Takashi Nishioka

Hiroshima University, Hirosima, Hiroshima, Japan

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Publications (148)264.44 Total impact

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    ABSTRACT: A Kondo semiconductor CeRu$_2$Al$_{10}$ with an orthorhombic crystal structure shows an unusual antiferromagnetic ordering at rather high temperature $T_0$ of 27.3 K, which is lower than the Kondo temperature $T_{\rm K}\sim$ 60 K. In optical conductivity [$\sigma(\omega)$] spectra that directly reflect electronic structure, the $c$-$f$ hybridization gap between the conduction and $4f$ states is observed at around 40 meV along the three principal axes. However, an additional peak at around 20 meV appears only along the $b$ axis. With increasing $x$ to 0.05 in Ce(Ru$_{1-x}$Rh$_x$)$_2$Al$_{10}$, the $T_0$ decreases slightly from 27.3 K to 24 K, but the direction of the magnetic moment changes from the $c$ axis to the $a$ axis. Thereby, the $c$-$f$ hybridization gap in the $\sigma(\omega)$ spectra is strongly suppressed, but the intensity of the 20-meV peak remains as strong as for $x=0$. These results suggest that the change of the magnetic moment direction originates from the decreasing of the $c$-$f$ hybridization intensity. The magnetic ordering temperature $T_0$ is not directly related to the $c$-$f$ hybridization but is related to the charge excitation at 20 meV observed along the $b$ axis.
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    ABSTRACT: We have studied the Pr- and La-doping effects on the magnetic anisotropy in the antiferro-magnetic (AFM) phase of ${\mathrm{CeRu}}_{2}{\mathrm{Al}}_{10}$. The crystalline electric field (CEF) splitting in ${\mathrm{PrRu}}_{2}{\mathrm{Al}}_{10}$ was found to be as large as $\ensuremath{\sim}800$ K with a singlet ground state. In ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$, the CEF level scheme of the Pr ion is not changed with $x$. The AFM moment (${m}_{\mathrm{AF}}$) is rotated from $c$ to $b$ axis in both systems at ${x}_{c}^{\mathrm{sr}}\ensuremath{\sim}0.03$ and $\ensuremath{\sim}0.07$ for Ln=Pr and La, respectively. As the ionic radius of La and Pr is larger and smaller than that of Ce, respectively, these results indicate that the chemical pressure effect is not associated with the rotation of ${m}_{\mathrm{AF}}$, but is caused by the suppression of the $c\ensuremath{-}f$ hybridization originating from the decrease of $4f$ electrons of Ce ions by Ce-site substitution. Since a small amount of Pr or La doping changes easily the magnetization easy axis of all the moments on Ce sites, the origin of the magnetic anisotropy is not the local single ion effect but the bandlike effect through the anisotropic $c\ensuremath{-}f$ hybridization. The magnetic phase diagrams of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Ln}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$ indicate that above ${x}_{c}^{\mathrm{sr}}$, the AFM order with ${m}_{\mathrm{AF}}\ensuremath{\parallel}b$ continues to exist up to ${x}_{c}$, which is $\ensuremath{\sim}0.4$ and $\ensuremath{\sim}0.6$ in Ln=Pr and Ln=La, respectively. This indicates that even in the sample with an AFM transition temperature (${T}_{0}$) near ${x}_{c}$, the anisotropic $c\ensuremath{-}f$ hybridization dominates the AFM order. A large positive transverse magnetoresistance is seen below ${T}_{0}$, but a very small one above ${T}_{0}$. Together with the results of Hall resistivity and the observation of Shubnikov\char21{}de Haas oscillation, we propose that there exist large Fermi surfaces above ${T}_{0}$ and small ones below ${T}_{0}$. A gap is opened by the AFM order on almost the area of the large Fermi surface, and small Fermi surfaces are constructed below ${T}_{0}$, although we do not know the mechanism, which might be specific to the AFM order in Kondo semiconductors. The largest suppression of the magnetic scattering below ${T}_{0}$ is observed for the current $I\ensuremath{\parallel}a$ and the smallest one for $I\ensuremath{\parallel}b$. This anisotropy may be associated with the anisotropic $c\ensuremath{-}f$ hybridization, which may contribute to the anisotropic magnetic scattering of the conduction electron below ${T}_{0}$.
    Physical Review B 06/2015; 91(23). DOI:10.1103/PhysRevB.91.235124 · 3.74 Impact Factor
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    ABSTRACT: This study investigates pressure effects on the magnetic properties of non-interacting single-domain (SD) magnetite. Using a high-pressure cell specially designed for a Magnetic Property Measurement System, magnetic hysteresis measurements were conducted under high pressures of up to 1 GPa on natural plagioclase crystals containing much acicular SD magnetite. Coercivity and saturation magnetization were nearly constant with pressure, while saturation remanent magnetization and coercivity of remanence decreased with pressure at moderate rates of −8 per cent GPa–1 and −18 per cent GPa–1, respectively. These results suggest that temperature effects govern the magnetic behaviour of acicular SD magnetite grains in the middle and lower crusts.
    Geophysical Journal International 05/2015; 202(1):394-401. DOI:10.1093/gji/ggv154 · 2.72 Impact Factor
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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 effect of electron doping by the substitution of Rh for Ru on unconventional magnetic order in CeRu2Al10 was investigated via neutron powder diffraction. In Ce(Ru1-xRhx)(2)Al-10 with x = 0.05, 0.12, and 0.2, reorientation of the ordered moment from the c-axis as in pure CeRu2Al10 to the a-axis takes place in all samples, while the ordering vector q = (0, 1, 0) remains unchanged within this concentration range. The moment reorientation is accompanied by an increase in its size by a factor of similar to 2.4, from mu = 0.43 mu(B) at x = 0 to mu = 1.06, 1.04, and 1.02 mu(B) for x = 0.05, 0.12, and 0.2, respectively. The continuous decrease in the Neel temperature T-0(T-N), despite an abrupt increase in mu, underlines the strong anisotropy in the exchange interaction in CeRu2Al10 and the fact that this anisotropy is easily suppressed by electron doping.
