論文 - 宮本 辰也
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Terahertz radiation by quantum interference of excitons in a one-dimensional Mott insulator 査読あり
Tatsuya Miyamoto, Akihiro Kondo, Takeshi Inaba, Takeshi Morimoto, Shijia You, Hiroshi Okamoto
Nature Communications 14 ( 1 ) 6229:1 - 6229:11 2023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：Springer Science and Business Media LLC
Abstract
Nearly monocyclic terahertz waves are used for investigating elementary excitations and for controlling electronic states in solids. They are usually generated via second-order optical nonlinearity by injecting a femtosecond laser pulse into a nonlinear optical crystal. In this framework, however, it is difficult to control phase and frequency of terahertz waves. Here, we show that in a one-dimensional Mott insulator of a nickel-bromine chain compound a terahertz wave is generated with high efficiency via strong electron modulations due to quantum interference between odd-parity and even-parity excitons produced by two-color femtosecond pulses. Using this method, one can control all of the phase, frequency, and amplitude of terahertz waves by adjusting the creation-time difference of two excitons with attosecond accuracy. This approach enables to evaluate the phase-relaxation time of excitons under strong electron correlations in Mott insulators. Moreover, phase- and frequency-controlled terahertz pulses are beneficial for coherent electronic-state controls with nearly monocyclic terahertz waves. -
Piezoelectric Transition in a Nonpyroelectric Gyroidal Metal–Organic Framework
Shunsuke Kitou, Hajime Ishikawa, Yusuke Tokunaga, Masato Ueno, Hiroshi Sawa, Yuiga Nakamura, Yuto Kinoshita, Tatsuya Miyamoto, Hiroshi Okamoto, Koichi Kindo, Taka-hisa Arima
Journal of the American Chemical Society 2025年03月
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Observation of nonreciprocal polarization modulations induced by an intense terahertz electric-field pulse in an electronic-type molecular ferroelectric
T. Kubo, T. Miyamoto, N. Takamura, M. Yamamoto, R. Ikeda, T. Sato, H. Okamoto
Physical Review Research 2025年02月
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Higher conductivity in doped ethylenedioxythiophene (EDOT) dimers with chalcogen-substituted end groups 査読あり
Kota Onozuka, Tomoko Fujino, Tatsuya Miyamoto, Takashi Yamakawa, Hiroshi Okamoto, Hiroshi Akiba, Osamu Yamamuro, Eiichi Kayahara, Shigeru Yamago, Hiroshi Oike, Hatsumi Mori
Journal of Materials Chemistry C 12 ( 35 ) 13956 - 13965 2024年08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 Society of Chemistry (RSC)
Chalcogen substitution at the end groups of doped EDOT oligomers impacts on the crystal and electronic structures, enhancing their conducting properties.
DOI： 10.1039/d4tc02346c
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Two-step growth of high-quality single crystals of the Kitaev magnet α−RuCl3 査読あり 国際共著
R. Namba, K. Imamura, R. Ishioka, K. Ishihara, T. Miyamoto, H. Okamoto, Y. Shimizu, Y. Saito, Y. Agarmani, M. Lang, H. Murayama, Y. Xing, S. Suetsugu, Y. Kasahara, Y. Matsuda, K. Hashimoto, T. Shibauchi
Physical Review Materials 8 ( 7 ) 074404:1 - 074404:8 2024年07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：American Physical Society (APS)
DOI： 10.1103/physrevmaterials.8.074404
その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.8.074404/fulltext
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Orbital hybridization of donor and acceptor to enhance the conductivity of mixed-stack complexes 査読あり
Tomoko Fujino, Ryohei Kameyama, Kota Onozuka, Kazuki Matsuo, Shun Dekura, Tatsuya Miyamoto, Zijing Guo, Hiroshi Okamoto, Toshikazu Nakamura, Kazuyoshi Yoshimi, Shunsuke Kitou, Taka-hisa Arima, Hiroyasu Sato, Kaoru Yamamoto, Akira Takahashi, Hiroshi Sawa, Yuiga Nakamura, Hatsumi Mori
Nature Communications 15 ( 1 ) 2024年04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：Springer Science and Business Media LLC
Abstract
Mixed-stack complexes which comprise columns of alternating donors and acceptors are organic conductors with typically poor electrical conductivity because they are either in a neutral or highly ionic state. This indicates that conductive carriers are insufficient or are mainly localized. In this study, mixed-stack complexes that uniquely exist at the neutral–ionic boundary were synthesized by combining donors (bis(3,4-ethylenedichalcogenothiophene)) and acceptors (fluorinated tetracyanoquinodimethanes) with similar energy levels and orbital symmetry between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Surprisingly, the orbitals were highly hybridized in the single-crystal complexes, enhancing the room-temperature conductivity (10<sup>−4</sup>–0.1 S cm<sup>−1</sup>) of mixed-stack complexes. Specifically, the maximum conductivity was the highest reported for single-crystal mixed-stack complexes under ambient pressures. The unique electronic structures at the neutral–ionic boundary exhibited structural perturbations between their electron-itinerant and localized states, causing abrupt temperature-dependent changes in their electrical, optical, dielectric, and magnetic properties. -
Band transport evidence in PEDOT:PSS films using broadband optical spectroscopy from terahertz to ultraviolet region 査読あり
Zijing Guo, Tetsu Sato, Yang Han, Naoki Takamura, Ryohei Ikeda, Tatsuya Miyamoto, Noriaki Kida, Makiko Ogino, Youtarou Takahashi, Naotaka Kasuya, Shun Watanabe, Jun Takeya, Qingshuo Wei, Masakazu Mukaida, Hiroshi Okamoto
Communications Materials 5 26:1 - 26:13 2024年03月
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Destabilization of spin-Peierls phase via a charge-spin modulated Floquet state induced by intramolecular vibrational excitation 査読あり
Daiki Sakai, Takashi Yamakawa, Hajime Ueda, Ryohei Ikeda, Tatsuya Miyamoto, Hiroshi Okamoto
Communications Physics 7 40:1 - 40:10 2024年01月
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Melting of excitonic insulator phase by an intense terahertz pulse in Ta2NiSe5
N. Takamura, T. Miyamoto, R. Ikeda, T. Kubo, M. Yamamoto, H. Sato, Y. Han, T. Ito, T. Sato, A. Nakano, H. Sawa, and H. Okamoto
Physical Review Research 2024年
担当区分：責任著者 記述言語：英語 掲載種別：研究論文（学術雑誌）
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Detecting Magnetoelectric Effect in a Metallic Antiferromagnet via Nonreciprocal Rotation of Reflected Light 査読あり
Keito Arakawa, Takeshi Hayashida, Kenta Kimura, Ryusuke Misawa, Takayuki Nagai, Tatsuya Miyamoto, Hiroshi Okamoto, Fumitoshi Iga, Tsuyoshi Kimura
Physical Review Letters 131 236702:1 - 236702:6 2023年12月
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Melting of excitonic insulator phase by an intense terahertz pulse in Ta$_2$NiSe$_5$
Naoki Takamura, Tatsuya Miyamoto, Ryohei Ikeda, Tetsushi Kubo, Masaki Yamamoto, Hiroki Sato, Yang Han, Takayuki Ito, Tetsu Sato, Akitoshi Nakano, Hiroshi Sawa, and Hiroshi Okamoto
arXiv 2309.05286:1 - 2309.05286:66 2023年09月
担当区分：責任著者 記述言語：英語 掲載種別：研究論文（学術雑誌）
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Dynamical aspects of excitonic Floquet states generated by a phase-locked mid-infrared pulse in a one-dimensional Mott insulator 査読あり
Takashi Yamakawa, Tatsuya Miyamoto, Daiki Sakai, Hiroshi Okamoto
New Journal of Physics 25 093044:1 - 093044:20 2023年09月
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Metallic State of a Mixed-Sequence Oligomer Salt That Models Doped PEDOT Family 査読あり
