Y. Taguchi

The University of Tokyo, Edo, Tōkyō, Japan

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Publications (203)477.12 Total impact

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    ABSTRACT: The coupling between spins and electric dipoles governs magnetoelectric phenomena in multiferroics. The dynamical magnetoelectric effect, which is an inherent attribute of the spin excitations in multiferroics, drastically changes the optical properties of these compounds compared with conventional materials where light-matter interaction is expressed only by the dielectric permittivity or magnetic permeability. Here we show via polarized terahertz spectroscopy studies on multiferroic Ca2CoSi2O7, Sr2CoSi2O7 and Ba2CoGe2O7 that such magnetoeletric spin excitations exhibit quadrochroism, that is, they have different colours for all the four combinations of the two propagation directions (forward or backward) and the two orthogonal polarizations of a light beam. We demonstrate that one-way transparency can be realized for spin-wave excitations with sufficiently strong optical magnetoelectric effect. Furthermore, the transparent and absorbing directions of light propagation can be reversed by external magnetic fields. This magnetically controlled optical-diode function of magnetoelectric multiferroics may open a new horizon in photonics.
    Nature Communications 02/2014; 5:3203. · 10.02 Impact Factor
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    ABSTRACT: The magnetic skyrmion is a topologically stable spin texture in which the constituent spins point to all the directions wrapping a sphere. Generation and control of nanometric magnetic skyrmions have large potential, for example, reduced power consumption, in spintronics device applications. Here we show the real-space observation of a biskyrmion, as defined by a molecular form of two bound skyrmions with the total topological charge of 2, realized under magnetic field applied normal to a thin plate of a bilayered manganite with centrosymmetric structure. In terms of a Lorentz transmission electron microscopy (TEM), we have observed a distorted-triangle lattice of biskyrmion crystal, each composed of two bound skyrmions with oppositely swirling spins (magnetic helicities). Furthermore, we demonstrate that these biskyrmions can be electrically driven with orders of magnitude lower current density (<10(8) A m(-2)) than that for the conventional ferromagnetic domain walls.
    Nature Communications 01/2014; 5:3198. · 10.02 Impact Factor
  • Y Tokunaga, Y Taguchi, T Arima, Y Tokura
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    ABSTRACT: In a multiferroic orthoferrite Dy_{0.7}Tb_{0.3}FeO_{3}, which shows electric-field-(E-)driven magnetization (M) reversal due to a tight clamping between polarization (P) and M, a gigantic effect of magnetic-field (H) biasing on P-E hysteresis loops is observed in the case of rapid E sweeping. The magnitude of the bias E field can be controlled by varying the magnitude of H, and its sign can be reversed by changing the sign of H or the relative clamping direction between P and M. The origin of this unconventional biasing effect is ascribed to the difference in the Zeeman energy between the +P and -P states coupled with the M states with opposite sign.
    Physical Review Letters 01/2014; 112(3):037203. · 7.94 Impact Factor
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    ABSTRACT: We report magnetotransport properties for epitaxial thin films of SrFeO3 and SrFe0.99Co0.01O3 with possible skyrmion-related spin textures. Resonant soft x-ray diffraction measurements revealed formation of helical spin structures for both samples (Q//<111>). From magnetotransport measurements we found several distinct helimagnetic phases with multiple/single Q vectors. A steep suppression of Hall resistivity is observed above the critical field to reach the high-field conical state, indicating the presence of skyrmionlike topological spin textures at lower fields responsible for the topological Hall effect.
    Physical Review B 12/2013; · 3.77 Impact Factor
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    ABSTRACT: The electron mobility is one of the key parameters that characterize the charge-carrier transport properties of materials, as exemplified by the quantum Hall effect as well as high-efficiency thermoelectric and solar energy conversions. For thermoelectric applications, introduction of chemical disorder is an important strategy for reducing the phonon-mediated thermal conduction, but is usually accompanied by mobility degradation. Here, we show a multilayered semimetal β-CuAgSe overcoming such a trade-off between disorder and mobility. The polycrystalline ingot shows a giant positive magnetoresistance and Shubnikov de Haas oscillations, indicative of a high-mobility small electron pocket derived from the Ag s-electron band. Ni doping, which introduces chemical and lattice disorder, further enhances the electron mobility up to 90,000 cm(2) V(-1) s(-1) at 10 K, leading not only to a larger magnetoresistance but also a better thermoelectric figure of merit. This Ag-based layered semimetal with a glassy lattice is a new type of promising thermoelectric material suitable for chemical engineering.
