I. Watanabe

RIKEN, Вако, Saitama, Japan

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Publications (312)475.9 Total impact

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    ABSTRACT: Muon positions in La2CuO4 were examined by using the density functional theory. Potential minimum positions near apical and plane oxygen have been determined as possible initial muon stopping positions. We found that final muon stopping positions were di↵erent from those initial positions due to e↵ects of the local deformation of crystal structure which was induced by injected muons. This means that injected muons relax their positions deforming local crystal structures and minimizing the total energy of the system. We also found that the estimation of those final muon positions had to be done in the large scale area as a supercell which contained 27 unit cells in order to achieve realistic situations of the system with the muon as a dilute impurity.
    13th International Conference on Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland; 12/2014
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    ABSTRACT: The electrostatic potential has been investigated in YBa2Cu3O6 by applying the density functional theory in order to estimate possible muon sites. We found five minimum potential positions around the apical oxygen of the CuO5 polyhedral. In addition to those, we also found another minimum potential position near the yttrium atom which is in between the CuO2 layers. Those estimated positions are di↵erent from those suggested from previous µSR studies on the basis of the dipole-field estimation.
    13th International Conference on Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland; 12/2014
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    ABSTRACT: Numerical investigations on muon sites in Ce-based Kondo semiconductors, Ce(Ru,Rh)2Al10 were carried out by using the Density Functional Theory. From the view point of simple electrostatic potential calculations, we found all the previously reported muon sites, suggested by di↵erent groups (Kambe S et al. 2010 J. Phys. Soc. Jpn. 79 053708 and Khalyavin D D et al., 2010 Phys. Rev. B 82 100405(R)), can be possibly chosen as muon stopping sites. We also investigated the changes in the potential of the Rh-doped case. We discovered that the electronic potential around the nearest Ru atom to the substituted Rh atom is a↵ected and the potential becomes asymmetric around the nearest Ru ion. Although big changes in hyperfine fields at muon sites have been reported (Guo H et al. 2013 Phys. Rev. B 88 115206), the muon positions estimated from the potential calculations do not change much.
    13th International Conference on Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland; 12/2014
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    ABSTRACT: The ab-initio density functional theory analysis was applied to metal-organic hybrids, (C2H5NH3)2CuCl4 (EA) and (C6H5CH2CH2NH3)2CuCl4 (PEA), in order to estimate possible muons stopping positions. Six potential minimum positions and eight ones were revealed in PA and PEA, respectively. Those potential minimum positions can be regarded as initial stopping positions of injected muons. All of expected potential minimum points in EA were near and around the apical Cl and the CuCl2 plane of the CuCl6 octahedra. Instead, in the case of PEA, two of eight positions were close to the phenyl ring giving a possibility that there would be muon states which couple surrounding electrons via a radical formation.
    13th International Conference on Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland; 12/2014
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    [Show abstract] [Hide abstract]
    ABSTRACT: Numerical investigations on muon sites in Ce-based Kondo semiconductors, Ce(Ru,Rh)2Al10 were carried out by using the Density Functional Theory. From the view point of simple electrostatic potential calculations, we found all the previously reported muon sites, suggested by di↵erent groups (Kambe S et al. 2010 J. Phys. Soc. Jpn. 79 053708 and Khalyavin D D et al., 2010 Phys. Rev. B 82 100405(R)), can be possibly chosen as muon stopping sites. We also investigated the changes in the potential of the Rh-doped case. We discovered that the electronic potential around the nearest Ru atom to the substituted Rh atom is a↵ected and the potential becomes asymmetric around the nearest Ru ion. Although big changes in hyperfine fields at muon sites have been reported (Guo H et al. 2013 Phys. Rev. B 88 115206), the muon positions estimated from the potential calculations do not change much.
    13th International Conference on Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland; 12/2014
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    ABSTRACT: Muon spin relaxation on Nd2Ti2O7 (NTO) and NdLaTi2O7 (NLTO) compounds are presented. The time spectra for both compounds are as expected for the paramagnetic state at high temperatures, but deviate from the exponential function below around 100 K. Firstly, the muon spin relaxation rate increases with decreasing temperature and then levels off below around 10 K, which is reminiscent of the frustrated systems. An enhancement of the relaxation rate by a longitudinal field in the paramagnetic state is observed for NTO and eliminated by a magnetic dilution for the NLTO sample. This suggests that the spectral density is modified by a magnetic dilution and thus indicates that the spins behave cooperatively rather than individually. The zero-field measurement at 0.3 K indicates that the magnetic ground state for NTO is ferromagnetic.
    Journal of Physics Condensed Matter 10/2014; 26(43):436002. · 2.22 Impact Factor
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    ABSTRACT: We have used the pulsed muon source at ISIS to study high-temperature Na-ion dynamics in the quasi-one-dimensional (Q1D) metallic antiferromagnet NaV2O4. By performing systematic zero-field and longitudinal-field measurements as a function of temperature we clearly distinguish that the hopping rate increases exponentially above Tdiff=250 K. The data is well fitted to an Arrhenius type equation typical for a diffusion process, showing that the Na-ions starts to be mobile above Tdiff . Such results makes this compound very interesting for the tuning of Q1D magnetism using atomic-scale ion-texturing through the periodic potential from ordered Na-vacancies. Further, it also opens the door to possible use of NaV2O4 and related compounds in energy related applications.