Kota Onozuka, Tomoko Fujino, Ryohei Kameyama, Shun Dekura, Kazuyoshi Yoshimi, Toshikazu Nakamura, Tatsuya Miyamoto, Takashi Yamakawa, Hiroshi Okamoto, Hiroyasu Sato, Taisuke Ozaki, Hatsumi Mori
Journal of the American Chemical Society 145 ( 28 ) 15152 - 15161 2023年07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：American Chemical Society (ACS)
DOI： 10.1021/jacs.3c01522
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Experimental verification of charge soliton excitations in the ionic Mott-Peierls ferroelectric TTF-CA 査読あり
R. Takehara, H. Adachi, K. Sunami, K. Miyagawa, T. Miyamoto, H. Okamoto, S. Horiuchi, K. Kanoda
Physical Review B 107 125164:1 - 125164:6 2023年03月
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Efficient Mott insulator-metal transition by an intense terahertz electric field pulse via quantum tunneling 査読あり
N. Takamura, T. Miyamoto, S. Liang, K. Asada, T. Terashige, Y. Takahashi, T. Hasegawa, H. Okamoto
Physical Review B 107 085147:1 - 085147:14 2023年02月
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Detecting magnetoelectric effect in a metallic antiferromagnet via nonreciprocal rotation of reflected light 査読あり
Keito Arakawa, Takeshi Hayashida, Kenta Kimura, Ryusuke Misawa, Takayuki Nagai, Tatsuya Miyamoto, Hiroshi Okamoto, Fumitoshi Iga, and Tsuyoshi Kimura
Physical Review Letters 2023年
記述言語：英語 掲載種別：研究論文（学術雑誌）
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Emission of Terahertz Electromagnetic Waves: A New Spectroscopic Method to Investigate Physical Properties of Solids 招待あり 査読あり
Noriaki Kida, Tatsuya Miyamoto, Hiroshi Okamoto
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 91 ( 11 ) 2022年11月
記述言語：英語 出版者・発行元：PHYSICAL SOC JAPAN
When a femtosecond laser pulse is applied to a solid with no spatial inversion symmetry, a low-frequency oscillation of polarization sometimes appears via the second-order nonlinear optical effect or equivalently the optical rectification. This results in an emission of terahertz electromagnetic waves via the electric-dipole radiation mechanism, which is usually called a terahertz radiation. In this review, we show that such a terahertz-radiation process itself can be used to reveal physical properties of solids. One example is the detections of ferroelectric domains and domain walls, which are demonstrated in various organic and inorganic ferroelectrics. The amplitude and sign of electric fields of the terahertz radiation are found to reflect the magnitude and direction of the original ferroelectric polarization, respectively. By using these characteristics, ferroelectric domains and domain walls can be visualized by measuring the position dependence of the electric-field waveform of the terahertz radiation over the sample. This method is also applied to determine the polarization vector in three dimensions in ferroelectric films. A similar terahertz radiation can be used as a probe to detect the conducting path in a negative differential resistance state, which is induced by the application of an external electric field on a ferroelectric charge-order material. Besides the electric-dipole radiation, the magnetic-dipole radiation is possible to occur in magnets. Using an emission of the terahertz electromagnetic waves originating from the magnetization modulation, visualization of magnetic domains can be achieved in ferrimagnets. In addition, we review new types of terahertz-radiation mechanisms based upon an impulsive stimulated Raman scattering process, a circular photogalvanic effect, and a photoinduced phase transition. We show that valuable information about the steady-state and dynamical electronic properties of solids can be derived by analyzing the amplitude, phase, and polarization of electromagnetic waves observed in various types of terahertz radiations.
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Hydrogen Bonding Propagated Phase Separation in Quasi-Epitaxial Single Crystals: A Pd–Br Molecular Insulator 査読あり
Takefumi Yoshida, Shinya Takaishi, Laurent Guérin, Tatsuhiro Kojima, Hiroyoshi Ohtsu, Masaki Kawano, Tatsuya Miyamoto, Hiroshi Okamoto, Kenichi Kato, Masaki Takata, Yuka Hosomi, Shoji Yoshida, Hidemi Shigekawa, Hisaaki Tanaka, Shin-ichi Kuroda, Hiroaki Iguchi, Brian K. Breedlove, Zhao-Yang Li, Masahiro Yamashita
Inorganic Chemistry 61 ( 35 ) 14067 - 14074 2022年09月
掲載種別：研究論文（学術雑誌） 出版者・発行元：American Chemical Society (ACS)
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Terahertz Radiation with Multi-Narrowband Components via Photoinduced Melting of Charge Order in an Electronic-Type Ferroelectric Organic <i>α</i>-(BEDT-TTF)<sub>2</sub>I<sub>3</sub> 査読あり
Yuto Kinoshita, Hiromichi Yamakawa, Tatsuya Miyamoto, Kaoru Yamamoto, Hatumi Mori, Hiroshi Okamoto, Noriaki Kida
Journal of the Physical Society of Japan 91 ( 9 ) 2022年08月
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Ni(III) Mott–Hubbard-like State Containing High-Spin Ni(II) in a Semiconductive Bromide-Bridged Ni-Chain Compound 査読あり
Masanori Wakizaka, Mohammad Rasel Mian, Takefumi Yoshida, Tetsu Sato, Hisaaki Tanaka, Tatsuya Miyamoto, Hiroshi Okamoto, Shinya Takaishi, Hiroaki Iguchi, Masahiro Yamashita
Inorganic Chemistry 2022年06月
掲載種別：研究論文（学術雑誌） 出版者・発行元：American Chemical Society (ACS)
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Phonon-dressed states in an organic Mott insulator 査読あり
Naoki Sono, Takashi Otaki, Takayuki Kitao, Takashi Yamakawa, Daiki Sakai, Takeshi Morimoto, Tatsuya Miyamoto, Hiroshi Okamoto
Communications Physics 5 ( 1 ) 2022年05月
担当区分：責任著者 掲載種別：研究論文（学術雑誌） 出版者・発行元：Springer Science and Business Media LLC
Abstract
In an electronic system of solids excited by a femtosecond laser pulse, new quantum states called photon-dressed states are created via the coupling of original electron wavefunctions and periodic electric fields of light. The study of photon-dressed states is a central issue in nonlinear optical science, as a coherent response associated with a photon-dressed state can cause ultrafast optical-switching phenomena. In general, however, phase-sensitive dynamics of photon-dressed states are difficult to observe due to their short lifetime. Here, we show that excitation of the organic Mott insulator potassium-tetracyanoquinodimethane with a strong mid-infrared pulse induces a couple of intramolecular vibrations, which add temporally periodic potentials to the electronic system, giving rise to phonon-dressed states in the picosecond time domain. Through sub-cycle spectroscopy using a phase-stable mid-infrared pulse as an excitation and an ultrashort visible pulse as a probe, we observe optical radiation consisting of four kinds of coherent oscillations with double, sum, and differential frequencies of two intramolecular vibrations. The probe-energy dependence of each oscillation can be well interpreted with phonon-dressed states in the framework of Floquet theory. These findings open a possibility for Floquet engineering in correlated electron materials with strong charge–phonon couplings. -
Band-filling effects in single-crystalline oligomer models for doped PEDOT: 3,4-ethylenedioxythiophene (EDOT) dimer salt with hydrogen-bonded infinite sulfate anion chains 査読あり
Ryohei Kameyama, Tomoko Fujino, Shun Dekura, Shusaku Imajo, Tatsuya Miyamoto, Hiroshi Okamoto, Hatsumi Mori
Journal of Materials Chemistry C 10 ( 19 ) 7543 - 7551 2022年04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 Society of Chemistry (RSC)
Band-filling modulation of single-crystalline 3,4-ethylenedioxythiophene dimer salt from the half-filled state based on hydrogen-bonded anion chain formation enhanced the conductivity.
