the anisotropic magnetic behaviors observed in the antiferromagnetic phase of the parent compound of iron-based superconductor.
    06/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The current-voltage characteristics in the charge order state of the two-dimensional organic conductor α-(BEDT-TTF)_{2}I_{3} exhibit power law behavior at low temperatures. The power law is understood in terms of the electric-field-dependent potential between electrons and holes, which are thermally excited from the charge order state. The power law exponent steeply changes from 1 to 3 in the range from 30 to 45 K with decreasing temperature, thereby suggesting the occurrence of a Kosterlitz-Thouless-type transition; many (few) unbound electron-hole pairs are thermally excited above (below) the transition. The effects of the finite size and interlayer coupling on the power law behavior are discussed.
    Physical Review Letters 05/2013; 110(19):196602. · 7.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have performed electrical resistivity measurements of EuFe2As2 under hydrostatic pressures (P) up to 3.2 GPa and magnetic fields up to 15 T. In zero field, the superconducting (SC) ground state with a sharp transition to zero resistivity, indicative of bulk superconductivity, emerges at Tc ~ 30 K in a pressure range from 2.5 GPa to ~ 3.0 GPa. Under applied field of 15 T, the SC transition is suppressed to lower temperatures with increasing pressure, suggesting that the application of pressure reduces the upper critical field Bc2 for the P-induced superconductivity. Meanwhile, the antiferromagnetic (AF) order of the Eu2+ moments survives up to 3.2 GPa, the highest pressure in the experiments, with no significant change in the AF ordering temperature and the magnetic field effect.
    Journal of Physics Conference Series 12/2012; 391(1):2132-.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Magnetic torque measurements have been carried out for two-dimensional magnetic-field-induced organic superconductor λ-(BETS)2FeCl4, where BETS stands for bis(ethylenedithio)tetraselenafulvalene, to investigate the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. The in-plane upper critical field of the field-induced-superconducting phase steeply decreases with decreasing temperature below 2.2 K. The in-plane field dependence of the diamagnetic susceptibility shows a significant decrease below 25 T at low temperatures, showing that magnetic fluxes are less excluded from the sample. These results show the presence of the FFLO phase with the tricritical point between the FFLO, homogeneous superconducting, and paramagnetic metallic phases at 2.2 K and 23 T. The stability of the FFLO phase is also investigated as a function of the magnetic field angle and compared with theories.
    Physical review. B, Condensed matter 05/2012; 85(17). · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Low-energy spin excitations on polycrystalline LaFeAsO1−xFx samples have been studied by inelastic neutron scattering. The Q-integrated dynamical spin susceptibility χ"(ω) at 11 meV decreases with increasing fluorine content x across the phase transition from antiferromagnetic to superconducting phases. For superconducting samples, a peak in χ"(ω) develops at ω = Eres below Tc, accompanied by its reduction below Eres. Fe 3d orbital contribution on the dynamical spin susceptibility and the electron scattering between Γ and M Fermi surfaces are discussed.
    Journal of Physics Conference Series 02/2012; 340(1):012075.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The tetragonally distorted antiperovskite Mn3CuN exhibits large magnetostriction of up to 2000 ppm, due to the ferromagnetic shape memory effect. Magnetocrystalline anisotropy, which is deeply involved in the magnetostriction, is investigated for Mn3CuN via quantitative evaluation of spin and orbital magnetic moments using soft x ray magnetic circular dichroism. Although Cu and Mn have finite magnetic contributions, the Cu contribution may be too small to dominate the magnetism of Mn3CuN. The orbital magnetic moment of Mn is found to be negligibly small, suggesting small magnetocrystalline anisotropy. Even with small magnetocrystalline anisotropy, the system exhibits ferromagnetic shape memory effects, presumably because the simultaneously small crystallographic anisotropy enhances the magnetic shear stress acting across the twinning plane.
    Journal of Applied Physics 07/2011; 110(2). · 2.21 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Magnetization studies in Nd1-xTbxAl2 (x
    07/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We studied X-ray Fourier transform holography using separated holography-mask and sample geometry. The method was successfully applied to the imaging of the magnetic nanostructure using soft X-ray in addition to the cross-sectional imaging of Cu interconnect lines using hard X-ray.
    Diamond Light Source Proceedings. 04/2011; 1.
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have developed a 25 T pulsed high magnetic field system for soft X-ray Magnetic Circular Dichroism: XMCD. The ultra-high vacuum chamber with a pulse magnet coil is installed. By using a newly developed bipolar capacitor bank, the XMCD of paramagnetic Gd2O3 at the M5 and the M4 edges was clearly observed at low temperatures. The present system is capable of measuring XMCD of field induced moments in various compounds including paramagnets and antiferromagnets.
