研究者詳細 - 中埜　彰俊

Cu Doping Effects on Magnetic Properties of Ba<inf>3</inf>ZnRu<inf>2</inf>O<inf>9</inf> 査読有り

Kimura A., Yasui Y., Nakano A., Taniguchi H., Terasaki I.

Journal of the Physical Society of Japan 94 巻 ( 3 ) 2025年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Physical Society of Japan

Magnetization and specific heat measurements were conducted on polycrystalline samples of the quantum spin liquid candidate material Ba3Zn1−xCuxRu2O9. Cu2+ doping at the Zn sites of Ba3ZnRu2O9 introduces paramagnetic behavior as observed in the temperature dependence of the magnetic susceptibility and magnetic specific heat of Ba3Zn1−xCuxRu2O9 (0 < x ≤ 0.03). The observed increase in magnetization and magnetic specific heat owing to Cu-doping can be explained by applying the Brillouin function and the Schottky specific heat model, which incorporate an inhomogeneous internal magnetic field. The Cu2+ spins in the doped samples exhibit a paramagnetic state and experience a fluctuating internal magnetic field. Based on these experimental findings, we discuss the interaction between the doped Cu2+ spins and the fluctuating Ru5+ spins.

DOI： 10.7566/JPSJ.94.034708

Titanite-type solid solutions (Ca,Pb)TiGeO5: A competition between triclinic and antiferroelectric distortions Open Access

Li Zerui, Kuwano Taro, Nakano Akitoshi, Hagiwara Manabu, Yasui Shintaro, Taniguchi Hiroki, Yokota Hiroko

Journal of the Ceramic Society of Japan advpub 巻 ( 0 ) 2025年

記述言語：英語出版者・発行元：The Ceramic Society of Japan

Titanite-type oxides, ABMO5, have been recently recognized as a novel vein of antiferroelectric (AFE) materials due to their unique crystal structure including an antiparallel arrangement of polar chains. This study presents the results of synthesis, structure analysis, and dielectric measurement of titanite-type solid solutions (Ca,Pb)TiGeO5. With the Pb composition increasing, structural symmetry at room temperature changes from monoclinic to triclinic. We identified the crystal structure of PbTiGeO5 by Rietveld refinement similar to that of triclinic titanite-type CaZrGeO5. In addition, the AFE phase transition temperature is decreased by Pb substitution. These indicate the competition between triclinic and AFE distortions, in other words, lone-pair electrons on Pb2+ ions and the second-order Jahn–Teller effect on the Ti4+ ions.

DOI： 10.2109/jcersj2.25007

CiNii Research

Melting of excitonic insulator phase by an intense terahertz pulse in Ta2NiSe5 査読有り Open Access

Takamura, N; Miyamoto, T; Ikeda, R; Kubo, T; Yamamoto, M; Sato, H; Han, Y; Ito, T; Sato, T; Nakano, A; Sawa, H; Okamoto, H

PHYSICAL REVIEW RESEARCH 6 巻 ( 4 ) 2024年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review Research

In this paper, the optical response to a terahertz pulse was investigated in the transition metal chalcogenide Ta2NiSe5, a candidate excitonic insulator. First, by irradiating a terahertz pulse with a relatively weak electric field (0.3 MV/cm), the spectral changes in reflectivity near the absorption edge due to third-order optical nonlinearity were measured, and the absorption peak characteristic of the excitonic phase just below the interband transition was identified. Next, by irradiating a strong terahertz pulse with a strong electric field of 1.65 MV/cm, the absorption of the excitonic phase was found to be reduced, and a Drude-like response appeared in the mid-infrared region. These responses can be interpreted as carrier generation by exciton dissociation induced by the electric field, resulting in the partial melting of the excitonic phase and metallization. The presence of a distinct threshold electric field for carrier generation indicates exciton dissociation via quantum-tunneling processes. The spectral change due to metallization by the electric field is significantly different from that due to the strong optical excitation across the gap, which can be explained by the different melting mechanisms of the excitonic phase in the two types of excitations.