DOI： 10.1039/d2tc01216b
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Charge and Lattice Dynamics in Excitonic Insulator Ta2NiSe5 Investigated Using Ultrafast Reflection Spectroscopy 査読あり
Tatsuya Miyamoto, Makoto Mizui, Naoki Takamura, Junya Hirata, Hiromichi Yamakawa, Takeshi Morimoto, Tsubasa Terashige, Noriaki Kida, Akitoshi Nakano, Hiroshi Sawa, Hiroshi Okamoto
Journal of the Physical Society of Japan 91 ( 2 ) 2022年02月
担当区分：筆頭著者, 責任著者 掲載種別：研究論文（学術雑誌） 出版者・発行元：Physical Society of Japan
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Scattering mechanism of hole carriers in organic molecular semiconductors deduced from analyses of terahertz absorption spectra using Drude–Anderson model 査読あり
Y. Han, T. Miyamoto, T. Otaki, N. Takamura, N. Kida, N. Osakabe, J. Tsurumi, S. Watanabe, T. Okamoto, J. Takeya, H. Okamoto
Applied Physics Letters 120 ( 5 ) 053302 - 053302 2022年01月
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Ionic to neutral conversion induced by resonant excitation of molecular vibrations coupled to intermolecular charge transfer 査読あり
T. Morimoto, H. Suzuki, T. Otaki, N. Sono, N. Kida, T. Miyamoto, H. Okamoto
Physical Review Research 3 ( 4 ) 2021年12月
担当区分：責任著者 掲載種別：研究論文（学術雑誌）
In organic molecular compounds, intramolecular vibration is sometimes coupled with intermolecular charge transfer (CT). In such materials, vibrational excitation by a midinfrared (MIR) pulse causes collective intermolecular CTs that can be a route to an electronic-state conversion. Here, we report that an ionic-to-neutral (IN) conversion in tetrathiafulvalene-p-chloranil (TTF-CA) can be driven by a strong vibrational excitation induced by an MIR pulse. Using MIR-pump subcycle-reflectivity-probe and second-harmonic-generation-probe measurements, we discuss the coherent electron and lattice dynamics during and after the IN conversion, which are distinct from the dynamics of the photoinduced transition by electronic excitation alone.
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Terahertz-field-induced polar charge order in electronic-type dielectrics 査読あり
H. Yamakawa, T. Miyamoto, T. Morimoto, N. Takamura, S. Liang, H. Yoshimochi, T. Terashige, N. Kida, M. Suda, H. M. Yamamoto, H. Mori, K. Miyagawa, K. Kanoda, H. Okamoto
Nature Communications 12 ( 1 ) 2021年12月
担当区分：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：Springer Science and Business Media LLC
<title>Abstract</title>Ultrafast electronic-phase change in solids by light, called photoinduced phase transition, is a central issue in the field of non-equilibrium quantum physics, which has been developed very recently. In most of those phenomena, charge or spin orders in an original phase are melted by photocarrier generations, while an ordered state is usually difficult to be created from a non-ordered state by a photoexcitation. Here, we demonstrate that a strong terahertz electric-field pulse changes a Mott insulator of an organic molecular compound in <italic>κ</italic>-(ET)<sub>2</sub>Cu[N(CN)<sub>2</sub>]Cl (ET = bis(ethylenedithio)tetrathiafulvalene), to a macroscopically polarized charge-order state; herein, electronic ferroelectricity is induced by the collective intermolecular charge transfers in each dimer. In contrast, in an isostructural compound, <italic>κ</italic>-(ET)<sub>2</sub>Cu<sub>2</sub>(CN)<sub>3</sub>, which shows the spin-liquid state at low temperatures, a similar polar charge order is not stabilized by the same terahertz pulse. From the comparative studies of terahertz-field-induced second-harmonic-generation and reflectivity changes in the two compounds, we suggest the possibility that a coupling of charge and spin degrees of freedom would play important roles in the stabilization of polar charge order.
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Ultrafast control of electronic states by a terahertz electric field pulse in the quasi-one-dimensional organic ferroelectric (TMTTF)2PF6 査読あり
K. Asada, T. Miyamoto, H. Yamakawa, J. Hirata, N. Takamura, T. Morimoto, K. Suzuki, T. Otaki, N. Kida, T. Nakamura, H. Okamoto
Physical Review B 104 ( 19 ) 2021年11月
担当区分：責任著者 掲載種別：研究論文（学術雑誌）
A strong terahertz pulse is effective for controlling the macroscopic polarization in ferroelectrics. In the present study, we investigated the response of an organic molecular compound, namely (TMTTF)2PF6 (TMTTF: tetramethyltetrathiafulvalene), to a strong electric field using terahertz pulse-pump optical-reflectivity probe spectroscopy. This compound undergoes a transition from Mott insulator to charge-order insulator with lowering temperature, and exhibits electronic ferroelectricity in the charge-order phase. When the terahertz pulse is applied in the Mott-insulator phase, an ultrafast reflectivity change proportional to the square of the electric field waveform of the terahertz pulse emerges, which is attributed to the generation of charge disproportionation in each dimer and the resultant creation of macroscopic polarization. When the terahertz pulse is applied in the charge-order phase, an ultrafast reflectivity change proportional to the electric field waveform of the terahertz pulse is observed, which originates from the modulation of the original charge disproportionation and polarization. These ultrafast reflectivity changes can be ascribed to purely electronic responses. In the midinfrared region, where totally symmetric (ag) modes of intramolecular vibrations coupled with intermolecular charge transfers exist, a large reflectivity change is commonly observed in the Mott-insulator and charge-order phases. To interpret this feature, we constructed a model that incorporates a charge-transfer transition and ag-mode intramolecular vibrations. The analyses of the results with this model revealed that the change in the reflectivity spectrum by the terahertz electric field can be explained by the energy shift of the charge-transfer transition caused by the electric field-induced change of charge disproportionation in each dimer, and the transfer of the spectral weight from the intradimer charge-transfer (CT) transition to the interdimer CT transition resulting from the weakening of the dimerization. Our model can be used to analyze the optical responses to electric fields in various organic molecular compounds with electron-intramolecular vibration couplings.
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Long Carbon-Carbon Bonding beyond 2 angstrom in Tris(9-fluorenylidene)methane 査読あり
Takashi Kubo, Yuki Suga, Daisuke Hashizume, Hiroki Suzuki, Tatsuya Miyamoto, Hiroshi Okamoto, Ryohei Kishi, Masayoshi Nakano
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 143 ( 35 ) 14360 - 14366 2021年09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 CHEMICAL SOC
We report on our investigation of C-C bonding longer than 2.0 angstrom, which can be realized by perpendicularly facing two fluorenyl rings in the title compound. A small orbital overlap between the distantly positioned carbon atoms is observed as a small concentration of electrons on the X-ray electron density map. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the compound originate from the in-phase and out-of-phase interactions of the overlapping orbitals, respectively, with a gap of 2.39 eV. Solid-state C-13 NMR spectroscopy shows a sharp peak at 82.9 ppm for the long-bonded carbons, and a CASSCF(6,6) calculation indicates small diradical character. The experimental and theoretical analyses reveal sufficient covalent-bonding interaction in the long-bonded carbon pair.
