Faculty Profiles - TANIGUCHI Hiroki

Papers - TANIGUCHI Hiroki

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Optical evidence for the spin-state disorder in LaCo<inf>1-x</inf>Rh<inf>x</inf>O<inf>3</inf> Reviewed Open Access

Ichiro Terasaki, Shinichiro Asai, Hiroki Taniguchi, Ryuji Okazaki, Yukio Yasui, Yuka Ikemoto, Taro Moriwaki

Journal of Physics Condensed Matter Vol. 29 ( 23 ) page： 235802 2017.5

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

© 2017 IOP Publishing Ltd. We have measured the infrared reflectivity of single-crystalline samples of LaCo1-xRhxO3 (x = 0, 0.05 and 0.10) from 10 to 300 K from 0.05 to 0.15 eV. We find that the optical phonons of the Co-O stretching mode depend on temperature and the Rh content. Analysis with three Lorentz oscillators reveals that the spin state of Co3+ in LaCo1-xRhxO3 can be understood in terms of a solid solution of low-spin- and high-spin-state Co3+ ions, and the substituted Rh ion retains some fraction of the high-spin Co3+ ions down to low temperature.

DOI： 10.1088/1361-648X/aa6ebc

Oxide heterostructure research in Korea Reviewed

Woo Seok Choi, Sungkyun Park, Hiroki Taniguchi, Tae Won Noh

Current Applied Physics Vol. 17 ( 5 ) page： 593 - 594 2017.5

COPD Assessment Test for measurement of health status in patients with idiopathic pulmonary fibrosis: A cross-sectional study

Matsuda Toshiaki, Taniguchi Hiroyuki, Ando Masahiko, Kondoh Yasuhiro, Kimura Tomoki, Kataoka Kensuke, Sakamoto Koji, Suzuki Atsushi, Furukawa Taiki, Hasegawa Yoshinori

RESPIROLOGY Vol. 22 ( 4 ) page： 721-727 2017.5

Improper ferroelectrics as high-efficiency energy conversion materials Reviewed

Toru Wakamatsu, Kenji Tanabe, Ichiro Terasaki, Hiroki Taniguchi

Physica Status Solidi - Rapid Research Letters Vol. 11 ( 5 ) page： 1700009 2017.5

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-V C H VERLAG GMBH

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim An improper ferroelectric is a certain type of ferroelectrics whose primary order parameter is not polarization but another physical quantity such as magnetization. In contrast to a conventional proper ferroelectrics as represented by Pb(Zr,Ti)O3 and BaTiO3, the improper ferroelectrics has been inconceivable for practical applications thus far. Herein, we illustrate the great potential of improper ferroelectrics for efficient conversion of temperature fluctuation to electric energy, as demonstrated with (Ca0.84Sr0.16)8[AlO2]12(MoO4)2 (CSAM-16). The present study has experimentally proven that CSAM-16 achieves an excellent electrothermal coupling factor and high electric field sensitivity for pyroelectric energy conversion that approach a practical level for application to self-powered autonomous electronic devices for rapidly spreading wireless sensor networks. The present results provide a novel approach to developing innovative pyroelectric energy harvesting devices using improper ferroelectrics.

DOI： 10.1002/pssr.201700009

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CiNii Research

Optical control of dielectric permittivity in LaAl<inf>0.99</inf>Zn<inf>0.01</inf>O<inf>3-δ</inf> Reviewed

Takayuki Nagai, Hidefumi Takahashi, Ryuji Okazaki, Kenji Tanabe, Ichiro Terasaki, Hiroki Taniguchi

Applied Physics Letters Vol. 110 ( 17 ) 2017.4

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

© 2017 Author(s). A photo-dielectric effect (i.e., a change in dielectric permittivity due to photo-irradiation) has been demonstrated in LaAl0.99Zn0.01O3-δ. Photo-irradiation with an incident energy of 3.4 eV was found to enhance the dielectric permittivity in LaAl0.99Zn0.01O3-δ over a wide frequency range from 100 Hz to 1 MHz. The change in dielectric permittivity in the high-frequency region hardly depended on frequency and was not accompanied by an increase in dielectric loss, indicating an intrinsic photo-dielectric effect in LaAl0.99Zn0.01O3-δ that is not due to photo-conduction. The dependence of the photo-dielectric effect on incident energy suggests the existence of deep in-gap states introduced by Zn substitution. The mechanism of the photo-dielectric effect in LaAl0.99Zn0.01O3-δ relates to the dielectric response of the photo-excited electrons trapped in the deep in-gap states, which work as effective polar displacements under an applied electric field. These findings are expected to contribute to the development of photo-capacitors that enable the remote control of the dielectric response via photo-irradiation.