DOI： 10.1103/PhysRevResearch.6.043187

Observation of converse flexoelectric effect in topological semimetals 査読有り Open Access

Takahashi, H; Kurosaka, Y; Kimura, K; Nakano, A; Ishiwata, S

COMMUNICATIONS MATERIALS 5 巻 ( 1 ) 2024年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Communications Materials

A strong coupling between electric polarization and elastic deformation in solids is an important　factor in creating useful electromechanical nanodevices. Such coupling is typically allowed in insulating materials with inversion symmetry breaking as exemplified by the piezoelectric effect in ferroelectric materials. Therefore, materials with metallicity and centrosymmetry have tended to be out of scope in this perspective. Here, we report the observation of giant elastic deformation by the application of an alternating electric current in topological semimetals (V,Mo)Te2, regardless of the centrosymmetry. Considering the crystal and band structures and the asymmetric measurement configurations in addition to the absence of the electromechanical effect in a trivial semimetal TiTe2, the observed effect is discussed in terms of a Berry-phase-derived converse flexoelectric effect in metals. The observation of the flexoelectric effect in topological semimetals paves a way for a new type of nanoscale electromechanical sensors and energy harvesting.

DOI： 10.1038/s43246-024-00677-z

Ca-induced phonon softening in BaTiO3 revealed by inelastic x-ray and neutron scattering 査読有り Open Access

Kimura, K; Tsutsui, S; Yamamoto, Y; Nakano, A; Kawamura, K; Kajimoto, R; Kamazawa, K; Martin, A; Webber, KG; Kakimoto, K; Hayashi, K

PHYSICAL REVIEW B 110 巻 ( 13 ) 2024年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review B

We performed inelastic x-ray and neutron scattering (IXS and INS) experiments for (Ba,Ca)TiO3 single crystals to elucidate the effect of Ca doping on the atomic dynamics. Dispersion relations of transverse phonons in BaTiO3, (Ba0.9Ca0.1)TiO3, and (Ba0.8Ca0.2)TiO3 were determined. The A-site-O vibrational mode as well as the acoustic mode were observed both by IXS and INS, whereas the Ti-O soft mode was detected only by INS. It was found that the A-site-O vibration exhibits significant softening with increasing Ca concentration, while the Ti-O soft mode is almost unaffected by the Ca doping. This finding is discussed in comparison with the polarization hysteresis loops and piezoelectric coefficients of (Ba,Ca)TiO3. It is indicated that the softening of the A-site-O vibration contributes to the enhancement of the piezoelectricity of Ca-doped BaTiO3-based materials.

DOI： 10.1103/PhysRevB.110.134314

Large Nernst Effect in the High-Mobility Semimetal Ta2PdSe6 査読有り Open Access

Nakano, A; Terasaki, I

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 93 巻 ( 10 ) 2024年10月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Physical Society of Japan

We have investigated the Nernst effect of the ternary transition-metal chalcogenide semimetal Ta2PdSe6, which has recently attracted much attention due to its giant Peltier conductivity at cryogenic temperatures. We observed that Nernst signals increase linearly with magnetic fields across the temperature and magnetic range investigated. The Nernst coefficient is higher than some kind of superconductors, and heavy fermion systems around 20 K. We discuss the origin of the large Nernst effect in relation to the small Fermi energy and high carrier mobility.

DOI： 10.7566/JPSJ.93.103703

Enhanced Cryogenic Thermoelectricity in Semimetal Ta2PdSe6 through Non-Fermi Liquid-Like Charge and Heat Transport 査読有り国際誌 Open Access

Kato, F; Maruoka, U; Nakano, A; Manjo, T; Ishikawa, D; Baron, AQR; Yasui, Y; Hasegawa, T; Terasaki, I

ADVANCED PHYSICS RESEARCH 2024年8月

Superconductivity in a ferroelectric-like topological semimetal SrAuBi 査読有り

Takahashi, H; Sasaki, T; Nakano, A; Akiba, K; Takahashi, M; Mayo, AH; Onose, M; Kobayashi, TC; Ishiwata, S