DOI： 10.1021/jacs.1c07431
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Fate of soliton matter upon symmetry-breaking ferroelectric order 査読あり
K. Sunami, R. Takehara, A. Katougi, K. Miyagawa, S. Horiuchi, R. Kato, T. Miyamoto, H. Okamoto, K. Kanoda
Physical Review B 103 ( 13 ) 2021年04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：American Physical Society (APS)
In a one-dimensional (1D) system with degenerate ground states, their domain boundaries, dubbed solitons, emerge as topological excitations often carrying unconventional charges and spins; however, the soliton excitations are vital in only the nonordered regime. Then a question arises: How do the solitons conform to a three-dimensional (3D) ordered state? Here, using a quasi-1D organic ferroelectric, tetrathiafulvalene-p-chloranil (TTF-CA), with degenerate polar dimers, we pursue the fate of spin-soliton charge-soliton composite matter in a 1D polar-dimer liquid upon its transition to a 3D ferroelectric order by resistivity, nuclear magnetic resonance (NMR), and nuclear quadrupole resonance (NQR) measurements. We demonstrate that the soliton matter undergoes neutral spin-spin soliton pairing and spin-charge soliton pairing to form polarons, coping with the 3D order. Below the ferroelectric transition, the former contributes to the magnetism through triplet excitations, which rapidly fade out on cooling, whereas the latter carries electrical current with paramagnetic spins that more moderately decrease with temperature. The nearly perfect scaling between NMR and NQR relaxation rates in the ferroelectric phase evidences that spin carriers diffuse with lattice distortion, namely, in the form of polarons. From the combined analyses of conductivity and NMR relaxation rate, we derive the excitation energies of polaron excitations and diffusion. Our results reveal the whole picture of soliton matter that condenses into the 3D ordered state.
DOI： 10.1103/physrevb.103.134112
その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.103.134112/fulltext
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Terahertz-Field-Induced Changes of Electronic States Associated with a Polarization Modulation in BiFeO3 査読あり
Naoki Sono, Yuto Kinoshita, Noriaki Kida, Toshimitsu Ito, Hiroshi Okamoto, Tatsuya Miyamoto
Journal of the Physical Society of Japan 90 ( 3 ) 033703 - 033703 2021年03月
担当区分：最終著者, 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：Physical Society of Japan
We performed terahertz-pulse-pump second-harmonic-generation (SHG)-probe and optical-reflectivity-probe measurements on a single crystal of a displacive-type ferroelectric, BiFeO3, to achieve an ultrafast control of the optical response by a terahertz pulse. By an application of a terahertz pulse with the amplitude of about 200kV/cm along the polarization direction, the SHG intensity was modulated completely along the electric-field waveform, suggesting that the relevant ionic motions are faster than the time resolution similar to 130 fs of the measurement as well as the period of the terahertz electric field. The electric-field-induced change of the reflectivity spectrum revealed that the intensities of d-d transitions in iron ions was strongly varied, while those of the charge-transfer transitions from oxygen-2p to iron-3d bands were almost unchanged. These results demonstrate that the electric field hardly modifies the wavefunction of valence electrons, but exclusively changes the distortions of iron-oxygen octahedrons. The latter effect shows that the octahedrons' distortions play dominant roles on the ferroelectric-polarization generation in BiFeO3.
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An unusual Pd(iii) oxidation state in the Pd-Cl chain complex with high thermal stability and electrical conductivity 査読あり
Mohammad Rasel Mian, Masanori Wakizaka, Takefumi Yoshida, Hiroaki Iguchi, Shinya Takaishi, Unjila Afrin, Tatsuya Miyamoto, Hiroshi Okamoto, Hisaaki Tanaka, Shin-ichi Kuroda, Brian K. Breedlove, Masahiro Yamashita
DALTON TRANSACTIONS 50 ( 5 ) 1614 - 1619 2021年02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
The Pd(iii) oxidation state is unusual and unstable since it strongly tends to disproportionate. We synthesized the quasi-one-dimensional (1D) halogen-bridged Pd(iii)-Cl complex [Pd(dabdOH)(2)Cl]Cl-2 (1-Cl; dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol) with multiple hydrogen bonds. From single-crystal X-ray diffraction, the bridging Cl- ions were located at the midpoint of the Pd-Cl-Pd moieties in the 1D chains, indicating that the Pd ions are in a Pd(iii) average valence (AV) state. Moreover, bright spots for the Pd(iii) d(z)(2) orbitals in the upper Hubbard band above the Fermi level were observed every similar to 5 angstrom using scanning tunnelling microscopy. These results clearly indicate that the Pd ions are in a Pd(iii) AV state in 1-Cl. In addition, 1-Cl has the highest thermal stability (470 K) among the Pd(iii) complexes reported and the highest electrical conductivity (0.6 S cm(-1) at 300 K) among the 1D Pd-Cl chains reported so far.
DOI： 10.1039/d0dt03848b
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Excitonic optical spectra and energy structures in a one-dimensional Mott insulator demonstrated by applying a many-body Wannier functions method to a charge model 査読あり
T. Yamaguchi, K. Iwano, T. Miyamoto, N. Takamura, N. Kida, Y. Takahashi, T. Hasegawa, H. Okamoto
Physical Review B 103 ( 4 ) 2021年01月
掲載種別：研究論文（学術雑誌）
We applied a many-body Wannier functions method to theoretically calculate the excitonic optical conductivity spectrum and energy structure in a one-dimensional (1D) Mott insulator at absolute zero temperature with a large system size. Focusing on full charge fluctuations associated with holon and doublon pairs, we employ a charge model, which is interpreted as an effective model for investigating the photoexcitations of a 1D extended Hubbard model under a half-filling of the spin-charge separation. As a result, theoretical spectra with the appropriate broadenings qualitatively reproduce the recent experimental data of ET-F2TCNQ at 294 K with and without a modulated electric field. Regarding the excitonic energy structure, we found that the excitons, particularly for even-parity, are weakly bound by many-body effects. This is also consistent with the fitting parameters reported in a recent experiment. Thus, the theoretical method presented in this paper is useful for understanding the physical roles of the charge fluctuations in many-body excited states of a 1D Mott insulator.
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Evaluations of nonlocal electron-phonon couplings in tetracene, rubrene, and C-10-DNBDT-NW based on density functional theory 査読あり
Takashi Otaki, Tsubasa Terashige, Junto Tsurumi, Tatsuya Miyamoto, Noriaki Kida, Shun Watanabe, Toshihiro Okamoto, Jun Takeya, Hiroshi Okamoto
PHYSICAL REVIEW B 102 ( 24 ) 2020年12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 PHYSICAL SOC
In organic molecular semiconductors (OSCs), fluctuation of transfer integrals originating from thermally induced molecular vibrations is suggested to cause large scatterings of carriers, and to be a most important factor for the suppression of their carrier mobility. The intrinsic carrier mobility under such a fluctuation of transfer integrals is calculated using the transient localization theory, in which the estimation of transfer-integral fluctuation depending on each OSC is indispensable. In the present study, we provide a methodology to evaluate nonlocal electron-phonon couplings in OSCs using the density functional theory, which enables us to evaluate precisely the fluctuation magnitude of transfer integrals. Our method is based upon the combination of the frequency correction to reduce numerical inaccuracies in normal-mode frequencies, the extraction of tight binding parameters using maximally localized Wannier functions, and the explicit consideration of anharmonicity of phonons. We apply this method to classical OSCs, tetracene and rubrene, and a recently developed high-mobility OSC, 3,11-didecyl-dinaphtho[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (C-10-DNBDT-NW). We succeeded in identifying the low-frequency vibrations dominating the fluctuation of transfer integrals at room temperature, which we consider to be the main factors to limit the intrinsic mobility.