    Nature Material 04/2013; · 35.75 Impact Factor
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    ABSTRACT: We have spectroscopically investigated the thermally and doping-induced metal-insulator transitions for pyrochlore-type Nd_{2}Ir_{2}O_{7} as well as its Rh-doped analogs Nd_{2}(Ir_{1-x}Rh_{x})_{2}O_{7}, where the spin-orbit interaction as well as the electron correlation is effectively tuned by the doping level (x). The charge dynamics dramatically changes on an energy scale of 1 eV in the course of thermally and doping-induced metal-insulator transitions, while the insulating ground state shows a small but well-defined charge gap of 45 meV. Anomalous doping variation of the low-energy (<0.3  eV) optical-conductivity spectra at the ground state can be interpreted in terms of the phase changes among the narrow-gap Mott insulator, Weyl semimetal, and correlated metal.
    Physical Review Letters 09/2012; 109(13):136402. · 7.94 Impact Factor
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    ABSTRACT: Infrared optical and inelastic x-ray scattering spectra have been systematically investigated in combination with first-principles calculations for paraelectric and antiferromagnetic perovskite Sr1−xBaxMnO3 (x=0–0.3) single crystals, which are close to a ferroelectric transition arising from off-center displacement of magnetic Mn4+ ions. All the phonon dispersions measured for the parent compound of x=0 agree well with the results of the first-principles calculation. As the Ba concentration increases, one optical phonon rapidly softens toward zero frequency at room temperature, while the other phonons are almost unchanged. This soft-mode behavior is also reproduced by the first-principles calculations, from which we have predicted the vibration mode of all the optical phonons. The results of the infrared measurements at various temperatures indicate that only the soft phonon mode shows marked temperature variation relevant to the antiferromagnetic transition for all x, whereas other optical modes are almost independent of temperature. The conventional evolution of a soft phonon with decreasing temperature is prevented by the onset of the magnetic order. Below the antiferromagnetic-transition temperature, the soft mode hardens with decreasing temperature and then resoftens toward the lowest temperature. A similar temperature dependence was observed in the nonzero-momentum region by means of the inelastic x-ray scattering measurements, although its magnitude decreases as the momentum is increased from zero. Such a nonmonotonic temperature profile of the soft-mode energy is well explained on the basis of a phenomenological spin-phonon coupling model, which suggests the largest coupling constant yet attained.
    Physical review. B, Condensed matter 09/2012; 86(10).
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    ABSTRACT: Single crystals of SrFe1−xCoxO3 solid solution with high valence ions (Fe4+/Co4+) were synthesized by combining floating-zone and high-pressure oxygen-annealing techniques. As the Co content (x) is increased, the ground state changes from the helimagnetic state (x ≤ 0.05) through cluster glass to the ferromagnetic state (x ≥ 0.2), with a high Curie temperature ranging between 245 and 337 K. We found that, within the helimagnetic state (x ≤ 0.05), several different magnetic phases, which possibly reflect versatile three-dimensional helical orders with multiple wave vectors, emerge depending on temperature and magnetic field strength. In addition, we observed Co-concentration–dependent systematic evolution of the complex magnetic phase diagram. Resonant soft x-ray diffraction measurements revealed a gradual decrease in the wave vector of the helix toward the ferromagnetic phase.
    Physical review. B, Condensed matter 08/2012; 86(6).
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    ABSTRACT: Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. One such example is the keen competition between phases with different spin/charge/orbital orders. Keen competition between antiferromagnetic metal and orbital-ordered insulator is found in the slightly electron-doped regime near Mn4+ state which is stabilized by the high oxygen-pressure condition. Another one is the emergence of ferroelectricity either induced by the magnetic ordering or independently of the magnetic ordering. As the respective examples, perovskite-type YMnO3 and Sr1−xBaxMnO3 are discussed. In the YMnO3, the ferroelectric lattice distortion associated with the E-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr0.5Ba0.5MnO3, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.
    Physica B Condensed Matter 06/2012; 407(11):1685–1688. · 1.33 Impact Factor
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    ABSTRACT: We have spectroscopically investigated the charge dynamics for the high-quality single crystals of perovskite AFeO3, which exhibit the helimagnetism (A = Sr and Ca) and/or the charge disproportionation (CD) transition (A = Ca). The metallic state at high temperatures, both for A= Sr and Ca, shows highly incoherent charge dynamics, while the optical spectra for A= Ca change in a wide energy upon the CD transition. These results are in accord with the picture of the phase change between the highly correlated metal and the anticorrelated CD insulator in this class of high Fe-valence perovskite.