NPJ QUANTUM MATERIALS 8 巻 ( 1 ) 2023年12月

記述言語：英語出版者・発行元：npj Quantum Materials

Given the rarity of metallic systems that exhibit ferroelectric-like transitions, it is apparently challenging to find a system that simultaneously possesses superconductivity and ferroelectric-like structural instability. Here, we report the observation of superconductivity at 2.4 K in a layered semimetal SrAuBi characterized by strong spin–orbit coupling (SOC) and ferroelectric-like lattice distortion. Single crystals of SrAuBi have been successfully synthesized and found to show a polar-nonpolar structure transition at 214 K, which is associated with the buckling of Au-Bi honeycomb lattice. On the basis of the band calculations considering SOC, we found significant Rashba-type spin splitting and symmetry-protected multiple Dirac points near the Fermi level. We believe that this discovery opens up new possibilities of pursuing exotic superconducting states associated with the semimetallic band structure without space inversion symmetry and the topological surface state with the strong SOC.

DOI： 10.1038/s41535-023-00612-4

Spark plasma sintering on the thermoelectric sulfide Ta2PdS6 査読有り

Nakano, A; Suekuni, K; Hattori, N; Terasaki, I

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 131 巻 ( 10 ) 頁： 669 - 674 2023年10月

担当区分：筆頭著者,　責任著者記述言語：英語出版者・発行元：Journal of the Ceramic Society of Japan

We have prepared a dense ceramic sample of a novel thermoelectric sulfide Ta2PdS6 by means of spark plasma sintering technique under a uniaxial pressure. The Rietveld analyses of the X-ray diffraction patterns have clarified occurrence of weak preferred orientation in the sintering process. Resistivity, thermopower, and Hall resistivity measurements on the ceramic sample have indicated that it shows higher carrier density and lower carrier mobility than a single crystal. The inferior carrier mobility in the ceramic sample may partly come from increased point-defect and/or grain-boundary scattering, to which poor power factor is attributed.

DOI： 10.2109/jcersj2.23038

CiNii Research

Ferroelectricity in oxygen-deficient perovskite-type oxide Sr10Ga6Sc4O25 査読有り

Nakano, A; Terasaki, I; Taniguchi, H

JOURNAL OF MATERIALS CHEMISTRY C 11 巻 ( 25 ) 頁： 8541 - 8546 2023年6月

担当区分：筆頭著者,　責任著者記述言語：英語出版者・発行元：Journal of Materials Chemistry C

We have investigated the physical properties of an oxygen-deficient perovskite-type Sr10Ga6Sc4O25 by means of dielectric permittivity, pyroelectric current, and X-ray diffraction (XRD) measurements using a polycrystalline sample. The real part of the dielectric permittivity shows a clear anomaly around 310 K. Switchable pyroelectric current is also observed around this temperature, indicating a ferroelectric phase transition in the title compound. Synchrotron powder XRD measurements reveal that no discontinuous jump takes place in the temperature dependent cell volume around 310 K, implying that the phase transition is of second order. The candidates of the low and high temperature space groups are discussed based on the group theory.

DOI： 10.1039/d2tc05118d

High-Pressure Thermopower of the Mott Insulator Ca2RuO4 査読有り

Sano I., Nakano T., Nakano A., Taniguchi H., Nakamura F., Terasaki I.

Journal of the Physical Society of Japan 91 巻 ( 3 ) 2022年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Physical Society of Japan

We have measured and analyzed the thermopower of single-crystal samples of the Mott insulator Ca2RuO4 in high pressure up to 0.9 GPa. We find that the thermopower rapidly changes at a metal-insulator transition temperature, which is explained in terms of a percolation theory. In contrast, the thermopower in the insulator phase slightly decreases with increasing pressure well below the transition temperature, and is associated with thermodynamic quantities through the Kelvin formula. The pressure dependence is compared with the current-density dependence, and a possible origin for the current-enhanced thermopower is proposed.

