Takumi Watanabe

Tokyo Institute of Technology, Edo, Tōkyō, Japan

Are you Takumi Watanabe?

Claim your profile

Publications (7)21.37 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Electronic and magnetic structures of MnP-based compounds, LaMnOP, BaMn2P2, KMnP and MnP, are calculated by density functional theory at the generalized gradient approximation (GGA) + U level. It reveals that the anti-ferromagnetic (AFM) configuration between the adjacent MnP layers is more stable by ∼80 meV than the ferromagnetic (FM) configuration for BaMn2P2, while LaMnOP and KMnP have almost no spin interaction between the MnP layers. This difference is explained by the shorter inter-layer spacing in BaMn2P2. The defect formation energies calculated for LaMnOP suggest that oxygen substitution at the P sites and P vacancies are the most easily formed point defects and explain the observed n-type conduction in nominally undoped polycrystalline samples.
    Materials Science and Engineering B 10/2010; 173(s 1–3):239–243. · 1.85 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Electronic and magnetic properties of a layered compound LaMnPO are examined in relation to a newly discovered iso-structural superconductor LaFeAs(P)O. Neutron diffraction measurements, together with temperature dependent magnetic susceptibility, clarify that LaMnPO is an antiferromagnet at least up to 375 K. The spin moment of a Mn ion is determined to be 2.26 μ<sub>B</sub> at room temperature, and the spin configuration is antiparallel in the Mn–P plane and parallel between the Mn–P planes, which is rather different from that of LaFeAsO. Optical absorption spectra, photoemission spectra, and temperature dependent electrical conductivity indicate that LaMnPO is a semiconductor. Furthermore, nominally undoped LaMnPO exhibits n -type conduction while the conduction type is changed by doping of Cu or Ca to the La sites, indicating that LaMnPO is a bipolar conductor. Density functional calculation using the GGA + U approximation supports the above conclusions; the electronic band structure has an open band gap and the antiferromagnetic spin configuration is more stable than the ferromagnetic one.
    Journal of Applied Physics 06/2009; · 2.21 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Rietveld analysis of the powder X-ray diffraction of a new layered oxyarsenide, LaNiOAs, which was synthesized by solid-state reactions, revealed that LaNiOAs belongs to the tetragonal ZrCuSiAs-type structure (P4/nmm) and is composed of alternating stacks of La-O and Ni-As layers. The electrical and magnetic measurements demonstrated that LaNiOAs exhibits a superconducting transition at 2.4 K, and above this, LaNiOAs shows metallic conduction and Pauli paramagnetism. The diamagnetic susceptibility measured at 1.8 K corresponded to {approx}20% of perfect diamagnetic susceptibility, substantiating that LaNiOAs is a bulk superconductor. - Graphical abstract: A new layered oxyarsenide, LaNiOAs, which exhibits a superconducting transition at 2.4 K, was synthesized by solid-state reactions. This compound had a tetragonal ZrCuSiAs-type structure similar to recently discovered superconductors, LaFeOP, LaFeOAs, and LaNiOP. Superconductivity was observed for undoped LaNiOAs, which differs from the case of LaFeOAs. Display Omitted.
    Journal of Solid State Chemistry 08/2008; 181(8). · 2.04 Impact Factor
  • Source
    Journal of the American Chemical Society 04/2008; 130(11):3296-7. · 10.68 Impact Factor
  • ChemInform 01/2008; 39(25).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Rietveld analysis of the powder X-ray diffraction of a new layered oxyarsenide, LaNiOAs, which was synthesized by solid-state reactions, revealed that LaNiOAs belongs to the tetragonal ZrCuSiAs-type structure (P4/nmm) and is composed of alternating stacks of La-O and Ni-As layers. The electrical and magnetic measurements demonstrated that LaNiOAs exhibits a superconducting transition at 2.4 K, and above this, LaNiOAs shows metallic conduction and Pauli paramagnetism. The diamagnetic susceptibility measured at 1.8 K corresponded to ~20% of perfect diamagnetic susceptibility, substantiating that LaNiOAs is a bulk superconductor.
    ChemInform 01/2008; 39(48).
  • [Show abstract] [Hide abstract]
    ABSTRACT: A layered oxyphosphide, LaNiOP, was synthesized by solid-state reactions. This crystal was confirmed to have a layered structure composed of an alternating stack of (La(3+)O(2-))(+) and (Ni(2+)P(3-))(-). We found that the resulting LaNiOP shows a superconducting transition at approximately 3 K. This material exhibited metallic conduction and Pauli paramagnetism in the temperature range of 4-300 K. The resistivity sharply dropped to zero and the magnetic susceptibility became negative at <4 K, indicating that a superconducting transition occurs. The volume fraction of the superconducting phase estimated from the diamagnetic susceptibility reached approximately 40 vol % at 1.8 K, substantiating that LaNiOP is a bulk superconductor.
    Inorganic Chemistry 09/2007; 46(19):7719-21. · 4.59 Impact Factor