    Physical review. B, Condensed matter 04/2012; 85(15).
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    ABSTRACT: The three-site spin correlation, S(i)·(S(j)×S(k)) on the neighboring triangular sites i, j and k, termed scalar spin chirality, can endow the conduction electron with a quantum Berry phase and resultant transverse (Hall) transport. The paramagnetic barely metallic state was prepared in hole-doped Y2Mo2O7 with pyrochlore lattice using a high-pressure synthesis method, which is further endowed with the spin chirality by partially replacing Y site with Tb (content x). The local spin chirality formed by the adjacent three Tb Ising moments on the pyrochlore lattice can couple to the conduction electrons to give rise to the topological Hall effect whose magnitude increases in proportion to x3 or the density of the Tb-moment triangular clusters.
    Physical Review Letters 04/2012; 108(15):156601. · 7.94 Impact Factor
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    ABSTRACT: Magnetic and magnetoelectric properties have been investigated for single crystals of Sc-doped M-type hexaferrites [1]. Magnetization and neutron diffraction studies have indicated that a longitudinal conical state is stabilized up to room temperature by tuning the Sc concentration. Magnetoelectric measurements have shown that electric polarization can be induced by applying a transverse magnetic field at lower temperatures, and that the spin helicity is nonvolatile and endurable up to near the transition temperature from conical to collinear state. In addition, the behavior of the polarization vector upon the reversal of magnetization varies with temperature, thereby allowing us to control the relation between spin helicity and magnetization vectors with magnetic field and temperature. This work was in part supported by FIRST program on Quantum Science on Strong Correlationfrom JSPS. [4pt] [1] Y. Tokunaga, Y. Kaneko, D. Okuyama, S. Ishiwata, T. Arima, S. Wakimoto, K. Kakurai, Y. Taguchi, and Y. Tokura, Phys. Rev. Lett. 105, 257201 (2010)
    02/2012;
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    ABSTRACT: Thermoelectric properties have been investigated for polycrystalline samples of layer-structured R1-xAxZnSbO (R=La, Ce; A=Ca, Sr) as two-dimensional analogues of a conventional thermoelectric semiconductor ZnSb[1]. By substituting A^2+ for R^3+ in the charge-reservoir layers, carrier concentration can be successfully controlled without lowering the carrier mobility. The hole doped materials showed low thermal conductivity and moderately high thermopower, whose temperature- and doping-dependence were well explained by theoretical calculation. The values of dimensionless figure of merit ZT were found to increase without showing any sign of saturation up to 390 K, and even higher values can be expected along the conducting ZnSb layers for a single crystal. These results indicate the potential of the hole-doped RZnSbO as a good thermoelectric material. This work was in part supported by FIRST program on Quantum Science on Strong Correlationfrom JSPS. [4pt] [1] T. Suzuki, M. S. Bahramy, R. Arita, Y. Taguchi, and Y. Tokura, Phys. Rev. B 83, 035204 (2011)
    02/2012;
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    ABSTRACT: The magnetic and electrical properties of Co3TeO6 single crystals with corundum-related structure reveal a magnetic-field-induced polarization below 21 K. A sharp peak in the specific heat at ≈18 K indicates a reconstructive-type first-order phase transition. From second-harmonic generation measurements, breaking of inversion symmetry is evident and the point-group symmetry was determined as m. The temperature and magnetic-field dependences of the magnetic and electrical polarizations are discussed in the light of the SHG results.
    Physical review. B, Condensed matter 11/2011; 84(18).
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    ABSTRACT: We have observed a persistent x-ray induced insulator-metal transition at low temperature for an epitaxial film of electron-doped VO2, a material as characterized by strong electron-electron and electron-lattice interactions. The volume fraction of the photo-generated metallic patches, ranging from 0 to 100% of the whole film, can be scaled well with the total dose of x-ray irradiation, irrespective of the x-ray intensity. This indicates the monomolecular process of the insulator-metal phase conversion that corresponds to the instantaneous creation of the metallic patch extending over as many as 105 V sites per one x-ray absorbed photon. The typical percolation behavior is observed in the conductivity change with the finely photo-controlled volume ratio of the metallic phase.
    Physical review. B, Condensed matter 10/2011; 84(16).
