Takashi Nishioka

Hiroshima University, Hirosima, Hiroshima, Japan

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Publications (128)246.4 Total impact

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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;
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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: 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 01/2014; · 2.38 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
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    ABSTRACT: We have performed the investigation of the charge density distribution of CeT2Al10 (T=Ru, Fe) and the crystal structure parameters of LnT2Al10. The lattice parameters of a-, b-, and c-axes exhibit the anisotropic contraction when Ru is replaced by Fe in LnT2Al10, different from the isotropic contraction simply expected from the smaller ionic radius of Fe than Ru. The contraction is larger in the a- and c-axes than in the b-axis. This anisotropic contraction of the YbFe2Al10-type crystal structure originates from the zigzag degree of the zigzag chain formed by T and Al bond along the a- and c-axes are larger than that along the b-axis. The lattice parameters of CeT2Al10 (T=Ru, Fe) exhibit the anisotropic deviation from the lanthanide contraction. The deviation is largest in the a-axis and is very small in the b-axis. Both the characteristic YbFe2Al10-type crystal structure and the anisotropic deviation towards the intermediate valence indicate that the largest c--f hybridization along the a-axis plays the important role and is associated with the unusual antiferromagnetic order in CeT2Al10 (T=Ru, Os).
    Journal of the Physical Society of Japan 02/2013; 82(2):4603-. · 2.09 Impact Factor
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    ABSTRACT: Spin dynamics in the new Kondo insulator compound CeRu_{2}Al_{10} has been studied using unpolarized and polarized neutron scattering on single crystals. In the unconventional ordered phase forming below T_{0}=27.3 K, two excitation branches are observed with significant intensities, the lower one of which has a gap of 4.8±0.3 meV and a pronounced dispersion up to ≈8.5 meV. Comparison with random-phase approximation magnon calculations assuming crystal-field and anisotropic exchange couplings captures major aspects of the data, but leaves unexplained discrepancies, pointing to a key role of direction-specific hybridization between 4f and conduction band states in this compound.
    Physical Review Letters 12/2012; 109(26):267208. · 7.73 Impact Factor
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    ABSTRACT: We have investigated the magnetic properties in the well localized compound NdFe2Al10 and the Kondo semiconductor CeT2Al10 (T=Ru, Os) to clarify the origin of the unusual magnetic order in CeT2Al10. In NdFe2Al10, the experimental results of the magnetic properties could be reproduced very well by the mean-field calculation for the two-sublattice model. In CeT2Al10 we could reproduce the anisotropic magnetic susceptibility in the paramagnetic region above 60–100 K very well by the mean-field calculation for the two-sublattice model introducing an anisotropic exchange interaction and the recently determined crystalline electric field (CEF) level scheme from Strigari et al. [ Phys. Rev. B 86 081105 (2012)]. However, in the antiferromagnetic (AFM) ordered state, we could not reproduce the experimental results at all in the framework of the mean-field calculation for the two-sublattice model. We propose that although the magnetic properties in the paramagnetic region above 60–100 K could be understood well by a localized picture, the ordered state could not, and that the c-f hybridization, especially along the a axis, is associated with the unusual magnetic order in CeT2Al10.
    Physical review. B, Condensed matter 12/2012; 86(24). · 3.77 Impact Factor
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    ABSTRACT: We have performed Co-NQR study to clarify the complex magnetic phases in approaching to the quantum critical point (QCP). The successive transitions at TN2=12 K and TN3=8 K after the ferrimagnetic-like order at TN1=21 K in ambient pressure are confirmed by the spectral changes of Co-NQR However no critical slowing down of the nuclear spin-lattice relaxation rate 1/T1 or the nuclear spin-spin relaxation rate 1/T2 at TN2 and TN3 was observed, suggesting 1st order transitions. In applying pressure, a large spectral change of Co-NQR occurs in relatively low pressure of 0.3 Gpa. The Co-NQR spectrum becomes simple above about 0.7 GPa, consisting of the two Co sites with spectral weight ratio of 2. No successive transitions were observed in 1.5 GPa, indicating that the successive transitions are confined to relatively low pressure region. The extremely slower decrease of the sublattice magnetization than that expected in the mean field approximation is seen in 1.5 GPa.
    Journal of Physics Conference Series 12/2012; 400(3):2026-.
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    ABSTRACT: We measured specific heat of NdRu2Al10 single crystal under magnetic field along a-axis in order to investigate basic physical properties of this compound. We found three anomalies at T1 ~ 2.4 K, T2 ~ 1.2 K and T3 ~ 0.9 K in the temperature dependence of specific heat. These anomalies are attributed to antiferromagnetic order. In addition, we observed a Schottky-type anomaly, which reflects splitting of crystalline electric field (CEF). The entropy reaches Rln2 just above T1 and almost reaches Rln10 at room temperature. These results indicate that these three phase transitions occur in a doublet ground state and that all CEF levels of J = 9/2 are below 300K. The results of specific heat are consistent with the CEF level scheme proposed by magnetization measurement.
    Journal of Physics Conference Series 12/2012; 391(1):2029-.
