Takashi Nishioka

Hiroshima University, Hirosima, Hiroshima, Japan

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Publications (137)249.7 Total impact

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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: 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: Hydrostatic pressure effects on magnetic parameters for crustal rock have been poorly investigated yet, while it is important for an understanding of source of long-wavelength magnetic anomaly, which is considered to reside in deep crust. In this study we have conducted the in-situ magnetic hysteresis measurements on multidomain (MD) magnetite under high pressure up to 1 GPa. With special attention to hydrostatic condition and sample preparation, pressure dependences of its magnetic hysteresis parameters (saturation magnetization, Ms; saturation remanence, Mrs; coercivity, Bc; coercivity of remanence, Bcr) are revealed as follows: (1) Bc monotonically increases with pressure at a rate of +91 %/GPa; (2) Ms is constant under high pressure up to 1 GPa; and (3) Mrs increases with pressure up to 0.5 GPa by ∼30 % and reaches to saturation above the pressure; (4) Bcr is nearly constant at low pressure, and it increases above ∼0.6 GPa; and (5) the changes in ratios Mrs/Ms and Bcr/Bc correlate with each other, resulting in systematic movement on the Day plot. These findings allow us to estimate change in a relaxation time of magnetic remanence carried by MD magnetite as a function of depth in the continental crust. The relaxation time monotonously decreases with depth, and primary remanence is considered to be replaced by a viscous remanent magnetization (VRM) over the Brunhes chron. Therefore, it is suggested that MD magnetite in deep crustal rocks can contribute to the source of the anomaly over the continental crust by VRM and induced magnetization.
    Physics of The Earth and Planetary Interiors 08/2014; · 2.38 Impact Factor
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    ABSTRACT: We present a new type of quantum critical material YbCo$_2$Ge$_4$, having the largest quantum-critical pseudospin size ever. The YbCo$_2$Ge$_4$-type structure is new, forms in the orthorhombic $Cmcm$ system, and is related to the well-known ThCr$_2$Si$_2$ structure. Heavy rare earth (Tm,Yb,Lu, or Y) members are also possible to be grown. YbCo$_2$Ge$_4$ possesses the Ising-type ground-state doublet, namely the simplest ones of uniaxially up or down, $|\pm \sim 7/2\rangle$. It is clearly manifested through comprehensive resistivity, magnetization, specific heat, and NQR/NMR experiments. Large pseudospin state usually tends to order in simple magnetisms, or hard to be screened by Kondo effect. Therefore, the discovery of the quantum criticality of the fluctuating large spins opens a new door to new-material search and theoretical studies.
    07/2014;
  • Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: To investigate the phase transitions in TbFe2Al10 which is an isomorphic compound of CeT2Al10 (T = Ru, Os) showing a novel second-order phase transition, we carried out specific heat and ultrasonic measurements on a single-crystalline sample. We found a λ-type sharp peak at 16.6 K and a shoulder-like anomaly at 8.0 K in the specific heat, indicating successive phase transitions in TbFe2Al10. Elastic modulus C22 increases monotonically with decreasing temperature at high temperatures. The modulus shows elastic hardening at 16.6 K, which is analogous to the hardening in the isomorphic compound CeT2Al10 and NdRu2Al10, suggesting a similar mechanism of the transition. On the other hand, no anomaly is observed at 8.0 K in the elastic modulus. There is a weaker coupling between the strain and an order parameter of the phase transition at 8.0 K.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: We performed a synchrotron X-ray study on CeRu2Al10 with pressure varies from 0 GPa to 9.2 GPa and temperature varies from 10 to 300 K using a diamond anvil pressure cell in order to investigate the mechanism of an anomalous antiferromagnetic order at TN 27 K from a structural point of view. No additional peaks appear except an impurity peak in the X-ray diffraction pattern below TN or above its extinct pressure PC ∼ 4 GPa. This result indicates that crystal retains the same crystal structure in this pressure and temperature region. In addition, the lattice parameters decrease monotonically with pressure at room temperature. Temperature dependence of the volume does not exhibit an abrupt shift at any pressures. These results indicate that drastic lattice distortion does not happen at TN or at PC of CeRu2Al10
