Faculty Profiles - TANIGUCHI Hiroki

Papers - TANIGUCHI Hiroki

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Spectroscopic signature of trimer Mott insulator and charge disproportionation in BaIrO3 Reviewed Open Access

R. Okazaki, S. Ito, K. Tanabe, H. Taniguchi, Y. Ikemoto, T. Moriwaki, I. Terasaki

Physical Review B Vol. 98 ( 20 ) 2018.11

Language：Japanese Publishing type：Research paper (scientific journal)

© 2018 American Physical Society. We have measured the reflectivity spectra of the barium iridate 9RBaIrO3, the crystal structure of which consists of characteristic Ir3O12 trimers. In the high-temperature phase above the transition temperature Tc≃180 K, we find that the optical conductivity involves two temperature-dependent optical transitions with an ill-defined Drude response. These features are reminiscent of the optical spectra in the organic dimer Mott insulators, implying a possible emergence of an unusual electronic state named trimer Mott insulator in BaIrO3, where the carrier is localized on the trimer owing to the strong Coulomb repulsion. Along with a pronounced splitting of the phonon peak observed below Tc, which is a hallmark of charge disproportionation, we discuss a possible phase transition from the trimer Mott insulator to a charge-ordered insulating phase in BaIrO3.

DOI： 10.1103/PhysRevB.98.205131

Open Access

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Scopus

Impurity-induced spin-state crossover in La<inf>0.8</inf>Sr<inf>0.2</inf>Co<inf>1−x</inf>Al<inf>x</inf>O<inf>3</inf> Reviewed Open Access

Ichiro Terasaki, Masamichi Ikuta, Takafumi D. Yamamoto, Hiroki Taniguchi

Crystals Vol. 8 ( 11 ) page： 411 2018.11

Language：English Publishing type：Research paper (scientific journal) Publisher：MDPI

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. We have prepared a set of polycrystalline samples of La0.8Sr0.2Co1−xAlxO3 (0 ≤ x ≤ 0.2), and have measured the magnetization as functions of temperature and magnetic field. We find that the average spin number per Co ion (SCo) evaluated from the room-temperature susceptibility is around 1.2–1.3 and independent of x. However, we further find that SCo evaluated from the saturation magnetization at 2 K is around 0.3–0.7, and decreases dramatically with x. This naturally indicates that a significant fraction of the Co3+ ions experience a spin-state crossover from the intermediate-to low-spin state with decreasing temperature in the Al-substituted samples. This spin-state crossover also explains the resistivity and the thermopower consistently. In particular, we find that the thermopower is anomalously enhanced by the Al substitution, which can be consistently explained in terms of an extended Heikes formula.

DOI： 10.3390/cryst8110411

Terasaki Ichiro, Misawa Kazuki, Tanabe Kenji, Taniguchi Hiroki

Journal of the Physical Society of Japan Vol. 87 ( 10 ) page： 104603 - 104603 2018.10

Competing structural instabilities in Bi2SiO5 Reviewed

A. Girard, H. Taniguchi, S. M. Souliou, M. Stekiel, W. Morgenroth, A. Minelli, A. Kuwabara, A. Bosak, B. Winkler

Physical Review B Vol. 98 ( 13 ) 2018.10

Language：Japanese Publishing type：Research paper (scientific journal)

© 2018 American Physical Society. The lattice dynamics of Bi2SiO5 has been studied by x-ray thermal diffuse scattering (TDS), inelastic x-ray scattering (IXS), and density functional theory calculations across the ferroelectric transition at TC=663 K. Rodlike diffuse features are observed along the [100] direction, perpendicular to the direction of the quasi-one-dimensional silicate chains. IXS measurements of the phonon dispersion along the [100] Γ-Y direction related these diffuse features to coupled unstable transverse optic and acoustic branches. A strong temperature-dependent TDS signal was also observed at the Brillouin zone edges, at the Y and S points, indicating a competition between ferroelectric and antiferroelectric distortions. We observed with IXS a substantial but finite phonon softening at the Y point and a minor softening at the S point, that are associated with the displacement of oxygen atoms. The good agreement between the experiment and the theoretical TDS and IXS allows us to provide a quantitative overview of the various lattice dynamical instabilities occurring at TC. The direct evidence for competing ferroelectric and antiferroelectric orders opens the way towards tuning the properties of Bi2SiO5 by application of pressure or electrical field.