    Journal of Physics Conference Series 09/2014; 551:012035.
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    ABSTRACT: We have studied low-temperature magnetic properties as well as high-temperature lithium ion diffusion in the battery cathode materials LixNi1/3Co1/3Mn1/3O2 by the use of muon spin rotation/relaxation. Our data reveal that the samples enter into a 2D spin-glass state below TSG=12 K. We further show that lithium diffusion channels become active for T>Tdiff=125 K where the Li-ion hopping-rate [nu(T)] starts to increase exponentially. Further, nu(T) is found to fit very well to an Arrhenius type equation and the activation energy for the diffusion process is extracted as Ea=100 meV.
    Journal of Physics Conference Series 09/2014; 551:012037.
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    ABSTRACT: Although stoichiometric lithium cobalt dioxide LiCoO2 (ST-LCO) is the most common positive electrode material for Li-ion batteries, the magnetic nature of ST-LCO is still not fully understood. Therefore, we measured susceptibility (\chi ), electron paramagnetic resonance (EPR), and μSR for ST-LCO, particularly above 100 K. The temperature dependence of \chi shows a Pauli paramagnetic behavior, supporting the previous conclusion that Co3+ ions are in a low-spin state with S = 0 (t_{2g}^6). However, the EPR and μSR measurements reveal a "dynamical" magnetic phase in ST-LCO above 100 K. Because the volume fraction of this magnetic phase reaches about 50% at 300 K, the appearance of the magnetic phase is not caused by impurities and/or muonium formation but is an intrinsic characteristic of ST-LCO. By considering the time windows of the three measurement techniques used in this study, we conclude that the origin of the dynamical magnetism is most likely spin fluctuations of the Co ions.
    Journal of Physics Conference Series 09/2014; 551:012008.
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    ABSTRACT: A diffusive behavior of Li+ ion in a garnet-type oxide, Li5+xLa3ZrxNb2−x012 with x=0–2, has been investigated by both a positive muon-spin relaxation (μ+SR) and quasi-elastic neutron scattering (QENS) technique using powder samples. The μ+SR results revealed that Li+ ions start to diffuse above ~150 K for the whole samples measured. The activation energy of Li diffusion (Ea) estimated from the μ+SR data was in good agreement with Ea obtained by the QENS measurements. However, both Eas were about a half of Ea of ionic conductivity of Li+ (σLi), whichwas evaluated by AC-impedance measurements on sintered pellets. This indicated the serious effect of grain boundary and/or surface on Ea. Furthermore, since the diffusion coefficient of Li+ estimated by μ+SR is roughly independent of x, the number of mobile Li+ in the garnet lattice was found to be the predominant parameter to determine σLi.
    Solid State Ionics 06/2014; 262:585-588. · 2.05 Impact Factor
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    ABSTRACT: Longitudinal-field muon-spin-relaxation (LF-μSR) measurements in TlCu1-xMgxCl3 were carried out to investigate the spin dynamics in the non-magnetic impurity doped spin gap system. As reported before, in the case of x ≥ 0.006, LF-μSR time spectra are well fitted by the two components function, which means the phase separation to a spin frozen region and to a spin flucuating region. In this report, we focus on the spin fluctuating region, and discuss the gradual change in its magnetic properties with increasing the Mg-doping.
    Journal of Physics Conference Series 04/2014; 502(1):012041.
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    ABSTRACT: Longitudinal-field muon-spin-relaxation measurements have revealed inhomogeneous distribution of the internal magnetic field at temperatures above the bulk superconducting (SC) transition temperature, $T_{\rm c}$, in slightly overdoped Bi$_2$Sr$_2$Ca$_{1-x}$Y$_x$Cu$_2$O$_{8+\delta}$. The distribution width of the internal magnetic field, $\Delta$, evolves continuously with decreasing temperature toward $T_{\rm c}$. The origin of the increase in $\Delta$ is discussed in terms of the creation of SC domains in a sample.
    04/2014;
  • Proceedings of the International Symposium on Science Explored by Ultra Slow Muon (USM2013); 03/2014
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    ABSTRACT: A diffusive behavior of Li+ ion in a garnet-type oxide, Li5+xLa3ZrxNb2-xO12 with x = 0-2, has been investigated by both a positive muon-spin relaxation (μ+SR) and quasi-elastic neutron scattering (QENS) technique using powder samples. The μ+SR results revealed that Li+ ions start to diffuse above ~150 K for the whole samples measured. The activation energy of Li diffusion (Ea) estimated from the μ+SR data was in good agreement with Ea obtained by the QENS measurements. However, both Eas were about a half of Ea of ionic conductivity of Li+ (sLi), which was evaluated by AC-impedance measurements on sintered pellets. This indicated the serious effect of grain boundary and/or surface on Ea. Furthermore, since the self-diffusion coefficient of Li+ estimated by μ+SR is roughly independent of x, the number of mobile Li+ in the garnet lattice was found to be the predominant parameter to determine sLi.
    JPS Conference Proceedings. 03/2014; 2:010303.
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    ABSTRACT: Lithium cobalt dioxide (LiCoO2) belongs to a family of layered CoO2-based materials and has considerable interests in both fundamental physics and technological applications in lithium-ion batteries. We report the results of structural, electrochemical, magnetic susceptibility (χ), electron paramagnetic resonance (EPR), and muon-spin rotation and relaxation (μSR) measurements on powder Lix 0CoO2 samples, where the nominal Li/Co ratios (x0) were 0.95, 1.00, 1.02, 1.05, and 1.10, respectively. Structural, electrochemical, and χ measurements suggested that the sample with x0 = 1.02 is very close to single stoichiometric LiCoO2 (ST LCO) phase and that the Co ions in the x0 = 1.02 sample are in a nonmagnetic low-spin state with S = 0 (t62g). However, both EPR and μSR revealed that the x0 = 1.02 (ST-LCO) sample includes a large amount of nonordered magnetic phase in the temperature (T ) range between 100 and 500 K. The volume fraction of such magnetic phase was found to be �45 vol% at 300 K by μSR, indicating an intrinsic bulk feature for ST-LCO. In fact, structural and photoelectron spectroscopic analyses clearly excluded the possibility that the nonordered magnetism is caused by impurities, defects, or surfaces. Because EPR and μSR sense static and dynamic nature of local magnetic environments, we concluded that Co spins in ST-LCO are fluctuating in the EPR and μSR time-windows. We also proposed possible origins of such nonordered magnetism, that is, a spin-state transition and charge disproportionation.