    Journal of Electron Spectroscopy and Related Phenomena 04/2011; 184(s 3–6):338–341. · 1.71 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Inelastic neutron scattering has been performed on powder sample of an iron-based superconductor BaFe2(As0.65P0.35)2 with superconducting transition temperature (Tc) = 30 K, whose superconducting (SC) order parameter is expected to have line node. In the normal state, constant-E scan of dynamical structure factor, S(Q, E), exhibits a peak structure centered at momentum transfer Q ∼ 1.20 Å−1, corresponding to antiferromagnetic wave vector. Below Tc, the redistribution of the magnetic spectral weight takes place, resulting in the formation of a peak at E ∼ 12 meV and a gap below 6 meV. The enhanced magnetic peak structure is ascribed to the spin resonance mode, evidencing sign change in the SC order parameter similar to other iron-based high-Tc superconductors. It suggests that fully-gapped s± symmetry dominates in this superconductor, which gives rise to high-Tc (=30 K) despite the nodal symmetry.Highlights► Inelastic neutron scattering has been performed on BaFe2(As0.65P0.35)2. ► Unexpectedly, we observed spin resonance at Q ∼ 1.2 A−1 and twice enhancement ratio. ► These are similar to other iron-based superconductors with fully-gapped s± symmetry. ► It suggests that fully-gapped s± symmetry dominates in this system. ► This situation makes BaFe2(As0.65P0.35)2 high-Tc (=30 K) despite nodal symmetry.
    Physica C Superconductivity 01/2011; 471:643-646. · 0.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We developed a scanning-type x-ray Fourier transform holography and applied it to both the soft and hard x-ray regions. For this approach, we prepared the holography mask and the imaging object separately and placed them in contact with each other. In this configuration, the illuminated area can be changed by moving the sample relative to the mask via the translation stage. In this case, an image can be recovered by Fourier inversion of the hologram as in the usual lensless Fourier transform holography. This method also reduces the relative vibration between the mask and sample, and provides good quality images. The method was successfully applied to large-area imaging of magnetic domains in a Co/Pt perpendicular magnetic film with soft x-rays. In addition, cross-sectional imaging of Cu interconnect lines was performed in the hard x-ray region.
    AIP Conference Proceedings 01/2011; 1365(1):277-280.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have performed powder neutron diffraction on the new superconductor, CeNi0.8Bi2 with a superconducting transtion temperature Tc~ 4.2 K. The structural parameters of this compound at room temperature are determined by Rietveld analysis. Below about 5 K, the clear magnetic Bragg peaks with propagation vector q = (0 0 0) are observed. The observed intensities of magnetic Bragg peaks can be explained by the magnetic structure that the two Ce moments in the unit cell are antiparallel along the c axis. The magnetic Bragg peaks are observed in the superconducting state, indicating the coexistence of the antiferromagnetic ordering and the superconductivity in this compound. The intensity of magnetic Bragg peak monotonously increases with decreasing temperature below TN and does not exhibit apparent anomaly at Tc, obviously different from cases of heavy fermion superconductors in which the magnetic ordering and the superconductivity coexist, for example, Cd-doped CeCoIn5. These results suggest that the 4f electron of the Ce atom is not coupled with the superconducting carrier, and the magnetic ordering is almost independent of the superconductivity in CeNi0.8Bi2.
    Physical review. B, Condensed matter 01/2011; 83. · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Low-energy spin excitations have been studied on polycrystalline LaFeAsO1−xFx samples by inelastic neutron scattering. The Q-integrated dynamical spin susceptibility χ″(ω) of the superconducting samples is found to be comparable to that of the magnetically ordered parent sample. On the other hand, χ″(ω) almost vanishes at x=0.158, where the superconducting transition temperature Tc is suppressed to 7K. In addition, χ″(ω) in optimally doped LaFeAsO0.918F0.082 with Tc=29K exhibits a spin resonance mode. The peak energy, Eres, when scaled by kBTc is similar to the value of about 4.7 reported in other high-Tc iron-based superconductors. This result suggests that there is intimate relationship between the dynamical spin susceptibility and high-Tc superconductivity in iron-based superconductors, and is consistent with a nesting condition between Fermi surfaces at the Γ and M points.
    Physica C Superconductivity 01/2011; 471(21):639-642. · 0.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Magnetic and local structures in an antiperovskite system, Mn3Cu1-xGexN, with a giant negative thermal expansion have been studied by neutron powder diffraction measurement. We discuss (1) an importance of an averaged cubic crystal structure and a ΓG5g antiferromagnetic spin structure for the large magneto-volume effect (MVE) in this itinerant electron system, (2) an unique role of a local lattice distortion well described by the low temperature tetragonal structure of Mn3GeN for the broadening of MVE.
    Journal of Physics Conference Series 12/2010; 251(1):012014.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the results of the angular-dependent magnetoresistance oscillations (AMROs), which can determine the shape of bulk Fermi surfaces (FSs) in quasi-two-dimensional (Q2D) systems, in a highly hole-doped Fe-based superconductor KFe2As2 with Tc ≈ 3.7  K. From the AMROs, we determined the two Q2D FSs with rounded-square cross sections, correspond to 12% and 17% of the first Brillouin zone. The rounded-squared shape of the FS cross section is also confirmed by the analyses of the interlayer transport under in-plane fields. From the obtained FS shape, we infer the character of the 3d orbitals that contribute to the FSs.