DOI： 10.1103/PhysRevB.98.134102

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Scopus

Dynamical coupling of dilute magnetic impurities with quantum spin liquid state in the S = 3/2 dimer compound Ba<inf>3</inf>ZnRu<inf>2</inf>O<inf>9</inf> Open Access

Takafumi D. Yamamoto, Hiroki Taniguchi, Ichiro Terasaki

Journal of Physics Condensed Matter Vol. 30 ( 35 ) 2018.9

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

© 2018 IOP Publishing Ltd. We have investigated the dilute magnetic impurity effect on the magnetic properties of a quantum spin liquid candidate Ba3ZnRu2O9 and a spin gapped compound Ba3CaRu2O9. The magnetic ground state of each compound stands against 2% substitution of magnetic impurities for Zn or Ca. We have found that the magnetic response of these impurities, which behave as paramagnetic spins, depends on the host materials and the difference of the two manifests itself in the Weiss temperature, which can hardly be explained by the dilute magnetic impurities alone in the case of Ba3ZnRu2O9. We consider a contribution from the Ru5+ ions which would appear only in the substituted Ba3ZnRu2O9 and discuss a possible physical meaning of the observed Weiss temperature.

DOI： 10.1088/1361-648X/aad5ac

Structural variations and dielectric properties of (B i1-x L ax)2Si O5 (0≤x≤0.1): Polycrystallines synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses STRUCTURAL VARIATIONS and DIELECTRIC ⋯ HIROKI TANIGUCHI et al. Reviewed

Hiroki Taniguchi, Shingo Tatewaki, Shintaro Yasui, Yasuhiro Fujii, Jun Ichi Yamaura, Ichiro Terasaki

Physical Review Materials Vol. 2 ( 4 ) 2018.4

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER PHYSICAL SOC

© 2018 American Physical Society. This paper focuses on effects of isovalent La substitution on the crystal structure and dielectric properties of ferroelectric Bi2SiO5. Polycrystalline samples of (Bi1-xLax)2SiO5 are synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses with a composition range of 0≤x≤0.1. The crystal structure changes from monoclinic to tetragonal with increasing La-substitution rate x at room temperature. This structural variation stems from the change in orientation of SiO4 tetrahedra that form one-dimensional chains when they are in the ordered configuration, thus suggesting that lone-pair electrons play an important role in sustaining one-dimensional chains of SiO4 tetrahedra. Synchronizing with the disordering of SiO4 chains, ferroelectric phase transition temperature of (Bi1-xLax)2SiO5 sharply decreases as x increases, and ferroelectricity finally vanishes at around x=0.03. The present results demonstrate that lone-pair electrons of Bi play an important role in the ferroelectricity of Bi2SiO5 through propping the ordered structure of one-dimensional SiO4 chains with stereochemical activity. Furthermore, an additional phase transition has been first discovered in the low-temperature region of (Bi1-xLax)2SiO5 with x≤0.01, where the ordered one-dimensional SiO4 chains remain.

DOI： 10.1103/PhysRevMaterials.2.045603

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Transport and magnetic properties of CaPd3-xCuxO4 (x ≤ 2.4) with interconnecting Cu(Pd)O4 chain structure Reviewed Open Access

Ichiro Terasaki, Kazuki Misawa, Kenji Tanabe, Hiroki Taniguchi

Journal of the Physical Society of Japan Vol. 87 ( 10 ) page： 104603 2018

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

© 2018 The Physical Society of Japan. We have successfully synthesized a set of polycrystalline samples of CaPd3-xCuxO4 from x = 0 to 2.4, and measured the resistivity, thermopower and susceptibility down to 4K. We find that the solid solution between Pd and Cu systematically evolves the electronic ground states from a conventional band insulator to a correlated Mott insulator. In particular, CaPd0.6Cu2.4O4 shows a magnetic phase transition at 20 K. A partial substitution of La and Y for Ca supplies electrons to CaPd0.6Cu2.4O4, and decreases the room-temperature resistivity smaller than 10ωcm.

DOI： 10.7566/JPSJ.87.104603

Optical control of dielectric permittivity in aluminates Open Access

Nagai Takayuki, Terasaki Ichiro, Taniguchi Hiroki

Meeting Abstracts of the Physical Society of Japan Vol. 73.2 ( 0 ) page： 897 - 897 2018

Spin-state control in La0.8Sr0.2Co1-xAlxO3 Open Access

Ikuta Masamichi, Yamamoto Takafumi D., Tanabe Kenji, Taniguchi Hiroki, Terasaki Ichiro

Meeting Abstracts of the Physical Society of Japan Vol. 73.1 ( 0 ) page： 2199 - 2199 2018

Structural Phase Transitions and Elastic Response of (Ca1-xSrx)8[AlO2]12(MoO4)2 Open Access

Nakamori Hiroki, Terasaki Ichiro, Taniguchi Hiroki

Meeting Abstracts of the Physical Society of Japan Vol. 73.2 ( 0 ) page： 2130 - 2130 2018

Photo-induced persistent enhancement of dielectric permittivity in Zn:BaAl<inf>2</inf>O<inf>4</inf> Reviewed

Takayuki Nagai, Yasusei Yamada, Kenji Tanabe, Ichiro Terasaki, Hiroki Taniguchi

Applied Physics Letters Vol. 111 ( 23 ) page： 232902 2017.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