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    ABSTRACT: We report a ferroelectric transition driven by the off-centering of magnetic Mn(4+) ions in antiferromagnetic Mott insulators Sr(1-x)Ba(x)MnO(3) with a perovskite structure. As x increases, the perovskite lattice shows the typical soft-mode dynamics, as revealed by the momentum-resolved inelastic x-ray scattering and far-infrared spectroscopy, and the ferroelectricity shows up for x ≥ 0.45. The observed polarization is comparable to that for a prototypical ferroelectric BaTiO(3). We further demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by ∼70% and increases the soft-phonon energy by ∼50%, indicating the largest magnetoelectric effects yet attained.
    Physical Review Letters 09/2011; 107(13):137601. · 7.94 Impact Factor
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    ABSTRACT: Circular dichroism in the angular distribution of photoelectrons from SrTiO(3):Nb and Cu(x)Bi(2)Se(3) is investigated by 7-eV laser angle-resolved photoemission spectroscopy. In addition to the well-known node that occurs in the circular dichroism pattern when the incidence plane matches the mirror plane of the crystal, we show that another type of node occurs when the mirror plane of the crystal is vertical to the incidence plane and the electronic state is two-dimensional. The flower-shaped circular dichroism patterns in the angular distribution occurring around the Fermi level of SrTiO(3):Nb and around the Dirac point of Cu(x)Bi(2)Se(3) are explained on equal footings. We point out that the penetration depth of the topological states of Cu(x)Bi(2)Se(3) depends on momentum.
    Physical Review Letters 08/2011; 107(7):077601. · 7.94 Impact Factor
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    ABSTRACT: Circular dichroism in the angular distribution (CDAD) of photoelectrons from SrTiO3:Nb and CuxBi2Se3 is investigated by 7-eV laser ARPES. In addition to the well-known node that occurs in CDAD when the incidence plane matches the mirror plane of the crystal, we show that another type of node occurs when the mirror plane of the crystal is vertical to the incidence plane and the electronic state is two dimensional. The flower-shaped CDAD's occurring around the Fermi level of SrTiO3:Nb and around the Dirac point of CuxBi2Se3 are explained on equal footings. We point out that the penetration depth of the topological states of CuxBi2Se3 depends on momentum.
    08/2011;
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    ABSTRACT: A helical spin texture is of great current interest for a host of novel spin-dependent transport phenomena. We report a rich variety of nontrivial, helimagnetic phases in the simple cubic perovskite SrFeO3 under magnetic fields up to 42 T. Magnetic and resistivity measurements revealed that the proper-screw spin phase proposed for SrFeO3 can be subdivided into at least five kinds of ordered phases. Near the multicritical point, an unconventional anomalous Hall effect was found to show up and was interpreted as due to a possible long-period noncoplanar spin texture with scalar spin chirality.
    Physical review. B, Condensed matter 07/2011; 84.
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    ABSTRACT: Transport and magnetic properties have been systematically investigated for SmMnAsO_{1-x} with controlled electron-doping. As the electron band-filling is increased with the increase of oxygen deficiency (x), the resistivity monotonically decreases and the transition from Mott-insulator to metal occurs between x=0.17 and 0.2. Seebeck coefficient at room temperature abruptly changes around the critical doping level from a large value (\sim -300 \mu V/K) to a small one (\sim -50 \mu V/K) both with negative sign. In the metallic compounds with x=0.2 and x=0.3, Mn spins order antiferromagnetically around 30 K, and the Hall coefficient with the negative sign shows a reduction in magnitude upon the magnetic transition, indicating the change in the multiple Fermi surfaces. Gigantic positive magnetoresistance effect is observed in a wide range of temperature, reaching up to 60 % at 2 K for the x=0.3 compound. The effect is attributed to the field-induced change of the complex Fermi surfaces in this multi-orbital correlated electron system.
    Physical review. B, Condensed matter 06/2011; 84.

Publication Stats

856 Citations
477.12 Total Impact Points

Institutions

  • 1993–2013
    • The University of Tokyo
      • • Department of Applied Physics
      • • Graduate School of Frontier Sciences
      • • Department of Physics
      Edo, Tōkyō, Japan
  • 2009–2011
    • RIKEN
      • Strong Correlation Interface Research Group
      Вако, Saitama, Japan
  • 2010
    • Japan Science and Technology Agency (JST)
      • Exploratory Research for Advanced Technology (ERATO)
      Edo, Tōkyō, Japan
  • 1993–2009
    • Tohoku University
      • • Institute for Materials Research
      • • Graduate School of Medicine
      Sendai-shi, Miyagi-ken, Japan
  • 1994–2006
    • Osaka Prefecture University
      • • Graduate School of Engineering
      • • College of Engineering
      • • Department of Mathematical Sciences
      Sakai, Osaka-fu, Japan
  • 2002
    • Stanford University
      • Department of Physics
      Stanford, CA, United States
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
  • 2000
    • Princeton University
      • Department of Physics
      Princeton, NJ, United States
  • 1996
    • Hokkaido University
      • Department of Physics
      Sapporo-shi, Hokkaido, Japan