    Physical Review Letters 12/2010; 105(24):246403. · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We performed inelastic neutron scattering on powder sample of the P-doped iron-based superconductor BaFe2(As0.65P0.35)2 with Tc = 30K, whose superconducting (SC) order parameter is expected to have line nodes. We have observed spin resonance at Q $\sim$ 1.2{\AA}^{-1} and E=12 meV in the SC state. The resonance enhancement, which can be a measure of the area of sign reversal between the hole and electron Fermi surfaces (FSs), is comparable to those of other iron-based superconductors without line nodes. This fact indicates that the sign reversal between the FSs is still dominant in this system, and the line nodes should create only limited area of sign-reversal on a single FS. Hence the system can hold relatively high-Tc. Comparison with theoretical calculation indicates horizontal line nodes may be a candidate to reproduce the observation.
    11/2010;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the results of the angular-dependent magnetoresistance oscillations (AMROs), which can determine the shape of bulk Fermi surfaces in quasi-two-dimensional (Q2D) systems, in a highly hole-doped Fe-based superconductor KFe$_2$As$_2$ with $T_c \approx$ 3.7 K. From the AMROs, we determined the two Q2D FSs with rounded-square cross sections, corresponding to 12% and 17% of the first Brillouin zone. The rounded-squared shape of the FS cross section is also confirmed by the analyses of the interlayer transport under in-plane fields. From the obtained FS shape, we infer the character of the 3d orbitals that contribute to the FSs. Comment: 4 pages, 4 figures, accepted in Phys. Rev. Lett
    11/2010;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: High-pressure electrical resistivity measurements up to 3.0GPa have been performed on EuFe2As2 single crystals with residual resistivity ratios RRR=7 and 15. At ambient pressure, a magnetic / structural transition related to FeAs-layers is observed at T0 =190K and 194K for samples with RRR=7 and 15, respectively. Application of hydrostatic pressure suppresses T0, and then induces similar superconducting behavior in the samples with different RRR values. However, the critical pressure 2.7GPa, where T0=0, for the samples with RRR=15 is slightly but distinctly larger than 2.5GPa for the samples with RRR=7.
    Journal of Physics Conference Series 08/2010; 273(1).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An optimally doped iron-based superconductor LaFeAsO$_{0.92}$F$_{0.08}$ with $T_c = 29$ K has been studied by inelastic powder neutron scattering. The magnetic excitation at $Q=1.15$ \AA$^{-1}$ is enhanced below $T_c$, leading to a peak at $E_{res}\sim13$ meV as the resonance mode, in addition to the formation of a gap at low energy below the crossover energy $\Delta_{c}\sim10 meV$. The peak energy at $Q=1.15$ \AA$^{-1}$ corresponds to $5.2 k_B T_c$ in good agreement with the other values of resonance mode observed in the various iron-based superconductors, even in the high-$T_c$ cuprates. Although the phonon density of states has a peak at the same energy as the resonance mode in the present superconductor, the $Q$-dependence is consistent with the resonance being of predominately magnetic origin.
    Physical Review B 06/2010; 82(17). · 3.66 Impact Factor

Publication Stats

2k Citations
497.18 Total Impact Points

Institutions

  • 2008–2014
    • Japan Atomic Energy Agency
      • Quantum Beam Science Directorate
      Muramatsu, Niigata, Japan
  • 2010–2013
    • National Institute for Materials Science
      Tsukuba, Ibaraki, Japan
    • University of Tsukuba
      Tsukuba, Ibaraki, Japan
  • 2010–2011
    • Japan Synchrotron Radiation Research Institute (JASRI)
      Tatsuno, Hyōgo, Japan
  • 2000–2006
    • The University of Tokyo
      • Institute for Solid State Physics
      Tokyo, Tokyo-to, Japan
  • 2005
    • Shiga University of Medical Science
      • Department of Medicine
      Ōtsu-shi, Shiga-ken, Japan
  • 1987–2005
    • Osaka Police Hospital
      Ōsaka, Ōsaka, Japan
  • 2004
    • University Joseph Fourier - Grenoble 1
      Grenoble, Rhône-Alpes, France
  • 1999–2001
    • Osaka City University
      • Department of Cardiology
      Ōsaka-shi, Osaka-fu, Japan
    • Gakushuin University
      • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 1997–2001
    • Osaka University
      • Division of Cardiovascular Medicine
      Ōsaka-shi, Osaka-fu, Japan
  • 1996–1999
    • Nagoya University
      • Department of Material Science
      Nagoya-shi, Aichi-ken, Japan
  • 1993
    • Kansai Rosai Hospital
      Itan, Hyōgo, Japan