DOI： 10.1063/1.4979644

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Improper Ferroelectricity in Stuffed Aluminate Sodalites for Pyroelectric Energy Harvesting Reviewed Open Access

Yusaku Maeda, Toru Wakamatsu, Ayako Konishi, Hiroki Moriwake, Chikako Moriyoshi, Yoshihiro Kuroiwa, Kenji Tanabe, Ichiro Terasaki, Hiroki Taniguchi

Physical Review Applied Vol. 7 ( 3 ) 2017.3

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

© 2017 American Physical Society. In the present study, we demonstrate ferroelectricity in stuffed aluminate sodalites (Ca1-xSrx)8[AlO2]12(WO4)2 (x≤0.2) (C1-xSxAW). Pyroelectric measurements clarify switchable spontaneous polarization in polycrystalline C1-xSxAW, whose polarization values are on the order of 10-2 μC/cm2 at room temperature. A weak anomaly in the dielectric permittivity at temperatures near the ferroelectric transition temperature suggests improper ferroelectricity of C1-xSxAW for all investigated values of x. A comprehensive study involving synchrotron x-ray powder diffraction measurements, molecular dynamics simulations, and first-principles calculations clarifies that the ferroelectric phase transition of C1-xSxAW is driven by the freezing of the fluctuations of WO4 tetrahedra in the voids of an [AlO2]1212- framework. The voltage response and electromechanical coupling factor of C1-xSxAW estimated from the present results indicate that this material exhibits excellent performance as a pyroelectric energy harvester, suggesting that aluminate sodalites exhibit great promise as a class of materials for highly efficient energy-harvesting devices.

DOI： 10.1103/PhysRevApplied.7.034012

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Absence of Magnetic Long Range Order in Ba3ZnRu2O9: A Spin-Liquid Candidate in the S = 3/2 Dimer Lattice

Terasaki Ichiro, Igarashi Taichi, Nagai Takayuki, Tanabe Kenji, Taniguchi Hiroki, Matsushita Taku, Wada Nobuo, Takata Atsushi, Kida Takanori, Hagiwara Masayuki, Kobayashi Kensuke, Sagayama Hajime, Kumai Reiji, Nakao Hironori, Murakami Youichi

Journal of the Physical Society of Japan Vol. 86 ( 3 ) page： 33702 - 33702 2017.3

Absence of magnetic long range order in Ba<inf>3</inf>ZnRu<inf>2</inf>O<inf>9</inf>: A spin-liquid candidate in the S = 3/2 dimer lattice 033702 Reviewed Open Access

Ichiro Terasaki, Taichi Igarashi, Takayuki Nagai, Kenji Tanabe, Hiroki Taniguchi, Taku Matsushita, Nobuo Wada, Atsushi Takata, Takanori Kida, Masayuki Hagiwara, Kensuke Kobayashi, Hajime Sagayama, Reiji Kumai, Hironori Nakao, Youichi Murakami

Journal of the Physical Society of Japan Vol. 86 ( 3 ) page： 033702 2017

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

©2017 The Physical Society of Japan. We have discovered a novel candidate for a spin liquid state in a ruthenium oxide composed of dimers of S = 3/2 spins of Ru5+, Ba3ZnRu2O9. This compound lacks a long range order down to 37 mK, which is a temperature 5000-times lower than the magnetic interaction scale of around 200 K. Partial substitution for Zn can continuously vary the magnetic ground state from an antiferromagnetic order to a spin-gapped state through the liquid state. This indicates that the spin-liquid state emerges from a delicate balance of inter- and intra-dimer interactions, and the spin state of the dimer plays a vital role. This unique feature should realize a new type of quantum magnetism.

DOI： 10.7566/JPSJ.86.033702

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CiNii Research

Effects of La-substitution in ferroelectric silicate Bi2SiO5 Open Access

Tatewaki S., Sakai Y., Fujii Y., Koreeda A., Yasui S., Azuma M., Tanabe K., Terasaki I., Taniguchi H.