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    ABSTRACT: Phonon spectra of LaRu2Al10 and CeRu2Al10 have been studied by Raman scattering from 3.4K to 300K. Polarization dependence at room temperature gives the precise mode assignment for the sake of first principles calculation results. The measured Raman spectra are well explained by the space group Cmcm and there is no remarkable difference between LaRu2Al10 and CeRu2Al10. Since we observed no anomaly at T0 = 27K in CeRu2Al10, the lattice distortion due to the structural transition has not been detected. From the temperature dependence of the vibrational frequency of Ce and La, their vibrational amplitude is not large in CeRu2Al10 and LaRu2Al10, that is, close to harmonic case.
    Journal of Physics Conference Series 12/2012; 391(1):2050-.
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    ABSTRACT: We carried out ultrasonic measurements on NdRu2Al10 single crystals in order to clarify what kind of phase transition occurs in this compound. The elastic moduli were measured as a function of temperature from 0.5 K to 150 K using the phase comparison-type pulse echo method. As the temperature is decreased from 150 K, longitudinal elastic modulus C11 and transverse elastic modulus C44 show a monotonic enhancement. With further decreasing temperature, both elastic moduli show abrupt hardening at 2.4 K and then show two-step softening at 1.3 K and 1.0 K. We found obvious hysteresis around both transitions at 1.3 K and 1.0 K in the temperature sweep, whereas no hysteresis was observed around the transition at 2.4 K within our experimental accuracy. The abrupt hardening at 2.4 K is very similar to that found in CeRu2Al10 and CeOs2Al10, suggesting that the transition at 2.4 K originates from the same mechanism of the transitions in CeRu2Al10 and CeOs2Al10. This is the first report that NdRu2Al10 undergoes the successive phase transitions.
    Journal of Physics Conference Series 12/2012; 391(1):2069-.
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    ABSTRACT: We have performed 27Al-NQR measurements in CeOs2Al10 which exhibits a novel phase transition at T0=29 K The NQR parameters determined for all the Al sites in ambient pressure were compared with those in CeRu2Al10 with T0=27 K and CeFe2Al10 with no phase transition. The distinct NQR splitting just below T0=32.5 K under pressure 0.66 GPa ensures an enhancement of T0 and a homogeneous transition. Despite the increase of T0, the nuclear spin-lattice relaxation rate 1/Tl is suppressed over whole range of temperature than in ambient pressure. The characteristic features of no critical slowing down at T0 and of the remarkable decrease of 1/T1T starting at T > T0 become prominent under pressure, suggesting an approach to Kondo semiconductor in a valence fluctuation regime.
    Journal of Physics Conference Series 12/2012; 400(3):2052-.
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    ABSTRACT: CeRu2Al10 causes a mysterious phase transition at T0~27 K In order to examine the feature of this transition, we have measured the magnetic susceptibility and electrical resistivity of Ce1-xRxRu2Al10 (R=La, La0.63Y0.37, Y) system in which Ce ions in CeRu2Al10 are replaced by non-magnetic rare earth ions with different ionic radii, where the relationship among the ionic radii is La3+ > (***La0.63Y0.37)3+ ~ Ce3+ > Y3+.*** The magnitude of the magnetic susceptibility becomes smaller with decreasing ionic radii, indicating Kondo temperature increases due to the chemical pressure effect. In the La- and La0.63Y0.37-substituted system, T0 decreases monotonically and disappears smoothly at x~0.7, while in the Y-substituted system, T0 decreases more slowly and disappears suddenly between x=0.4 and 0.5. These behaviors cannot be understood by a simple magnetic phase transition, suggesting that the change in valence of Ce ion plays an important role in the mysterious phase transition.
    Journal of Physics Conference Series 12/2012; 400(3):2073-.
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    ABSTRACT: Spin dynamics in the new Kondo insulator compound CeRu2Al10 has been studied using unpolarized and polarized neutron scattering on single crystals. In the unconventional ordered phase forming below T0 = 27.3 K, two excitation branches are observed with significant intensities, the lower one of which has a gap of 4.8 +/- 0.3 meV and a pronounced dispersion up to about 8.5 meV. Comparison with RPA magnon calculations assuming crystal-field and anisotropic exchange couplings captures major aspects of the data, but leaves unexplained discrepancies, pointing to a key role of direction-specific hybridization between 4f and conduction band states in this compound.
    Physical Review Letters 09/2012; 109(26). · 7.73 Impact Factor
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    ABSTRACT: We performed the detailed studies of the pressure effect on the anisotropic electrical resistivity (ρ) of CeRu2Al10 in order to clarify if the pressure effect is also anisotropic or not. The results clearly showed the large anisotropic pressure effect on ρ. By applying the pressure, the large peak of ρ along the c axis below the antiferromagnetic transition temperature (T0) is rapidly suppressed, and becomes small and similar to the large peak along the a axis where the peak is already small at the ambient pressure. On the other hand, that along the b axis at the ambient pressure is enhanced up to 2 GPa. Also, in the temperature dependence of ρ between T0 and ∼50 K, the similar anisotropic pressure effect is observed. Along the a and c axes, the semiconducting behavior is suppressed, but that along the b axis is enhanced by pressure. From these results, we concluded that the two-dimensional electronic nature of CeRu2Al10 is reinforced by pressure and becomes close to that of CeOs2Al10. The pressure effect of the magnetization along the a axis and the thermal expansion of CeRu2Al10 along the three crystal axes were also investigated. These results and the pressure effect on ρ indicate that the anisotropic c-f hybridization effect is important to clarify the unusual long-range order in this system.
    Physical review. B, Condensed matter 08/2012; 86(8). · 3.77 Impact Factor

Publication Stats

649 Citations
246.40 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