    Physical Review B 03/2014; 89(9):094406. · 3.66 Impact Factor
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    ABSTRACT: The most fundamental limitations of a muon-spin relaxation experiment can be the lack of knowledge of the implantation site of the muon and the uncertainty about the muon's perturbation of its host. Here we review some of the work done on the 'muon site problem' in the solid state and highlight some recent applications of electronic structure calculations that have successfully characterized the quantum states of muons in a number of insulating compounds containing fluorine, in a number of pnictide superconductors, and in ZnO.
    Physica Scripta 12/2013; 88:068510. · 1.03 Impact Factor
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    ABSTRACT: We present the muon spin relaxation/rotation spectra in the multiferroic compound (Cu,Zn)$_{3}$Mo$_{2}$O$_{9}$. The parent material Cu$_{3}$Mo$_{2}$O$_{9}$ has a multiferroic phase below $T_{\rm N}$ = 8.0 K, where the canted antiferromagnetism and the ferroelectricity coexist. The asymmetry time spectra taken at RIKEN-RAL pulsed muon facility show a drastic change at $T_{\rm N}$. At low temperatures the weakly beating oscillation caused by the static internal magnetic fields in the antiferromagnetic phase was observed in Cu$_{3}$Mo$_{2}$O$_{9}$ and the lightly ($0.5\%$) Zn-doped sample. From the fitting of the oscillating term, we obtain the order parameter in these samples: ferromagnetic moment in two sublattices of antiferromagnet. In the heavily ($5.0\%$) Zn-doped sample, the muon-spin oscillation is rapidly damped. The frequency-domain spectrum of this sample suggests a formation of magnetic superstructure.
    12/2013;
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    ABSTRACT: The magnetic ground state of the Rh-doped Kondo semiconductor CeRu2Al10 [Ce(Ru1−xRhx)2Al10] is investigated with the muon spin relaxation method. Muon spin precession with two frequencies is observed in the x=0 sample, while only one frequency is present in the x=0.05 and 0.10 samples, which is attributed to the broad static field distribution at the muon site. The internal field at the muon site is enhanced from about 180 G in the x=0 sample to about 800 G in the Rh-doped samples, supporting the spin-flop transition as suggested by the magnetization measurement, and the boundary of different magnetic ground states is identified around x=0.03. The drastic change of magnetic ground state by a small amount of Rh doping (3%) indicates that the magnetic structure in CeRu2Al10 is not robust and can be easily tuned by external perturbations such as electron doping. The anomalous temperature dependence of the internal field in CeRu2Al10 is suggested to be attributed to the hyperfine interaction between muons and conduction electrons.
    Physical Review B 09/2013; 88(11). · 3.66 Impact Factor
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    ABSTRACT: The magnetic ground state of the Rh-doped Kondo semiconductor CeRu2Al10 [Ce(Ru1−xRhx)2Al10] is investigated with the muon spin relaxation method. Muon spin precession with two frequencies is observed in the x=0 sample, while only one frequency is present in the x=0.05 and 0.10 samples, which is attributed to the broad static field distribution at the muon site. The internal field at the muon site is enhanced from about 180 G in the x=0 sample to about 800 G in the Rh-doped samples, supporting the spin-flop transition as suggested by the magnetization measurement, and the boundary of different magnetic ground states is identified around x=0.03. The drastic change of magnetic ground state by a small amount of Rh doping (3%) indicates that the magnetic structure in CeRu2Al10 is not robust and can be easily tuned by external perturbations such as electron doping. The anomalous temperature dependence of the internal field in CeRu2Al10 is suggested to be attributed to the hyperfine interaction between muons and conduction electrons.
    Physical Review B 09/2013; 88(11). · 3.66 Impact Factor
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    ABSTRACT: It has been nearly 27 years since the discovery of the first high temperature (high-Tc) superconductor on LaBaCuO by Benorz & Muller in 1986 [1], in which the mechanism of how high-Tc superconductor happening remain unsolved up until now. La2CuO4 are a parent compound for La2-xSrxCuO4 superconductor, where show antiferromagnetic behavior on based temperature. By adding Sr doping about 0.05 its properties become superconductor. According to previous experimental and theoretical study [2,3], magnetism plays quite an important role for LaBaCuO. μSR was one of the good tools to investigate magnetic properties of magnetic materials, but its utiliy is limited by the lack of knowledge of the muon implantation site and because of that we need to know where muon was staying on crystals. For many years many methods already proposed to determine the muon site in La2CuO4, but always lead to different sites from one to another. This lead to many confusion for researcher who want to continue and go to deeper understanding about magnetic behavior using μSR. On the present study, we reinvestigated the previous study sites by using dipole field and density functional theory calculation to see the relaxation and energy behavior for each sites. For the dipole calculation we used two kind of magnetic moment from experiment and theoretical, and expand the structure to be 50 A˚ radius. The potential minima position also included to be tested as the possible muon sites, GGA PBE correction was used to see the different behavior between them. The purpose of this investigation is the first step to find out the general method for muon sites determination, that can applied for many cases.
    09/2013;