Meeting Abstracts of the Physical Society of Japan Vol. 72.1 ( 0 ) page： 2719 - 2719 2017

Impurity-induced Ferromagnetism of CaRu1-xScxO3Probed by Thermal Conductivity Measurements in Magnetic Fields Open Access

Yamamoto T. D., Daou Ramzy, Taniguchi Hiroki, Hebert Sylvie, Maignan Antoine, Terasaki Ichiro

Meeting Abstracts of the Physical Society of Japan Vol. 72.2 ( 0 ) page： 2057 - 2057 2017

Colossal Seebeck effect enhanced by quasi-ballistic phonons dragging massive electrons in FeSb 2 Reviewed Open Access

H. Takahashi, R. Okazaki, S. Ishiwata, H. Taniguchi, A. Okutani, M. Hagiwara, I. Terasaki

Nature Communications Vol. 7 2016.9

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

© 2016 The Author(s). Phonon transport is an essential property of thermoelectric materials. Although the phonon carries heat, which reduces the thermoelectric efficiency, it contributes positively to the Seebeck coefficient S through the phonon-drag effect, as typified by the high-purity semiconductors, which show fairly large S at cryogenic temperatures. Although such a large S is attractive in terms of Peltier cooling, a clear guiding principle for designing thermoelectric materials enriched by the phonon-drag effect remains to be established. Here we demonstrate that a correlated semiconductor, FeSb2, is a promising thermoelectric material featuring quasi-ballistic phonons dragging d electrons with large effective mass. By changing the sample size within the sub-millimetre order for high-purity single crystals, we succeed in substantially increasing S to as much as -27 mV K-1 at low temperatures. Our results exemplify a strategy for exploring phonon-drag-based thermoelectric materials, the performance of which can be maximized by combining heavy electrons with ballistic phonons.

DOI： 10.1038/ncomms12732

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Optical sheet conductivities of layered oxides Reviewed Open Access

Kenji Tanabe, Hiroki Taniguchi, Ichiro Terasaki

Journal of Physics Condensed Matter Vol. 28 ( 32 ) page： 325501 2016.6

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

© 2016 IOP Publishing Ltd. We report on the optical properties of the layered Co oxides Bi2-xPbxSr2Co2O8 with x = 0 and 0.4 and discuss similarities among optical sheet conductivities of layered Co and Cu oxides. Optical sheet conductivity is defined as the product of the optical conductivity and the lattice parameter along the cross-layer direction. Although the optical conductivity spectra of both Bi2-xPbxSr2Co2O8 with x = 0 and 0.4 are similar in shape to Na0.75CoO2 and Ca3Co4O9 below 3 eV, they are much smaller in magnitude. In contrast, optical sheet conductivities are roughly identical among the four Co oxides below 3 eV, which indicates that the common CoO2 layer in these oxides has the same electronic state. In addition, we find that optical sheet conductivities are identical among the layered Cu oxides with a four-fold coordinated CuO4 plane. We suggest using optical sheet conductivity as a key concept to discuss the similarity among the layered materials.

DOI： 10.1088/0953-8984/28/32/325501

Two competing soft modes and an unusual phase transition in the stuffed tridymite-type oxide BaAl2 O4 Reviewed Open Access

Y. Ishii, S. Mori, Y. Nakahira, C. Moriyoshi, J. Park, B. G. Kim, H. Moriwake, H. Taniguchi, Y. Kuroiwa

Physical Review B Vol. 93 ( 13 ) 2016.4

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

© 2016 American Physical Society. We investigated the structural phase transition of BaAl2O4, which has a network structure with corner-sharing AlO4 tetrahedra, via synchrotron x-ray thermal diffuse scattering measurements and first-principles calculations. BaAl2O4 shows the structural phase transition at TC=451.4 K from the P6322 parent crystal structure to the low-temperature superstructure with a cell volume of 2a×2b×c. This phase transition is unusual, in which two energetically competing phonon modes at M and K points soften simultaneously. When approaching TC from above, the K-point mode appears first. However, this K-point mode is overcome by the later-developed M-point mode. The thermal diffuse scattering intensities from both modes increase sharply at TC; therefore, both modes soften simultaneously. The first-principles calculations demonstrate that the M-point mode is electrostatically more preferable than the K-point mode and determines the eventual low-temperature structure, although these two modes are competing energetically. This competition is characteristic of BaAl2O4, which is ascribed to the structurally flexible network structure of this compound.