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Observation of the Three-Dimensional Polarization Vector in Films of Organic Molecular Ferroelectrics Using Terahertz Radiation Emission 査読あり
Yuto Kinoshita, Masato Sotome, Tatsuya Miyamoto, Yohei Uemura, Shunto Arai, Sachio Horiuchi, Tatsuo Hasegawa, Hiroshi Okamoto, Noriaki Kida
Physical Review Applied 14 ( 5 ) 2020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：American Physical Society (APS)
For the application of a ferroelectric thin film to optical and electric devices, it is important to align its ferroelectric polarization parallel or perpendicular to the film surface. In this context, an effective method to characterize the polarization directions in the whole area of devices is needed. Here, we report an alternative method to determine a polarization vector in three dimensions using an emission of terahertz waves. The target material is an organic hydrogen-bonded molecular ferroelectric 2-methylbenzimidazole (MBI). By the irradiation of a femtosecond laser pulse on a bulk single-crystal thin film of MBI, we observe the emission of the terahertz waves originating from the polarization modulation of infrared-active phonons via the impulsive stimulated Raman-scattering mechanism. By measuring the terahertz electric field parallel to the ferroelectric polarization with rotating the single-crystal film around two independent axes, we succeed in determining the polarization vector in three dimensions, the direction of which is tilted by 45 degrees normal to the film surface and substrate. This method can be a powerful tool to characterize directions of ferroelectric polarizations in any kinds of thin-film samples.
DOI： 10.1103/physrevapplied.14.054002
その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevApplied.14.054002/fulltext
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Conductive zigzag Pd(iii)-Br chain complex realized by a multiple-hydrogen-bond approach 査読あり
Mohammad Raset Mian, Unjila Afrin, Hiroaki Iguchi, Shinya Takaishi, Takefumi Yoshida, Tatsuya Miyamoto, Hiroshi Okamoto, Hisaaki Tanaka, Shin-ichi Kuroda, Masahiro Yamashita
CRYSTENGCOMM 22 ( 23 ) 3999 - 4004 2020年06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
Coexistence of zigzag structure and the uncommon Pd(iii) oxidation state in quasi-1D halogen-bridged metal complexes was realized in a conductive Br-bridged Pd chain complex, [Pd(dabdOH)(2)Br]SO4 center dot 3H(2)O (2), for the first time. Intra- and interchain hydrogen-bond networks among the (2S,3S)-2,3-diaminobutane-1,4-diol (dabdOH) ligand and SO(4)(2-)and H2O molecules support this unusual structure and electronic state. The electrical conductivity of2reached 0.05 S cm(-1)at room temperature.
DOI： 10.1039/d0ce00332h
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Long-term stabilization of carrier envelope phases of mid-infrared pulses for the precise detection of phase-sensitive responses to electromagnetic waves 査読あり
T. Yamakawa, N. Sono, T. Kitao, T. Morimoto, N. Kida, T. Miyamoto, H. Okamoto
AIP ADVANCES 10 ( 2 ) 2020年02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 a high performance mid-infrared pump visible probe measurement system, which can measure phase-sensitive responses to a mid-infrared pulse along the oscillating electromagnetic field. In this system, the pump light is a phase-locked mid-infrared pulse with a temporal width of 100 fs, which is produced via difference frequency generation (DFG) from two idler pulses of two optical parametric amplifiers (OPAs) that are excited by the same Ti:sapphire regenerative amplifier. The probe pulse is a visible pulse with a temporal width of 9 fs and is generated from a custom-built non-collinear OPA. By measuring the electric-field waveforms of mid-infrared pump pulses with electro-optic sampling and evaluating their carrier envelope phase (CEP) and the temporal positions of their envelopes relative to ultrashort visible probe pulses, we are able to perform double feedback corrections that eliminate both the following sources of drift. The CEP drift in mid-infrared pulses originating from fluctuations in the difference of optical-path lengths of the two idler pulses before the DFG is corrected by inserting a wedge plate in one idler path, and the drift in pump-probe delay times due to fluctuations in the difference of the overall optical-path lengths of the pump and probe pulses is corrected with mechanical delay lines. In this double feedback system, the absolute carrier phase of mid-infrared pulses can be fixed within 200 mrad and errors in the measurement of phase-sensitive responses can be reduced to within 1 fs over a few tens of hours.
DOI： 10.1063/1.5143049
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Strong Terahertz Radiation via Rapid Polarization Reduction in Photoinduced Ionic-To-Neutral Transition in Tetrathiafulvalene-p-Chloranil 査読あり
Yuto Kinoshita, Noriaki Kida, Yusuke Magasaki, Takeshi Morimoto, Tsubasa Terashige, Tatsuya Miyamoto, Hiroshi Okamoto
PHYSICAL REVIEW LETTERS 124 ( 5 ) 2020年02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 PHYSICAL SOC
Terahertz lights are usually generated through the optical rectification process within a femtosecond laser pulse in noncentrosymmetric materials. Here, we report a new generation mechanism of terahertz lights based upon a photoinduced phase transition, in which an electronic structure is rapidly changed by a photoirradiation. When a ferroelectric organic molecular compound, tetrathiafulvalene-p-chloranil, is excited by a femtosecond laser pulse, the ionic-to-neutral transition is driven and simultaneously a strong terahertz radiation is produced. By analyzing the terahertz electric-field waveforms and their dependence on the polarization direction of the incident laser pulse, we demonstrate that the terahertz radiation originates from the ultrafast decrease of the spontaneous polarization in the photoinduced ionic-to-neutral transition. The efficiency of the observed terahertz radiation via the photoinduced phase transition mechanism is found to be much higher than that via the optical rectification in the same material and in a typical terahertz emitter, ZnTe.
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Biexciton in one-dimensional Mott insulators 査読あり
T. Miyamoto, T. Kakizaki, T. Terashige, D. Hata, H. Yamakawa, T. Morimoto, N. Takamura, H. Yada, Y. Takahashi, T. Hasegawa, H. Matsuzaki, T. Tohyama, H. Okamoto
Communications Physics 2 ( 1 ) 2019年12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：Springer Science and Business Media LLC
<title>Abstract</title>
Mott insulators sometimes show dramatic changes in their electronic states after photoirradiation, as indicated by photoinduced Mott-insulator-to-metal transition. In the photoexcited states of Mott insulators, electron wave functions are more delocalized than in the ground state, and long-range Coulomb interactions play important roles in charge dynamics. However, their effects are difficult to discriminate experimentally. Here, we show that in a one-dimensional Mott insulator, bis(ethylenedithio)tetrathiafulvalene-difluorotetracyanoquinodimethane (ET-F<sub>2</sub>TCNQ), long-range Coulomb interactions stabilize not only excitons, doublon-holon bound states, but also biexcitons. By measuring terahertz-electric-field-induced reflectivity changes, we demonstrate that odd- and even-parity excitons are split off from a doublon-holon continuum. Further, spectral changes of reflectivity induced by a resonant excitation of the odd-parity exciton reveals that an exciton-biexciton transition appears just below the exciton-transition peak. Theoretical simulations show that long-range Coulomb interactions over four sites are necessary to stabilize the biexciton. Such information is indispensable for understanding the non-equilibrium dynamics of photoexcited Mott insulators. -
Topological charge transport by mobile dielectric-ferroelectric domain walls 査読あり
R. Takehara, K. Sunami, K. Miyagawa, T. Miyamoto, H. Okamoto, S. Horiuchi, R. Kato, K. Kanoda
Science Advances 5 ( 11 ) 2019年11月
掲載種別：研究論文（学術雑誌）
The concept of topology has been widely applied in condensed matter physics, leading to the identification of peculiar electronic states on three-dimensional (3D) surfaces or 2D lines separating topologically distinctive regions. In the systems explored so far, the topological boundaries are built-in walls; thus, their motional degrees of freedom, which potentially bring about new paradigms, have been experimentally inaccessible. Here, working with a quasi-1D organic material with a charge-transfer instability, we show that mobile neutral-ionic (dielectric-ferroelectric) domain boundaries with topological charges carry strongly 1D-confined and anomalously large electrical conduction with an energy gap much smaller than the one-particle excitation gap. This consequence is further supported by nuclear magnetic resonance detection of spin solitons, which are required for steady current of topological charges. The present observation of topological charge transport may open a new channel for broad charge transport-related phenomena such as thermoelectric effects.