Publication Stats

1k Citations
475.90 Total Impact Points

Institutions

  • 2001–2014
    • RIKEN
      Вако, Saitama, Japan
    • The University of Electro-Communications
      • Department of Applied Physics and Chemistry
      Tokyo, Tokyo-to, Japan
  • 1999–2010
    • The University of Tokyo
      • Department of Basic Medical Sciences
      Edo, Tōkyō, Japan
    • Tokyo University of Science
      • Department of Fire Science and Technology
      Edo, Tōkyō, Japan
    • Tohoku University
      • Department of Applied Physics
      Sendai, Kagoshima, Japan
  • 2009
    • Toyota Central R & D Labs., Inc.
      Nagoya, Aichi, Japan
  • 2008
    • University of Hyogo
      • Graduate School of Material Science
      Akō, Hyogo-ken, Japan
  • 1999–2001
    • High Energy Accelerator Research Organization
      • Institute of Materials Structure Science
      Tsukuba, Ibaraki, Japan
  • 1998
    • Tokyo Institute of Technology
      • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 1988–1994
    • Hokkaido University
      • • Department of Physics
      • • Department of Internal Medicine II
      • • Department of Medicine II
      Sapporo-shi, Hokkaido, Japan
  • 1989–1993
    • Hokkaido University Hospital
      • Division of Internal Medicine II
      Sapporo, Hokkaidō, Japan