DOI： 10.1103/PhysRevB.93.134108

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Novel charge ordering in the trimer iridium oxide BaIrO<inf>3</inf> Reviewed Open Access

Ichiro Terasaki, Shun Ito, Taichi Igarashi, Shinichiro Asai, Hiroki Taniguchi, Ryuji Okazaki, Yukio Yasui, Kensuke Kobayashi, Reiji Kumai, Hironori Nakao, Youichi Murakami

Crystals Vol. 6 ( 3 ) 2016.3

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

© 2016 by the authors; licensee MDPI, Basel, Switzerland. We have prepared polycrystalline samples of the trimer Ir oxide BaIrO3 with face-shared Ir3O12 trimers, and have investigated the origin of the phase transition at 182 K by measuring resistivity, thermopower, magnetization and synchrotron X-ray diffraction. We propose a possible electronic model and transition mechanism, starting from a localized electron picture on the basis of the Rietveld refinement. Within this model, BaIrO3 can be basically regarded as a Mott insulator, when the Ir3O12 trimer is identified to one pseudo-atom or one lattice site. The transition can be viewed as a transition from the Mott insulator phase to a kind of charge ordered insulator phase.

DOI： 10.3390/cryst6030027

Yamamoto Takafumi D., Kotani Atsuhiro, Nakajima Hiroshi, Okazaki Ryuji, Taniguchi Hiroki, Mori Shigeo, Terasaki Ichiro

Journal of the Physical Society of Japan Vol. 85 ( 3 ) page： 34711 - 34711 2016.3

High temperature stability of the dielectric and insulating properties of Ca(Ti, Zr)SiO<inf>5</inf> ceramics Reviewed

Junichi Kimura, Hiroki Taniguchi, Takashi Iijima, Takao Shimizu, Shintaro Yasui, Mitsuru Itoh, Hiroshi Funakubo

Applied Physics Letters Vol. 108 ( 6 ) 2016.2

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

© 2016 AIP Publishing LLC. Useful dielectric properties for high-temperature ceramic capacitors are demonstrated in a non-perovskite oxide, Ca(Ti0.85 Zr0.15)SiO5, which is mainly composed of one-dimensional chains of oxygen octahedra that are mutually linked by SiO4 tetrahedra. Its dielectric constant and low temperature coefficient of capacitance were found to be 43 and -102 ppm/K, respectively, over the wide temperature range of 300-780 K. The high insulating performance was also indicated by the high resistivity, exceeding 1011 Ω cm up to 523 K. The systematic dielectric measurements for Ca(Ti1-x Zrx)SiO5 as functions of the composition and temperature indicate that the suppression of the anti-ferroelectric phase transition of CaTiSiO5 by Zr4+-substitution is a key to improve the temperature-stability and the high-resistivity in Ca(Ti1-x Zrx)SiO5. The present results shed light on the development of a designing principle for ceramic capacitors for the high-temperature use.

DOI： 10.1063/1.4941803

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Optical conductivity of layered calcium cobaltate Ca<inf>3</inf>Co<inf>4</inf>O<inf>9</inf> Reviewed

Kenji Tanabe, Ryuji Okazaki, Hiroki Taniguchi, Ichiro Terasaki

Journal of Physics Condensed Matter Vol. 28 ( 8 ) page： 085601 2016.1

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

© 2016 IOP Publishing Ltd. We report the optical properties of layered calcium cobaltate, Ca3Co4O9, which is regarded as a promising candidate for use as a thermoelectric material. The optical conductivity shows three broad peaks related to the inter-band transition below 4 eV, which are quite similar to those in the spectra of NaxCoO2. This similarity implies that the CoO2 layer, which is an essential unit for both Ca3Co4O9 and NaxCoO2, is dominant in the energy band structure below 4 eV. In addition, we estimate the effective carrier number per Co site and find similarity between the CoO2 layers of Ca3Co4O9 and Na0.75CoO2, which is consistent with the similarity in their Seebeck coefficients. To discuss the contribution of the rocksalt-type Ca2CoO3 layer in Ca3Co4O9, we propose the concept of optical sheet conductivity in the layered materials and estimate its value in the Ca2CoO3 layer. A comparison with the spin-polarized band calculation of the LDA + Hubbard U formalism with U = 5 eV suggests that the Ca2CoO3 layer has the inter-band transition of 2.6 eV in the spin-down band structure. Evaluation of the valences of Co 3d orbitals indicates the existence of charge transfer from the Ca2CoO3 layer to the CoO2 layer and mixing of Co3+ and Co4+ in the CoO2 layer, which may be the origin of the large thermoelectric effect.