DOI： 10.7566/JPSJ.91.033703

Charge and Lattice Dynamics in Excitonic Insulator Ta2NiSe5 Investigated Using Ultrafast Reflection Spectroscopy

Miyamoto, T; Mizui, M; Takamura, N; Hirata, J; Yamakawa, H; Morimoto, T; Terashige, T; Kida, N; Nakano, A; Sawa, H; Okamoto, H

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 91 巻 ( 2 ) 2022年2月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Physical Society of Japan

An excitonic insulator, in which an optical gap is formed by exciton condensation, is expected to exhibit large photoresponses. Here, we report the results of femtosecond reflection spectroscopy covering a wide infrared region on a typical excitonic insulator, Ta2NiSe5, which gives direct information not only about a change of excitonic gap but also a metallization. With a weak excitation, the excitonic gap decreases due to the weakening of excitonic effect by photocarriers. With a strong excitation, a Drude-like high-reflection band appears, showing an excitonic-insulator-to-metal transition. Analyses of spectral and temporal changes of reflectivity revealed that the metallization is accompanied by large structural changes. This demonstrates an important role of electron–lattice interaction on the stabilization of excitonic phase.

DOI： 10.7566/JPSJ.91.023701

Nontrivial Spin Dimer in the Hexagonal Ruthenate Ba3CaRu2O9 Revealed by Nonmagnetic Ion Substitution

Nishihara, D; Kimura, A; Nakano, A; Taniguchi, H; Terasaki, I

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 90 巻 ( 11 ) 2021年11月

出版者・発行元：Journal of the Physical Society of Japan

We have prepared a set of polycrystalline samples of Ba3Ca(Ru1−xNbx)2O9 consisting of the face-shared (Ru,Nb)2O9 dimer block from x = 0 to 0.12, and have measured the x-ray diffraction and magnetization. We find that all the samples of x > 0 show Curie–Weiss-like susceptibility below 30 K with the Weiss temperature θNb of 2–7 K, which suggests that the Nb-substituted NbRuO9 dimer interacts the Ru2O9 dimers magnetically. We also find the NbRuO9 dimer effectively works as S = 1 in the dilution limit, which is also suggested by the first-principle calculation. The Ru2O9 dimer lattice is nontrivial in the sense that S = 1 moment in the NbRuO9 dimer is screened for large x without any magnetic long range order. A possible origin is discussed.

DOI： 10.7566/JPSJ.90.114706

A large piezoelectric voltage coefficient in aluminate-sodalite-type improper ferroelectric oxides 査読有り

Taniguchi Hiroki, Hattori Taiga, Isobe Toshihiro, Nakano Akitoshi, Terasaki Ichiro, Hagiwara Manabu

JOURNAL OF MATERIALS CHEMISTRY C 2021年10月

Unconventional high-temperature ferromagnetic semiconductor PbPd1-x-yFeyLixO2 査読有り

He, YC; Sato, D; Misawa, K; Nishihara, D; Kimura, A; Nakano, A; Taniguchi, H; Terasaki, I

JOURNAL OF APPLIED PHYSICS 129 巻 ( 20 ) 2021年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Applied Physics

We have prepared a set of polycrystalline samples of PbPd 1 - x - y Fe y Li x O 2 (x = 0, 0.01, 0.02, 0.03, and 0.04; y = 0, 0.02, and 0.05) and have measured synchrotron x-ray diffraction and magnetization systematically. We have found high-temperature ferromagnetism in Fe- and Li-substituted samples and the largest magnetic moment of 0.023 μ B per formula unit in PbPd 0.93 Fe 0.05 Li 0.02 O 2. The ferromagnetism survives at 700 K, and the Curie temperature will be far above 800 K. Although similar but controversial high-temperature ferromagnets have been reported in many thin-film samples thus far, the discovered ferromagnetism is macroscopic in the sense that bulk samples stick to a permanent magnet at room temperature. The Fe and Li dependence of the ferromagnetism is complicated, implying that the ferromagnetism is truly unconventional.

DOI： 10.1063/5.0051283