研究者詳細 - 池田　直

PubMed

Present status of the experimental aspect of RFe2O4 study

Naoshi Ikeda, Tomoko Nagata, Jun Kano, Shigeo Mori

JOURNAL OF PHYSICS-CONDENSED MATTER 27 ( 5 ) 2015年2月

記述言語：英語出版者・発行元：IOP PUBLISHING LTD

We give a brief review of the experimental research on a triangular mixed valence iron oxide RFe2O4 (R = Y, Dy, Ho, Er, Tm, Yb, Lu, Sc or In). Interest in this material has been increasing every year because of the fascinating but complicated interaction between spin, charge and the orbital state of iron ions in frustrated geometry. Reports collected in this review cover experimental research on crystallography, chemical analysis, bulk and thin film preparation, magnetic, dielectric, diffraction with neutrons, x-ray and electron, optical and x-ray absorption, Mossbauer spectroscopy and other methods that incorporate the use of modern scientific technology and knowledge. The report mainly focuses on experimental facts since 1990 on which an early review by Siratori has been published (Kimizuka et al 1990 Handbook on the Physics and Chemistry of Rare Earths vol 13, ed K A Gschneidner Jr and L Eyring

DOI： 10.1088/0953-8984/27/5/053201

Superconductivity in alkali-metal-doped picene

Ryoji Mitsuhashi, Yuta Suzuki, Yusuke Yamanari, Hiroki Mitamura, Takashi Kambe, Naoshi Ikeda, Hideki Okamoto, Akihiko Fujiwara, Minoru Yamaji, Naoko Kawasaki, Yutaka Maniwa, Yoshihiro Kubozono

NATURE 464 ( 7285 ) 76 - 79 2010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：NATURE PUBLISHING GROUP

Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds(1), boron-doped diamond(2) and-most prominently-iron arsenides such as LaO(1-x)F(x)FeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (T(c)) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (K(x)picene) shows T(c) values of 7K and 18 K, depending on the metal content. The drop of magnetization in Kxpicene solids at the transition temperature is sharp (<2 K), similar to the behaviour of Ca-intercalated graphite(1). The Tc of 18K is comparable to that of K-intercalated C(60) (ref. 4). This discovery of superconductivity in K(x)picene shows that organic hydrocarbons are promising candidates for improved T(c) values.

DOI： 10.1038/nature08859

An organic thyristor

F Sawano, Terasaki, I, H Mori, T Mori, M Watanabe, N Ikeda, Y Nogami, Y Noda

NATURE 437 ( 7058 ) 522 - 524 2005年9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：NATURE PUBLISHING GROUP

Thyristors are a class of nonlinear electronic device that exhibit bistable resistance - that is, they can be switched between two different conductance states(1). Thyristors are widely used as inverters ( direct to alternating current converters) and for the smooth control of power in a variety of applications such as motors and refrigerators. Materials and structures that exhibit nonlinear resistance of this sort are not only useful for practical applications: they also provide systems for exploring fundamental aspects of solid- state and statistical physics. Here we report the discovery of a giant nonlinear resistance effect in the conducting organic salt(2) theta- ( BEDT- TTF) 2CsCo( SCN)(4), the voltage- current characteristics of which are essentially the same as those of a conventional thyristor. This intrinsic organic thyristor works as an inverter, generating an alternating current when a static direct- current voltage is applied. Whereas conventional thyristors consist of a series of diodes ( their nonlinearity comes from interface effects at the p- n junctions), the present salt exhibits giant nonlinear resistance as a bulk phenomenon. We attribute the origin of this effect to the current-induced melting of insulating charge- order domains, an intrinsically non- equilibrium phenomenon in the sense that ordered domains are melted by a steady flow.

DOI： 10.1038/nature04087

Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe2O4

Naoshi Ikeda, Hiroyuki Ohsumi, Kenji Ohwada, Kenji Ishii, Toshiya Inami, Kazuhisa Kakurai, Youichi Murakami, Kenji Yoshii, Shigeo Mori, Yoichi Horibe, Hijiri Kitô

Nature 436 ( 7054 ) 1136 - 1138 2005年8月

記述言語：英語掲載種別：研究論文（学術雑誌）

Ferroelectric materials are widely used in modern electric devices such as memory elements, filtering devices and high-performance insulators. Ferroelectric crystals have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles (specifically, a polar displacement of anions and cations). First-principles calculations and electron density analysis of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the orbital hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, an alternative model-electronic ferroelectricity-has been proposed in which the electric dipole depends on electron correlations, rather than the covalency. This would offer the attractive possibility of ferroelectric materials that could be controlled by the charge, spin and orbital degrees of freedom of the electron. Here we report experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe 2O4. Using resonant X-ray scattering measurements, we determine the ordering of the Fe2+ and Fe3+ ions. They form a superstructure that supports an electric polarization consisting of distributed electrons of polar symmetry. The polar ordering arises from the repulsive property of electrons-electron correlations-acting on a frustrated geometry.