    Journal of the Physical Society of Japan 10/2014; 83(10):104707. DOI:10.7566/JPSJ.83.104707 · 1.48 Impact Factor
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    ABSTRACT: We have performed Ru-NQR measurements on CeRu2Al10 exhibiting novel phase transition at an abnormally high temperature T-0 = 27.3K and on NdRu2Al10 with a magnetic transition temperature T-m = 2.4K as a reference RKKY system. The splitting of the NQR line due to internal fields below T-0 shows a mean-field-like monotonic increase, indicating no change in the magnetic structure below T-0. The internal field strength is one order larger than those at Al sites in CeRu2Al10, and is comparable to that at the Ru site in NdRu2Al10 despite the sevenfold smaller magnitude of the 4 f moment, being indicative of an enhanced conduction electron polarization at the Ru site. One of the causes of the high T-0 might be the enhanced exchange coupling through the Ce-Ru-Ce path mediated by the enhanced conduction electron polarization. Being similar to that in the Al site, the nuclear spin-lattice relaxation rate 1/T-1 shows a gap-like decrease below T-0 without enhancement owing to a critical slowing down at T-0, in contrast to the mean field 2nd-order transition.
    Journal of the Physical Society of Japan 10/2014; 83(10):103705. DOI:10.7566/JPSJ.83.103705 · 1.48 Impact Factor
  • H. Tanida · H. Nohara · M. Sera · T. Nishioka · M. Matsumura · R. Kobayashi
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    ABSTRACT: We investigated the Ce- and Ru-site substitution effects on the spin gap in CeRu2Al10 by measuring the magnetic and thermal properties of Ce(1-y)Ln(y)Ru(2)Al(10) (Ln = La, Pr, Y) and Ce(Ru1-xTx)(2)Al-10 (T = Re, Rh) system, respectively. The magnetic susceptibility shows that the itinerant and localized nature is enhanced by Re and Rh doping, respectively. In the latter, the orientation of the magnetic moment in the antiferromagnetic ordered phase suddenly changes from the c to a axis at x = 0.03. The orientation of the magnetic moment to the a axis is consistent with the large anisotropy of the magnetic susceptibility in the paramagnetic region. The specific heat shows that the exponential temperature dependence, e(-Delta/kBT), and the electronic specific coefficient gamma is not changed in the Ce-site substituted systems at least up to y = 0.1 but in the Ru-site substituted systems, the e(-Delta/kBT) dependence disappears, and the gamma value increases rapidly with x. These indicate that although the spin gap is robust against the Ce-site substitution by Ln ion at least up to y = 0.1, the spin gap is rapidly collapsed by the Ru-site substitution and in place, the conduction electron with heavy effective mass appears at the Fermi level. The spin gap which is formed under the subtle balance between the localized and itinerant nature is easily collapsed by a small amount of Re or Rh doping.
    Physical Review B 10/2014; 90(16). DOI:10.1103/PhysRevB.90.165124 · 3.74 Impact Factor
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    ABSTRACT: We examined the magnetic and transport properties of the Kondo semiconductor CeFe2Al10. A large increase in the electrical resistivity along the a- and b-axes (rho(a), rho(b)) is observed below 20 K, but only a small increase in rho(c). A clear shoulder is seen at T similar to 20 K in the thermoelectric power along the b-axis. The upturn of rho(a) and rho(b) below 20 K is quite easily suppressed by pressure as well as by a magnetic field along the a-axis. From these results, we conclude that the c-f hybridization gap below 20 K is characterized by a charge gap along the a- and b-axes, and a spin gap along the a-axis, while a small charge gap effect is observed along the c-axis.
    Journal of the Physical Society of Japan 08/2014; 83(8):084708. DOI:10.7566/JPSJ.83.084708 · 1.48 Impact Factor
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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; 233. DOI:10.1016/j.pepi.2014.06.001 · 2.40 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.
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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: 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
  • 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: 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: 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
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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
  • 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.
    Physical Review B 01/2014; 89(16). DOI:10.1103/PhysRevB.89.161103 · 3.74 Impact Factor
  • M. Sera · H. Nohara · M. Nakamura · H. Tanida · T. Nishioka · M. Matsumura
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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). DOI:10.1103/PhysRevB.88.100404 · 3.74 Impact Factor

Publication Stats

1k Citations
264.44 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
    • University of Hyogo
      • Department of Material Science
      Kōbe, Hyōgo, Japan
  • 1994–2004
    • Nagoya University
      • • Graduate School of Science
      • • Department of Material Science
      Nagoya-shi, Aichi-ken, Japan
    • The University of Tokyo
      Tōkyō, Japan