© 2017 Author(s). Photo-induced persistent enhancement of dielectric permittivity has been demonstrated in Ba(Al0.97Zn0.03)2O4-δ (BAZ) with a stuffed-Tridymite-Type structure under photo-irradiation with an energy of 3.4 eV (365 nm). The dielectric permittivity is enhanced by more than 20% and shows a weak frequency dependency. The dielectric loss (tanδ) shows only a slight increment for frequencies up to 1 MHz, indicating that the incremental change in dielectric permittivity of BAZ is not caused by photoconduction. This enhancement persists on termination of photo-irradiation. The results of this study suggest that the origin of the enhancement of dielectric permittivity is related to the dielectric response of the photo-excited dipole moments, which are composed of photo-excited electrons trapped in in-gap states and photo-excited holes generated in the valence band maximum. These findings shed light on the development of devices that can control dielectric properties by photo-irradiation.

DOI： 10.1063/1.5001369

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CiNii Research

Intrinsic Enhancement of Dielectric Permittivity in (Nb + In) co-doped TiO<inf>2</inf> single crystals Reviewed Open Access

Masaru Kawarasaki, Kenji Tanabe, Ichiro Terasaki, Yasuhiro Fujii, Hiroki Taniguchi

Scientific Reports Vol. 7 ( 1 ) page： 5351 2017.12

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

© 2017 The Author(s). The development of dielectric materials with colossal permittivity is important for the miniaturization of electronic devices and fabrication of high-density energy-storage devices. The electron-pinned defect-dipoles has been recently proposed to boost the permittivity of (Nb + In) co-doped TiO2 to 105. However, the follow-up studies suggest an extrinsic contribution to the colossal permittivity from thermally excited carriers. Herein, we demonstrate a marked enhancement in the permittivity of (Nb + In) co-doped TiO2 single crystals at sufficiently low temperatures such that the thermally excited carriers are frozen out and exert no influence on the dielectric response. The results indicate that the permittivity attains quadruple of that for pure TiO2. This finding suggests that the electron-pinned defect-dipoles add an extra dielectric response to that of the TiO2 host matrix. The results offer a novel approach for the development of functional dielectric materials with large permittivity by engineering complex defects into bulk materials.

DOI： 10.1038/s41598-017-05651-z

Magneto-thermopower in the weak ferromagnetic oxide CaRu<inf>0.8</inf>Sc<inf>0.2</inf>O<inf>3</inf>: An experimental test for the Kelvin formula in a magnetic material Reviewed Open Access

Takafumi D. Yamamoto, Hiroki Taniguchi, Yukio Yasui, Satoshi Iguchi, Takahiko Sasaki, Ichiro Terasaki

Journal of the Physical Society of Japan Vol. 86 ( 10 ) page： 104707 2017.10

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

©2017 The Physical Society of Japan. We have measured the resistivity, the thermopower, and the specific heat of the weak ferromagnetic oxide CaRu0.8Sc0.2O3 in external magnetic fields up to 140 kOe below 80 K. We have observed that the thermopower Q is significantly suppressed by magnetic fields at around the ferromagnetic transition temperature of 30 K, and have further found that the magneto-thermopower QðH; TÞ ¼ QðH; TÞ Qð0; TÞ is roughly proportional to the magneto-entropy SðH; TÞ ¼ SðH; TÞ Sð0; TÞ. We discuss this relationship between the two quantities in terms of the Kelvin formula, and find that the observed ΔQ is quantitatively consistent with the values expected from the Kelvin formula, a possible physical meaning of which is discussed.

DOI： 10.7566/JPSJ.86.104707

Magneto-thermopower in the Weak Ferromagnetic Oxide CaRu0.8Sc0.2O3: An Experimental Test for the Kelvin Formula in a Magnetic Material Open Access

Yamamoto Takafumi D., Taniguchi Hiroki, Yasui Yukio, Iguchi Satoshi, Sasaki Takahiko, Terasaki Ichiro

Journal of the Physical Society of Japan Vol. 86 ( 10 ) page： 104707 - 104707 2017.10

Enhancement of the dielectric permittivity of (Nb<inf>1/2</inf>In<inf>1/2</inf>)<inf>0.02</inf>Ti<inf>0.98</inf>O<inf>2</inf> single crystals at low temperatures due to (Nb + In) codoping Reviewed

Hiroki Taniguchi, Kako Ando, Ichiro Terasaki

Japanese Journal of Applied Physics Vol. 56 ( 10 ) 2017.10

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IOP PUBLISHING LTD

© 2017 The Japan Society of Applied Physics. Dielectric measurements are performed on (Nb1/2In1/2)0.02Ti0.98O2 (NITO-2.0) single crystals grown by a floating zone method to address the nature of the colossal permittivity recently reported in (Nb + In) co-doped TiO2 ceramics. The colossal permittivity of the order of 105, which is also observed in the NITO-2.0 single crystals, disappears in the lowest temperature region, indicating an extrinsic contribution from thermally excited carriers to the colossal permittivity. Even at low temperatures where the thermally excited carriers are expected to be frozen out, a high permittivity of the order of 103 remains. This finding suggests that an intrinsic contribution from electron-pinned defect dipoles boosts the dielectric permittivity of TiO2.