DOI： 10.1088/0953-8984/28/8/085601

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PubMed

Ferromagnetic cluster glass phase embedded in a paramagnetic and metallic host in non-uniform magnetic system CaRu<inf>1-x</inf>Sc<inf>x</inf>O<inf>3</inf> Reviewed Open Access

Takafumi D. Yamamoto, Atsuhiro Kotani, Hiroshi Nakajima, Ryuji Okazaki, Hiroki Taniguchi, Shigeo Mori, Ichiro Terasaki

Journal of the Physical Society of Japan Vol. 85 ( 3 ) page： 034711 2016

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

© 2016 The Physical Society of Japan. We have investigated both static and dynamic magnetic properties of polycrystalline CaRu1-xScxO3 system in order to clarify the role of Sc ions as a disorder for magnetic ordering. We have observed typical features of a ferromagnetic cluster glass state below around 40 K: (i) a broad, frequency-dependent peak in the ac magnetic susceptibility, (ii) a slow relaxation of the magnetization, and (iii) a continuous increase in the dc magnetic susceptibility in field cooling process. The composition dependence of characteristic parameters for the cluster glass state suggests that chemical segregation can hardly explain the clustering mechanism. We propose a possible picture that the ferromagnetic clusters are distributed uniformly and form the glassy state embedded in the paramagnetic and metallic host of CaRuO3.

DOI： 10.7566/JPSJ.85.034711

Heterovalent Pb-substitution in ferroelectric bismuth silicate Bi<inf>2</inf>SiO<inf>5</inf> Reviewed

Hiroki Taniguchi, Tomohiro Nakane, Takayuki Nagai, Chikako Moriyoshi, Yoshihiro Kuroiwa, Akihide Kuwabara, Masaichiro Mizumaki, Kiyofumi Nitta, Ryuji Okazaki, Ichiro Terasaki

Journal of Materials Chemistry C Vol. 4 ( 15 ) page： 3168 - 3174 2016

Publishing type：Research paper (scientific journal)

© The Royal Society of Chemistry 2016. Systematic tuning of the ferroelectric phase transition in Bi2SiO5 was demonstrated using element substitution, where nominally heterovalent Pb was successfully substituted for Bi up to 20%. Comprehensive studies using powder X-ray diffraction analysis, dielectric measurements, X-ray absorption spectroscopy, and first-principles calculations revealed that charge compensation for heterovalent substitution was achieved via introduction of a considerable number of oxygen vacancies in Bi2O2 layers, resulting in the nominal composition (Bi1-x3+Pbx2+)2SiO5-x. The oxygen vacancies were found to induce local disordering of SiO4 tetrahedral chains, sandwiched between the Bi2O2 layers, leading to a decrease in the ferroelectric phase transition temperature and broadening of the phase transition in Bi2SiO5.

DOI： 10.1039/c6tc00584e

Scopus

Heterovalent Pb-substitution in ferroelectric bismuth silicate Bi2SiO5 Reviewed

Hiroki Taniguchi, Tomohiro Nakane, Takayuki Nagai, Chikako Moriyoshi, Yoshihiro Kuroiwa, Akihide Kuwabara, Masaichiro Mizumaki, Kiyofumi Nitta, Ryuji Okazaki, Ichiro Terasaki

JOURNAL OF MATERIALS CHEMISTRY C Vol. 4 ( 15 ) page： 3168 - 3174 2016

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

Systematic tuning of the ferroelectric phase transition in Bi2SiO5 was demonstrated using element substitution, where nominally heterovalent Pb was successfully substituted for Bi up to 20%. Comprehensive studies using powder X-ray diffraction analysis, dielectric measurements, X-ray absorption spectroscopy, and first-principles calculations revealed that charge compensation for heterovalent substitution was achieved via introduction of a considerable number of oxygen vacancies in Bi2O2 layers, resulting in the nominal composition (Bi1-x Pb-3+(x)2+)(2)SiO5-x. The oxygen vacancies were found to induce local disordering of SiO4 tetrahedral chains, sandwiched between the Bi2O2 layers, leading to a decrease in the ferroelectric phase transition temperature and broadening of the phase transition in Bi2SiO5.

DOI： 10.1039/c6tc00584e

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Papers - TANIGUCHI Hiroki