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Evaluating intrinsic mobility from transient terahertz conductivity spectra of microcrystal samples of organic molecular semiconductors 査読あり
H. Yada, H. Sekine, T. Miyamoto, T. Terashige, R. Uchida, T. Otaki, F. Maruike, N. Kida, T. Uemura, S. Watanabe, T. Okamoto, J. Takeya, H. Okamoto
Applied Physics Letters 115 ( 14 ) 143301 - 143301 2019年09月
担当区分：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：AIP Publishing
In microcrystal samples of the prototypical organic molecular semiconductors rubrene and C10-DNTT (2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene), we measured spectra of complex optical conductivity caused by photocarriers with optical-pump terahertz-probe spectroscopy. We analyzed the σ - ω spectra using the Drude-Smith model and evaluated the DC mobility μ DC, which includes extrinsic effects, and the intrinsic mobility μ int. The resulting μ DC and μ int values are comparable to the mobility values obtained using transport measurements of polycrystalline and single-crystalline field-effect transistors, respectively. This correspondence demonstrates that the intrinsic mobility values of organic molecular semiconductors can be evaluated from the measurements of microcrystal samples using transient terahertz spectroscopy.
DOI： 10.1063/1.5118262
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Doublon-holon pairing mechanism via exchange interaction in two-dimensional cuprate Mott insulators 査読あり
T. Terashige, T. Ono, T. Miyamoto, T. Morimoto, H. Yamakawa, N. Kida, T. Ito, T. Sasagawa, T. Tohyama, H. Okamoto
SCIENCE ADVANCES 5 ( 6 ) 2019年06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 ASSOC ADVANCEMENT SCIENCE
Coupling of charge and spin degrees of freedom is a critical feature of correlated electron oxides, as represented by the spin-related mechanism of a Cooper pair under high-Tc superconductivity. A doublon-holon pair generated on an antiferromagnetic spin background is also predicted to attract each other via the spin-spin interaction J, similar to a Cooper pair, while its evidence is difficult to obtain experimentally. Here, we investigate such an excitonic effect by electroreflectance spectroscopy using terahertz electric field pulses in undoped cuprates: Nd2CuO4, Sr2CuO2Cl2, and La2CuO4. Analyses of the spectral changes of reflectivity under electric fields reveal that the splitting of odd-parity and even-parity excitons, a measure of doublon-holon binding energy, increases with J. This trend is reproduced by t-J-type model calculations, providing strong evidence of the spin-related doublon-holon pairing. Agreement with the calculations supports the s-wave symmetry of the doublon-holon pair in contrast to the d-wave Cooper pair in doped cuprates.
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Photoexcitation of a one-dimensional polarization-inverted domain from the charge-ordered ferroelectric ground state of (TMTTF)(2)PF6 査読あり
T. Yamaguchi, K. Asada, H. Yamakawa, T. Miyamoto, K. Iwano, T. Nakamura, N. Kida, H. Okamoto
PHYSICAL REVIEW B 99 ( 24 ) 2019年06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 PHYSICAL SOC
We theoretically revealed that a weak photoexcitation achieves the electric polarization-inversion with approximately 18% of all the charges, which was interpreted as a superimposition of multiexciton states, from the charge-ordered ferroelectric ground state of (TMTTF)(2)PF6 at absolute zero temperature. Regarding a relative change of electric polarization (Delta P/P), the photoexcitation corresponds to 36%, which is much larger than Delta P/P of other typical organic materials. The photoexcitation of Delta P/P similar to 36% corresponds to the single peak of the optical conductivity in the low-energy region, which was also observed at 10 K. Therefore, the value of Delta P/P similar to 36% can be achieved in the early stage of the ultrafast photoinduced dynamics of the material. This fact is useful not only for applications of this material and other analogous materials in optical devices, but also for research toward controlling electric polarizations by light, which is one of the recent attracting issues in photoinduced phase transition phenomena. Theoretical calculations are based on a quarter-filled one-dimensional effective model with appropriate parameters and 50 unit cells.
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MX-type single chain complexes with an aromatic in-plane ligand: incorporation of aromatic interactions for stabilizing the chain structure 査読あり
Unjila Afrin, Hiroaki Iguchi, Mohammad Rasel Mian, Shinya Takaishi, Hiromichi Yamakawa, Tsubasa Terashige, Tatsuya Miyamoto, Hiroshi Okamoto, Masahiro Yamashita
DALTON TRANSACTIONS 48 ( 22 ) 7828 - 7834 2019年06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
MX-type one-dimensional complexes [Pt-IV(amp)(2)Br-2][Pt-II/IV(amp)(2)Br](2)(HSO4)(2)(SO4)(2)center dot 13H(2)O (3) and [Pt-IV(amp)(2)Br-2][Pt-II/IV(amp)(2)Br](2)(H2PO4)(6)center dot 8H(2)O (4) were synthesized as the first analogue containing only an aromatic in-plane ligand. The Pt-Br chain structures of 3 and 4 are stabilized by both the hydrogenbond network along the chain and the p-stacking via intercalated Pt(IV) complexes. Structural and spectroscopic studies indicated that both 3 and 4 form the Pt(II/IV) mixed valence state.
DOI： 10.1039/c9dt00784a
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Hysteretic Current-Voltage Characteristics in the Deuterium-Dynamics-Triggered Charge-Ordered Phase of kappa-D-3 (Cat-EDT-TTF)(2) 査読あり
Akira Ueda, Kouki Kishimoto, Yoshiya Sunairi, Junya Yoshida, Hiromichi Yamakawa, Tatsuya Miyamoto, Tsubasa Terashige, Hiroshi Okamoto, Hatsumi Mori
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 88 ( 3 ) 2019年03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 SOC JAPAN
A purely organic material, kappa-D-3 (Cat-EDT-TTF)(2), undergoes a peculiar charge ordering transition triggered by deuterium localization in hydrogen bonds between two-dimensional conducting layers. Here, we report that the current density-electric field characteristics of this charge-ordered phase exhibit negative differential resistance and also hysteresis, which is considered to be induced by the deuterium dynamics. Upon the application of a pulsed voltage, the resistance irreversibly changes; namely, the initial charge-ordered state is changed to a metastable state through a high-conducting (excited) state, which results in the appearance of the hysteresis. Raman spectroscopy suggests that this metastable state is a non-charge-ordered dimer-Mott state. Interestingly, this state does not appear at low temperatures, and instead, the initial charge-ordered state reappears. These results are well understood by considering the temperature-dependent dynamics of hydrogen-bonded deuterium (i.e., localization/fluctuations) coupled to the pi-electrons in the conducting layers. In contrast, the hydrogen analogue kappa-D-3 (Cat-EDT-TTF)(2), which is a dimer-Mott insulator without proton localization, does not show such hysteretic behavior.
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Large terahertz magnetization response in ferromagnetic nanoparticles 査読あり
Tomoaki Ishii, Hiromichi Yamakawa, Toshiki Kanaki, Tatsuya Miyamoto, Noriaki Kida, Hiroshi Okamoto, Masaaki Tanaka, Shinobu Ohya
APPLIED PHYSICS LETTERS 114 ( 6 ) 2019年02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
Ultra-fast coherent magnetization reversal, which is indispensable for high-speed spintronic devices, is expected to be realized by irradiation with a nearly monocyclic terahertz pulse. In this letter, using MnAs ferromagnetic nanoparticles embedded in a GaAs host matrix, we observe a large modulation up to 20% of the magnetization of these nanoparticles with terahertz pump pulse irradiation. This modulation ratio is the largest value ever reported for terahertz magnetization modulation. We reveal that the modulation is induced by the electric-field component of the terahertz pulse via spin-carrier interactions. Ferromagnetic nanoparticle systems are promising, and material designing is the key for ultrafast magnetization reversal using terahertz pulses. Published under license by AIP Publishing.