DOI： 10.1038/nature04039

Synthesis of Carbon Nanowalls using Plasma-Irradiated Solid Carbon and Absorption of Cs in Water

Yukimasa Fukada, Yumito Aoyagi, Misaki Yokoyama, Yoichi Horibe, Jun Kano, Miyu Kaneda, Tatsuo Fujii, Akitaka Yoshigoe, Masaaki Kobata, Tatsuo Fukuda, Kenji Yoshii, Naoshi Ikeda

Journal of Electronic Materials 54 ( 1 ) 686 - 692 2024年11月

Ultrafast anisotropic polarization dynamics of electronic ferroelectric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>LuFe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>

H. Yu, Y. Fukada, G. Nishida, K. Takubo, T. Ishikawa, S. Koshihara, N. Ikeda, Y. Okimoto

Physical Review Materials 8 ( 6 ) 2024年6月

Structural Characterization of Graphite Analogue BC_x Synthesized Under Various Conditions and Its Application to Ti Intercalation

Kazumasa Horigane, Masayuki Tadokoro, Ritsuko Eguchi, Hirofumi Ishii, Shinichi Nakamura, Takashi Kambe, Naoshi Ikeda, Hidenori Goto, Yoshihiro Kubozono, Jun Akimitsu

Inorganic Chemistry 62 ( 48 ) 19466 - 19473 2023年11月

Nonlinear Optical Properties in an Epitaxial YbFe₂O₄ Film Probed by Second Harmonic and Terahertz Generation 査読国際誌

Hongwu Yu, Yoichi Okimoto, Atsuya Morita, Shuhei Shimanuki, Kou Takubo, Tadahiko Ishikawa, Shin-ya Koshihara, Ryusei Minakami, Hirotake Itoh, Shinichiro Iwai, Naoshi Ikeda, Takumi Sakagami, Mayu Nozaki, Tatsuo Fujii

Materials 16 ( 5 ) 1989 - (1-11) 2023年2月

Valence control of charge and orbital frustrated system YbFe2O4 with electrochemical Li+ intercalation

S. Murase, Y. Yoshikawa, K. Fujiwara, Y. Fukada, T. Teranishi, J. Kano, T. Fujii, Y. Inada, M. Katayama, K. Yoshii, T. Tsuji, D. Matsumura, N. Ikeda

Journal of Physics and Chemistry of Solids 162 110468 - 110468 2022年3月

掲載種別：研究論文（学術雑誌）出版者・発行元：Elsevier BV

We report an attempt valence control of the mixed valence iron triangular oxide YbFe2O4 to develop an effective technique controling the frustration of charges in strongly correlated electron systems. The electrochemical doping of Li + into YbFe2O4 was examined on a cell-type sample similar to the Li-ion secondary battery cell. Systematic changes in the lattice constant and Fe – Fe and Fe–Yb distance were observed with Li doping. Maximum value of the doping was over 300 mAh/g. An EXAFS experiment indicated that Li positioned between Yb octahedron layer (U-layer) and Fe-bipyramidal layer (W-layer). However, detailed change of iron valence state of YbFe2O4was not clearly observed because of the superimpose of the signal from iron metal nano particles in XANES observation. We discuss that the uncertainty might arise from the inhomogeneous distribution of the sample particle size, which might prevent the homogeneous doping of Li because the doping occurs on the surface of each nano-particles.