DOI： 10.7567/JJAP.56.10PC02

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Magnetoelectric coupling in the pyrochlore ruthenate Gd<inf>2</inf>Ru<inf>2</inf>O<inf>7</inf> Reviewed Open Access

Takuma Okamura, Ryuji Okazaki, Hiroki Taniguchi, Yukio Yasui, Ichiro Terasaki

Journal of the Physical Society of Japan Vol. 86 ( 8 ) page： 084708 2017.8

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

©2017 The Physical Society of Japan. We have prepared polycrystalline samples of Gd2Ru2O7, and have measured the dielectric constant, magnetization and magnetostriction in external fields from 0 to 7 T below 15 K. We find that the dielectric constant of Gd2Ru2O7 significantly changes with external fields, and reveal a finite magnetoelectric coupling in this oxide. Using a simple model in which the spins on the Gd ions feel an internal magnetic field of 7 T from the Ru clusters, we have explained magnetization and magnetoelectric effects qualitatively.

DOI： 10.7566/JPSJ.86.084708

Magnetoelectric Coupling in the Pyrochlore Ruthenate Gd2Ru2O7 Open Access

Okamura Takuma, Okazaki Ryuji, Taniguchi Hiroki, Yasui Yukio, Terasaki Ichiro

Journal of the Physical Society of Japan Vol. 86 ( 8 ) page： 84708 - 84708 2017.8

Giant inductance in non-ohmic conductor Reviewed

Kenji Tanabe, Hiroki Taniguchi, Fumihiko Nakamura, Ichiro Terasaki

Applied Physics Express Vol. 10 ( 8 ) 2017.8

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

© 2017 The Japan Society of Applied Physics. An inductor is composed of a coil, and its design has seen little progress since its inception more than one hundred years ago. We propose a novel method of inducing inductance in a non-ohmic conductor. In this paper, we report a giant inductance of 42H in Ca2RuO4 in the low frequency region at room temperature, as an example. The quality (Q) factor, defined as the ratio between the imaginary and real parts of impedance, approaches infinity in the static limit. Our inductor, therefore, displays ideal characteristics such as a large inductance and high Q factor, and it may be a prime candidate for next-generation inductors.

DOI： 10.7567/APEX.10.081801

Web of Science

Scopus

Local structure analysis of relaxor Pb(Mg<inf>1/3</inf>Nb<inf>2/3</inf>)O<inf>3</inf> Reviewed

Y. Yoneda, H. Taniguchi, D. Fu

Ferroelectrics Vol. 513 ( 1 ) page： 1 - 6 2017.6

Language：English Publishing type：Research paper (scientific journal) Publisher：TAYLOR & FRANCIS LTD

© 2017 Taylor & Francis Group, LLC. We performed local structure analyses of relaxor ferroelectric of lead magnesium niobate (PMN) by combining an extended X-ray absorption fine structure (EXAFS) and atomic pair-distribution function (PDF) methods. EXAFS study revealed that Pb atoms of PMN are disordered largely compared with those of PbTiO3, though the local environment of the Nb atoms is not significantly different from that for NaNbO3 and AgNbO3. The PDF analysis also revealed the short-range order structure of Pb, which is deviated from the perovskite framework.

DOI： 10.1080/00150193.2017.1350050

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Broadband micro light scattering observation of ferroelectric sodium metavanadate Reviewed

Yasuhiro Fujii, Daisuke Katayama, Mitsuyoshi Morita, Akitoshi Koreeda, Hiroki Taniguchi, Ichiro Terasaki

Ferroelectrics Vol. 512 ( 1 ) page： 14 - 19 2017.5

Language：English Publishing type：Research paper (scientific journal) Publisher：TAYLOR & FRANCIS LTD

© 2017 Taylor & Francis Group, LLC. Sodium metavanadate has pyroxene-type one dimensional VO4-tetrahedral chains, which is analogous to the SiO4-chains in Bi2SiO5. Although it is known to undergo ferroelectric phase transition, its mechanism has not been clarified. This paper shows the spectroscopic results of simultaneous measurement of Raman/Brillouin scattering by utilizing ultra-narrowband holographic notch filters. The spectroscopic anomalies accompanying to the phase transition were observed for the first time in both Raman and Brillouin regimes.

DOI： 10.1080/00150193.2017.1355146

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Papers - TANIGUCHI Hiroki