DOI： 10.1063/1.5088227
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Smallest Optical Gap for Pt(II)-Pt(IV) Mixed-Valence Pt-Cl and Pt-Br Chain Complexes Achieved by Using a Multiple-Hydrogen-Bond Approach 査読あり
M. Rasel Mian, H. Iguchi, S. Takaishi, U. Afrin, T. Miyamoto, H. Okamoto, M. Yamashita
Inorganic Chemistry 58 ( 1 ) 114 - 120 2019年01月
掲載種別：研究論文（学術雑誌）
A multiple-hydrogen-bond approach was applied to shorten Pt-X-Pt distances in Cl- and Br-bridged Pt chain complexes. [Pt(dabdOH) 2 Cl]Cl 2 (5) and [Pt(dabdOH) 2 Br]Br 2 (6) (dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol) contain hydroxy groups, which form additional hydrogen bonds with counteranions. 5 has the shortest Pt-Cl-Pt distance (5.0747(8) Å) of all Cl-bridged Pt chain complexes reported to date. Furthermore, the smallest optical gap (1.45 eV for 5 and 1.19 eV for 6) in any Cl- or Br-bridged Pt chain complex was achieved. 6 has the highest electrical conductivity (1.9 × 10 -5 S cm -1 at room temperature) of all Br-bridged Pt chain complexes. This study shows that the introduction of additional hydrogen bonds between the ligands and halides is effective to enhance the electronic properties of halogen-bridged metal complexes.
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Ultrafast polarization control by terahertz fields via π-electron wavefunction changes in hydrogen-bonded molecular ferroelectrics 査読あり
T. Miyamoto, D. Hata, T. Morimoto, H. Yamakawa, N. Kida, T. Terashige, K. Iwano, H. Kishida, S. Horiuchi, H. Okamoto
Scientific Reports 8 ( 1 ) 2018年12月
担当区分：筆頭著者 掲載種別：研究論文（学術雑誌）
Rapid polarization control by an electric field in ferroelectrics is important to realize high-frequency modulation of light, which has potential applications in optical communications. To achieve this, a key strategy is to use an electronic part of ferroelectric polarization. A hydrogen-bonded molecular ferroelectric, croconic acid, is a good candidate, since π-electron polarization within each molecule is theoretically predicted to play a significant role in the ferroelectric-state formation, as well as the proton displacements. Here, we show that a sub-picosecond polarization modulation is possible in croconic acid using a terahertz pulse. The terahertz-pulse-pump second-harmonic-generation-probe and optical-reflectivity-probe spectroscopy reveal that the amplitude of polarization modulation reaches 10% via the electric-field-induced modifications of π-electron wavefunctions. Moreover, the measurement of electric-field-induced changes in the infrared molecular vibrational spectrum elucidates that the contribution of proton displacements to the polarization modulation is negligibly small. These results demonstrate the electronic nature of polarization in hydrogen-bonded molecular ferroelectrics. The ultrafast polarization control via π-electron systems observed in croconic acid is expected to be possible in many other hydrogen-bonded molecular ferroelectrics and utilized for future high-speed optical-modulation devices.
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Probing ultrafast spin-relaxation and precession dynamics in a cuprate Mott insulator with seven-femtosecond optical pulses 査読あり
T. Miyamoto, Y. Matsui, T. Terashige, T. Morimoto, N. Sono, H. Yada, S. Ishihara, Y. Watanabe, S. Adachi, T. Ito, K. Oka, A. Sawa, H. Okamoto
Nature Communications 9 ( 1 ) 2018年12月
担当区分：筆頭著者 記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Springer Science and Business Media LLC
A charge excitation in a two-dimensional Mott insulator is strongly coupled with the surrounding spins, which is observed as magnetic-polaron formations of doped carriers and a magnon sideband in the Mott-gap transition spectrum. However, the dynamics related to the spin sector are difficult to measure. Here, we show that pump-probe reflection spectroscopy with seven-femtosecond laser pulses can detect the optically induced spin dynamics in Nd2CuO4, a typical cuprate Mott insulator. The bleaching signal at the Mott-gap transition is enhanced at ~18 fs. This time constant is attributable to the spin-relaxation time during magnetic-polaron formation, which is characterized by the exchange interaction. More importantly, ultrafast coherent oscillations appear in the time evolution of the reflectivity changes, and their frequencies (1400–2700 cm−1) are equal to the probe energy measured from the Mott-gap transition peak. These oscillations can be interpreted as the interference between charge excitations with two magnons originating from charge–spin coupling.
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Evidence for solitonic spin excitations from a charge-lattice–coupled ferroelectric order 査読あり
K. Sunami, T. Nishikawa, K. Miyagawa, S. Horiuchi, R. Kato, T. Miyamoto, H. Okamoto, K. Kanoda
Science Advances 4 ( 11 ) 2018年11月
掲載種別：研究論文（学術雑誌）
Topological defects have been explored in different fields ranging from condensed matter physics and particle physics to cosmology. In condensed matter, strong coupling between charge, spin, and lattice degrees of freedom brings about emergent excitations with topological characteristics at low energies. One-dimensional (1D) systems with degenerate dimerization patterns are typical stages for the generation of topological defects, dubbed “solitons”; for instance, charged solitons are responsible for high electrical conductivity in doped trans-polyacetylene. Here, we provide evidence based on a nuclear magnetic resonance (NMR) study for mobile spin solitons deconfined from a strongly charge-lattice–coupled spin-singlet ferroelectric order in a quasi-1D organic charge-transfer complex. The NMR spectral shift and relaxation rate associated with static and dynamic spin susceptibilities indicate that the ferroelectric order is violated by dilute solitonic spin excitations, which were further demonstrated to move diffusively by the frequency dependence of the relaxation rate. The traveling solitons revealed here may promise the emergence of anomalous electrical and thermal transport.
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Control of electronic states by a nearly monocyclic terahertz electric-field pulse in organic correlated electron materials 査読あり
Tatsuya Miyamoto, Hiromichi Yamakawa, Takeshi Morimoto, Hiroshi Okamoto
Journal of Physics B: Atomic, Molecular and Optical Physics 51 ( 16 ) 162001 - 162001 2018年08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：IOP Publishing
Recent developments in femtosecond laser technology have enabled the generation of a nearly monocyclic strong terahertz (THz) pulse with an amplitude greater than 100 kV cm(-1). Such a THz pulse can be used to control not only the elementary excitations in solids such as phonons, magnons, and excitons but also the electronic phases. To achieve ultrafast phase control in the sub-ps time domain with a THz pulse, correlated electronic materials that demonstrate electronic phase transitions without large structural changes induced by external stimuli, such as by temperature, pressure, and light can be used. In this paper, we review recent studies on electronic phase controls using a nearly monocyclic THz pulse in organic molecular compounds with correlated electron systems, TTF-CA (TTF: tetrathiafulvalene and CA: p-chloranil), alpha-(ET)(2)I-3 (ET: bis(ethylenedithio)tetrathiafulvalene), and kappa-(ET)(2)Cu[N(CN)(2)]Br. TTF-CA undergoes a neutral-to-ionic phase transition as the temperature decreases. It demonstrates an electronic-type ferroelectricity in the ionic phase, in which ferroelectric polarization is generated from intermolecular charge transfers across the neutral-to-ionic phase transition. THz-pulse pump second-harmonic-generation probe and optical-reflectivity probe measurements show that ferroelectric polarization in the ionic phase can be rapidly modulated by a THz pulse via charge transfers induced by an electric field. In alpha-(ET)(2)I-3, rapid polarization modulation by a THz pulse was also achieved in the ferroelectric charge-order phase. Detailed analyses of reflectivity changes induced with THz electric fields revealed that the ferroelectric polarization originated from intermolecular charge transfers and was oriented diagonally to the crystal axes. These results demonstrate that the ferroelectricity of this compound was electronic, similar to that of the ionic phase of TTF-CA. In the para-electric neutral phase of TTF-CA, a macroscopic polarization was generated by a THz pulse via the dynamics induced by an electric field on microscopic ionic domains. In kappa-(ET)(2)Cu[N(CN)(2)] Br, a transition from a Mott insulator to a metal by a THz pulse was demonstrated by observing Drude-like low-energy spectral weights induced by the electric field. A THz pulse induced doublon-holon pair production by quantum tunnelling processes, which collapsed the original Mott gap in under a picosecond. These results suggest that strong THz-pulse irradiation is an effective approach for the ultrafast control of electronic phases in correlated electron materials.