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: The magnetoresistance of CeT2Al10 (T = Ru and Os) and the Hall resistivity of CeOs2Al10 have been measured using non destructive pulsed magnets. The longitudinal magnetoresistance of CeRu2Al10 shows a negative magnetoresistance for the magnetic field (H) along the a- and c-axes but shows less field dependence for H||b. These behaviors are probably attributed to the suppression of the conduction electron scattering by the localized moments. On the other hand, the transverse magnetoresistance exhibits a complex H dependence, which cannot be explained simply by a cyclotron motion of the conduction electrons. The longitudinal magnetoresistance of CeOs2Al10 for H||a rapidly decreases with increasing field below ∼10 K, indicating that the semiconducting behavior below ∼15 K at zero field is rapidly suppressed by the magnetic field up to ∼15 T. The Hall resistivity of CeOs2Al10 exhibits a non linear behavior below 4.2 K, which reflects the fact that CeOs2Al10 is a two-carrier system.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
  • Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: The evolution of the complex magnetic order in non-centrosymmetric CeCoGe3 toward to QCP was investigated by Co-NQR measurements. We found that the complex successive transitions disappear already under 2.7 GPa. The simple NQR spectrum corresponding to one Co site at 2.7 GPa contrasting to 2–4 Co sites at lower pressures suggests a simple antiferromagnetic order at least under 2.7 GPa, which implies that the complex magnetic order is not caused specifically by the absence of inversion symmetry. The disappearance of the successive transition at 2.7 GPa is not necessarily consistent with the magnetic phase diagram in approaching QCP proposed by the specific heat measurements. The T-dependence of the nuclear spin–lattice relaxation rate, 1/T1, is interpreted in terms of the localized moment picture still under 2.7 GPa.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: Orthorhombic YbFe2Al10-type Kondo semiconductor CeT2Al10 (T = Ru and Os) have anomalous high phase transition temperatures in spite of small Ce content. We have succeeded to synthesize new dilute rare earth hexagonal compounds Ce2Ru3Al15-type R2Ru3Al15 (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) closely related to YbFe2Al10-type structure, and investigated their transport and magnetic properties. The electrical resistivity measurements indicate that all the compounds are metal, and R = Ce shows a logarithmic increase below 20 K due to Kondo effect. R = Ce, Nd, Sm, Gd, Tb, Dy, Ho, and Er show antiferromagnetic transitions at TN = 3.5, 2.7, 9.2, 19.0, 15.7, 7.85, 3.0, and 1.6 K, respectively, and the TN almost follows the de Gennes’ law except for R = Ce and Sm. R = Pr has nonmagnetic ground state and R = Tm is expected to order magnetically below 1.2 K from the susceptibility measurement. We have found that the Néel temperature of R = Ce is about 20 times larger than an expected value by the de Gennes’ law, indicating that Ce2Ru3Al15 is also a member of high magnetic transition temperature materials.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
  • Harukazu Kato, Takashi Nishioka, Masahiro Matsumura
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    ABSTRACT: Substitution effects of the nonmagnetic ion in an A-site-ordered perovskite system, CaCu3Ru4O12 (CCRO), have been investigated by means of nuclear quadrupole resonance (NQR) measurements. An isovalent substituted system, SrCu3Ru4O12 (SCRO), shows a valence transition as well as CCRO, although it vanishes in LaCu3Ru4O12 (LCRO) which is substituted of divalent Ca by trivalent La. The valence transition is presumably influenced by the electron numbers in the Cu-3d and Ru-4d subbands. The non-Fermi-liquid behavior is pronounced in SCRO, which is demonstrated as non-Korringa behavior of the spin–lattice relaxation rate below 20 K.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: The CeT2Al10 family of orthorhombic compounds exhibits a very peculiar evolution from a Kondo-insulator (T: Fe) to an unconventional long-range magnetic order (T: Ru, Os). Inelastic neutron scattering experiments performed on single-crystal CeFe2Al10 reveal that this material develops a spin-gap in its magnetic spectral response below ~ 50 K, with a magnetic excitation dispersing from $E = 10.2 \pm 0.5$ meV at the Y zone-boundary point [q = (0,1,0)] to $\approx 12$ meV at the top of the branch. The excitation shows a pronounced polarization of the magnetic fluctuations along a, the easy anisotropy axis. Its behavior is contrasted with that of the (magnonlike) modes previously reported for CeRu2Al10, which have transverse character and exist only in the antiferromagnetic state. The present observation is ascribed to a "magnetic exciton" mechanism invoked to explain a similar magnetic response previously discovered in YbB12.
    01/2014; 89(16).
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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: We investigated the magnetic properties of the antiferromagnetic (AFM) compound GdT2Al10 (T=Ru and Fe). Although GdRu2Al10 could be understood well as a simple AFM compound, the exchange interaction in GdFe2Al10 is found to be greatly varied with temperature. The magnitude of the AFM exchange interaction is reduced with decreasing temperature. We ascribed its origin to the exchange enhancement of the Fe ion with a decrease of temperature as was observed in YFe2Al10. The ordered moment is found to be along the [011] direction in the bc plane in both compounds from the anisotropic magnetic susceptibility. The origin of the magnetic anisotropy below TN could not be understood by the magnetic dipole interaction, which might come from the AFM order on the zigzag chain.