DOI： 10.1016/j.jpcs.2021.110468

Observation of magnetoelectric effect of charge-ordered ferroelectric LuFe<inf>2</inf>O<inf>4</inf> by inverse capacitance analysis

Yukimasa Fukada, Naoshi Ikeda

Journal of the Physical Society of Japan 90 ( 11 ) 2021年11月

掲載種別：研究論文（学術雑誌）

We report the magnetic field effect on the dielectric constant of the charge-ordered ferroelectric LuFe2O4. Below the Néel temperature of 240 K, we found decreases in capacitance and resistance by the magnetic field. The variation in dielectric constant with the magnetic field was investigated by inverse complex capacitance analysis with a series of equivalent electric circuits having a log-normal distribution of the relaxation time. The phenomenon can be explained by the spin- and charge-ordered domain model, in which the entropy of Fe spins affects the arrangement of Fe charges.

DOI： 10.7566/JPSJ.90.113705

Quantum Interference of Totally Reflected Mössbauer γ-Rays from a 57Fe Monolayer Embedded in a Thin Film

Kosuke Fujiwara, Takaya Mitsui, Yumito Aoyagi, Yoshitaka Yoda, Naoshi Ikeda

Journal of the Physical Society of Japan 90 ( 8 ) 084705 - 084705 2021年8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 Society of Japan

Total reflection gamma-ray resonant spectra of a Cr/Fe/Cr/MgO thin film in which a Fe-57 monolayer was embedded at a depth of 6 nm below the surface were studied using a synchrotron Mossbauer source. Due to an interference between electron and nuclear resonant scatterings, the spectra showed asymmetric absorption and scattering profiles depending on beam incidence angles. The spectra were reproduced via theoretical simulations based on an anomalous scattering factor associated with nuclear resonance. The origin of the asymmetric spectral profiles was explained by a change in the phase factor of nuclear resonant scattering amplitude at the Fe-57 enriched monolayer. These results pave the way for advanced local magnetic analyses on the deeply buried interface of functional magnetic thin films and direct phase determination of gamma-rays in thin films.

DOI： 10.7566/jpsj.90.084705

Single domain growth and charge ordering of epitaxial YbFe2O4 films

Takumi Sakagami, Reika Ota, Jun Kano, Naoshi Ikeda, Tatsuo Fujii

CRYSTENGCOMM 2021年8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：ROYAL SOC CHEMISTRY

YbFe<INF>2</INF>O<INF>4</INF> is a charge-ordered ferroelectric that exhibits coupling between magnetization and electric polarization near room temperature and crystallizes in a rhombohedral structure (R3m). This study presents an attempt to fabricate stoichiometric and epitaxial YbFe<INF>2</INF>O<INF>4-delta</INF> films with a nearly single-domain structure using an RF magnetron sputtering method. The (0001)-oriented epitaxial films of YbFe<INF>2</INF>O<INF>4-delta</INF> on YSZ (111) substrates via reactive sputtering method exhibited clear three-fold symmetry normal to the substrate without the formation of twin domains rotated by 60 degrees. The oxygen stoichiometry of the epitaxial YbFe<INF>2</INF>O<INF>4-delta</INF> was improved by controlling an oxygen partial pressure (P<INF>O<INF>2</INF></INF>) during the deposition. The films showed a sharp ferrimagnetic transition, and the transition temperature (T<INF>N</INF>) increased linearly to approximately 245 K with decreasing P<INF>O<INF>2</INF></INF>. The magnitude of magnetization of the obtained films was comparable to that of bulk single crystals. Further, the electron diffraction pattern of the stoichiometric films confirmed the presence of three-dimensional charge order, which is consistent with the behavior of the bulk crystals as well.

DOI： 10.1039/d1ce00834j

High valence states of Pd supported on ferroelectric BaTiO3 driven by electric polarization

Tasuku Yoshida, Jun Kano, Masaichiro Mizumaki, Yusuke Tamenori, Kiyofumi Nitta, Kazuo Kato, Satoshi Hinokuma, Norihiro Oshime, Satoshi Hirose, Hitoshi Mikami, Naoshi Ikeda, Tatsuo Fujii, Yuta Nishina, Tomoko Okubo

APPLIED PHYSICS LETTERS 119 ( 9 ) 2021年8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：AIP Publishing

At a metal-ferroelectric junction, it has been considered that the electric polarization of the ferroelectric material can affect the electronic structure of the neighboring metal. Here, we demonstrate that the valence state of Pd can be shifted to the unstable high value of 4+ by the electric field of electric polarization in ferroelectric BaTiO3. Study of the absorption fine structure of both hard and soft x rays revealed the existence of Pd4+ states on the surface of Pd oxide nanoparticles. The positions of Pd and oxygen atoms are shifted in opposite directions by the electric field due to the electric polarization of ferroelectric BaTiO3. The atomic displacement of Pd and O forms a zigzag structure, in which the coordination number of Pd atoms is changed from four to six, producing a quadrivalent state. This report presents experimental evidence that ferroelectric polarization can control the electronic states of neighboring metal atoms, and we suggest that using the ferroelectric support effect may produce a new type of catalyst.