    Physical Review B 09/2013; 88(10). · 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: We have studied the Rh substitution (electron doping) effect in the Kondo semiconductor CeRu2Al10, which orders antiferromagnetically below (T0=27 K), by specific heat C, magnetic susceptibility χ, electrical resistivity ρ, and Hall resistivity ρH measurements of Ce(Ru1-xRhx)2Al10 (x=0.12, 0.2, 0.23, 0.34) single crystals. T0 decreases monotonically with an increase in x. In addition, new anomalies occur at T1˜ 6.5 and T2˜ 3.0 K in all the Rh substitution samples. However, T1 and T2 are almost independent of x. The shape of the anomaly at T0 in χ(T) changes with Rh substitution, which implies the reorientation of the ordered moment. These phenomena indicate that the magnetic properties of CeRu2Al10 are strongly associated with the number of 4d electrons.
    Journal of the Physical Society of Japan 09/2013; 82(9):3702-. · 2.09 Impact Factor
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    ABSTRACT: We have examined the La substitution and its pressure effect on the Kondo semiconductor CeRu2Al10, which shows an unusual antiferromagnetic (AFM) order at T0=27 K with the AFM ordered moment (mAF) parallel to the orthorhombic c axis, not expected from the large anisotropy of the magnetic susceptibility (χ) of χa≫χc≫χb in the paramagnetic state. mAF could not be aligned along the a axis, but could only be in the bc plane. By only 10% La substitution corresponding to a negative chemical pressure effect, the direction of mAF in the AFM ordered phase is changed from the c to the b axis. However, by applying a small pressure of P∼0.3 GPa, this mAF∥b is easily changed to mAF∥c. Thus, the magnetic anisotropy could be easily controlled by tuning the pressure slightly. This indicates that the c-f hybridization dominates the magnetic anisotropy in the AFM ordered state and plays an essential role in the unusual AFM order in CeT2Al10 (T=Ru,Os).
    Physical Review B. 07/2013; 88(4).
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    ABSTRACT: We have investigated the marked change in the magnetic properties of CeRu2Al10 induced by a small amount of Rh substitution [Ce(Ru0.95Rh0.05)2Al10]. The magnetic susceptibility along the a-axis (χa) shows a large decrease with decreasing temperature after showing a sharp peak at T0=24 K and the high field magnetization for the magnetic field along the a-axis exhibits a spin-flop transition at ˜13 T. These indicate that the antiferromagnetic (AFM) order where the AFM moment is parallel to the a-axis takes place, which is consistent with the anisotropy of χa>χc>χb in the paramagnetic region, being different from the AFM order in CeRu2Al10. These results were analyzed by mean field calculation for the two-sublattice model using the crystalline electric field level scheme by Strigari et al. [Phys. Rev. B 86 (2012) 081105]. On the basis of both experimental and calculated results, we conclude that, by a small amount of Rh substitution with one extra 4d electron, the large c--f hybridization along the a-axis, leading to the unusual ground state of CeRu2Al10, is markedly suppressed and the Ce ion in a Rh-substituted sample becomes close to the normal localized magnetic ion.
    Journal of the Physical Society of Japan 05/2013; 82(5):4709-. · 2.09 Impact Factor
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    ABSTRACT: In order to uncover the 4f electron state as a background for the emergence of the novel phase below T0 in CeT2Al10 (T = Ru, Os), the nuclear spin--lattice relaxation rate 1/T1 in the well-localized RKKY system NdRu2Al10 was measured and compared with that in CeRu2Al10. The value of 1/T1 in NdRu2Al10 with TN=2.4 K, which follows de Gennes scaling from TN=16 K in GdRu2Al10, shows a characteristic T-dependence for localized systems, 1/T1=a/(b+cT), with the constants a, b, and c. On the other hand, in CeRu2Al10, 1/T1 at temperatures higher than T*˜ 60 K also follows the same function if the Korringa relaxation is subtracted, indicating that the Ce 4f electron in CeRu2Al10 is in a localized state at least down to T*. Below the characteristic temperature T*, the downward deviation of 1/T1 from the above function might be ascribed to the beginning of Kondo coherence as proposed previously. Further comparison with CeT2Al10 compounds suggests that T* becomes more than 200 K in T = Os and that a coherent Kondo semiconducting state finally emerges in T = Fe.
    Journal of the Physical Society of Japan 02/2013; 82(2):3702-. · 2.09 Impact Factor

Publication Stats

652 Citations
249.70 Total Impact Points

Institutions

  • 2014
    • Hiroshima University
      • Department of Quantum Matter
      Hirosima, Hiroshima, Japan
  • 2005–2014
    • Kochi University
      • • Graduate School of Integrated Arts and Sciences
      • • Faculty of Science
      Kôti, Kōchi, Japan
  • 2010
    • Wuhan National High Magnetic Field Center
      Wu-han-shih, Hubei, China
  • 1996–2004
    • Nagoya University
      • Graduate School of Science
      Nagoya-shi, Aichi-ken, Japan