DOI： 10.1063/5.0066289

Light reflectance and photoelectron yield spectroscopy enable acceptor level measurement in p-type Ba1-xTiO3 semiconductor

Saya Fujii, Jun Kano, Norihiro Oshime, Tohru Higuchi, Yuta Nishina, Tatsuo Fujii, Naoshi Ikeda, Hiromi Ota

JOURNAL OF APPLIED PHYSICS 129 ( 8 ) 2021年2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：AMER INST PHYSICS

We report the band structure of Ba-deficient BaTiO3 as a p-type semiconductor, studied by a combination of light reflectance and photoelectron yield spectroscopy. Two acceptor levels were observed at the tail of a valence band. As the quantity of Ba vacancies increased, the density of state of the two acceptor levels also increased. The levels of the conduction band minimum and the valence band maximum shifted far away from the vacuum level, but the bandgap seems to be independent of Ba deficient concentration. For classical semiconductors such as Si and GaAs, the observation of impurity levels is restricted to low temperatures (similar to 20K) owing to their narrow bandgaps. Oxide semiconductors have now been demonstrated with wide bandgaps and acceptor levels, at normal operating temperatures, which could lead to new device designs in the future.

DOI： 10.1063/5.0033761

Crystallographical and morphological changes in charge-ordering transition of RFe<inf>2</inf>O<inf>4</inf> (R: Y, Lu) investigated by transmission electron microscopy

Y. Horibe, S. Mori, N. Ikeda, K. Yoshii, H. Maeno, Y. Murakami

Ferroelectrics 584 ( 1 ) 20 - 30 2021年

掲載種別：研究論文（学術雑誌）

Temperature dependence of charge-ordered crystal structures and domain structures in RFe2O4 (R: Y and Lu) was investigated by energy-filtered transmission electron microscopy, combined with conventional transmission electron microscopy. The presence of three-dimensional to two-dimensional charge ordering transition were observed in both YFe2O4 and LuFe2O4 on heating. Furthermore, real-space images obtained with the energy-filtered transmission electron microscopy revealed that YFe2O4 has less anisotropic nanometer-scale charge-ordered domains than LuFe2O4. These findings in RFe2O4 indicate the importance of the interchange interactions between Fe-O bilayers in addition to those within bilayers in the structural phase transitions associated with charge ordering in this system.

DOI： 10.1080/00150193.2021.1984762

Analysis of glass behavior of lu<inf>2</inf>fe<inf>3</inf>o<inf>7</inf> with distributed equivalent circuit model

Yukimasa Fukada, Ryota Fukuyama, Kosuke Fujiwara, Kenji Yoshii, Kei Shigematsu, Masaki Azuma, Naoshi Ikeda

Journal of the Physical Society of Japan 90 ( 2 ) 2021年

掲載種別：研究論文（学術雑誌）

We report an analysis of the dielectric glasslike state of the spin- and charge-ordered system Lu2Fe3O7, which has structural and physical properties analogous to those of the electronic ferroelectrics RFe2O4 (R: rare earth). Aging and memory effects of resistivity were found in the electric impedance response. We performed a numerical analysis with equivalent electric circuits that have a logarithmic Gaussian distribution of resistance and capacitance. The distribution parameters well explained the glassy nature of the electric and dielectric properties between 100 and 200 K and the distribution variations of charge- and magnetic-ordered domains underlying the glass nature.

DOI： 10.7566/JPSJ.90.024710

Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO3

Norihiro Oshime, Jun Kano, Eiji Ikenaga, Shintaro Yasui, Yosuke Hamasaki, Sou Yasuhara, Satoshi Hinokuma, Naoshi Ikeda, Pierre-Eymeric Janolin, Jean-Michel Kiat, Mitsuru Itoh, Takayoshi Yokoya, Tatsuo Fujii, Akira Yasui, Hitoshi Osawa

SCIENTIFIC REPORTS 10 ( 1 ) 2020年7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：NATURE RESEARCH

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

DOI： 10.1038/s41598-020-67651-w