その他リンク： http://iopscience.iop.org/article/10.1088/1361-6455/aad023/pdf
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Revisited phase diagram on charge instability and lattice symmetry breaking in the organic ferroelectric TTF-QCl(4) 査読あり
Ryosuke Takehara, Keishi Sunami, Fumitatsu Iwase, Masayuki Hosoda, Kazuya Miyagawa, Tatsuya Miyamoto, Hiroshi Okamoto, Kazushi Kanoda
PHYSICAL REVIEW B 98 ( 5 ) 2018年08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 PHYSICAL SOC
There has been increasing interest in electronic ferroelectricity, which stems from deformation of the electronic wave function coupled with lattice symmetry breaking, invoking a Berry-phase description of electronic polarization beyond the conventional notion of ionic displacement. How does a ferroelectric transition of an electronic nature cross over to a conventional one of an ionic nature? This is a fundamental issue of ferroelectricity that has yet to be answered. Here, investigating a quasi-one-dimensional organic ferroelectric material, TTF-QCl(4), under pressures of up to 35 kbar, we reveal a pressure-temperature phase diagram that spans the electronic and ionic regimes of ferroelectric transitions in a single material. The global phase diagram revealed by nuclear quadrupole resonance experiments demonstrates that electronic ferroelectricity crosses over to conventional ionic ferroelectricity through the separation of charge-transfer instability and lattice symmetry breaking (dimerization), and that the previously reported dimer liquid phase is largely extended to high pressures.
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Structural Study of Bromide-Bridged Pd Chain Complex with Weak CH center dot center dot center dot O Hydrogen Bonds 査読あり
Mohammad Rasel Mian, Hiroaki Iguchi, Momoka Miyata, Shinya Takaishi, Hiromichi Yamakawa, Tsubasa Terashige, Tatsuya Miyamoto, Hiroshi Okamoto, Masahiro Yamashita
ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE 644 ( 14 ) 646 - 651 2018年08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：WILEY-V C H VERLAG GMBH
A novel Br-bridged Pd chain complex, [Pd(hxn)(2)Br](TsO)(2) (2) (TsO- = p-toluenesulfonate), was synthesized from a new in-plane ligand (3S,4S)-3,4-diaminohexane (hxn). 2 forms Pd-II/Pd-IV mixed-valence state, as confirmed by single-crystal X-ray structure analysis and polarized Raman spectra. The hxn ligand provides the additional hydrogen bonds (CH center dot center dot center dot O) between the methyl group of the ligands and the O atoms of TsO- anions, which are weaker than those observed in the chain complex with hydroxy group, [Pd(dabdOH)(2)Br]Br-2 (1).
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Ultrafast magnetization modulation induced by the electric field component of a terahertz pulse in a ferromagnetic-semiconductor thin film 査読あり
Tomoaki Ishii, Hiromichi Yamakawa, Toshiki Kanaki, Tatsuya Miyamoto, Noriaki Kida, Hiroshi Okamoto, Masaaki Tanaka, Shinobu Ohya
SCIENTIFIC REPORTS 8 ( 1 ) 2018年05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
High-speed magnetization control of ferromagnetic films using light pulses is attracting considerable attention and is increasingly important for the development of spintronic devices. Irradiation with a nearly monocyclic terahertz pulse, which can induce strong electromagnetic fields in ferromagnetic films within an extremely short time of less than -1 ps, is promising for damping-free high-speed coherent control of the magnetization. Here, we successfully observe a terahertz response in a ferromagnetic-semiconductor thin film. In addition, we find that a similar terahertz response is observed even in a non-magnetic semiconductor and reveal that the electric-field component of the terahertz pulse plays a crucial role in the magnetization response through the spin-carrier interactions in a ferromagnetic-semiconductor thin film. Our findings will provide new guidelines for designing materials suitable for ultrafast magnetization reversal.
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Terahertz radiation by subpicosecond spin-polarized photocurrent originating from Dirac electrons in a Rashba-type polar semiconductor 査読あり
Yuto Kinoshita, Noriaki Kida, Tatsuya Miyamoto, Manabu Kanou, Takao Sasagawa, Hiroshi Okamoto
PHYSICAL REVIEW B 97 ( 16 ) 2018年04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 PHYSICAL SOC
The spin-splitting energy bands induced by the relativistic spin-orbit interaction in solids provide a new opportunity to manipulate the spin-polarized electrons on the subpicosecond timescale. Here, we report one such example in a bulk Rashba-type polar semiconductor BiTeBr. Strong terahertz electromagnetic waves are emitted after the resonant excitation of the interband transition between the Rashba-type spin-splitting energy bands with a femtosecond laser pulse circularly polarized. The phase of the emitted terahertz waves is reversed by switching the circular polarization. This suggests that the observed terahertz radiation originates from the subpicosecond spin-polarized photocurrents, which are generated by the asymmetric depopulation of the Dirac state. Our result provides a way for the current-induced terahertz radiation and its phase control by the circular polarization of incident light without external electric fields.
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Generation of a carrier-envelope-phase-stable femtosecond pulse at 10 mu m by direct down-conversion from a Ti:sapphire laser pulse 査読あり
Takeshi Morimoto, Naoki Sono, Tatsuya Miyamoto, Noriaki Kida, Hiroshi Okamoto
APPLIED PHYSICS EXPRESS 10 ( 12 ) 2017年12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：IOP PUBLISHING LTD
We report the generation of a carrier-envelope-phase-stable femtosecond pulse at 10 mu m with 800 nJ by the down-conversion of a 25-fs-long 800 nm pulse with 1.2 mJ obtained from a standard Ti:sapphire regenerative amplifier. We discuss a simple optical setup for the spectral broadening of the 800 nm pulse by filamentation in air and the intrapulse difference frequency generation in a wide-bandgap nonlinear optical crystal, LiGaS2. From the measurement of the electric-field waveform of the generated mid-infrared pulse by electrooptic sampling, the maximum electric-field amplitude is estimated to be approximately 11MV/cm at a beam diameter of 70 mu m. (C) 2017 The Japan Society of Applied Physics
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Visualization of a nonlinear conducting path in an organic molecular ferroelectric by using emission of terahertz radiation 査読あり
M. Sotome, N. Kida, Y. Kinoshita, H. Yamakawa, T. Miyamoto, H. Mori, H. Okamoto
PHYSICAL REVIEW B 95 ( 24 ) 2017年06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 PHYSICAL SOC
Nonlinear electric transport and switching to a negative resistance state are typical electric-field-induced phenomena in correlated electron materials, while their mechanisms are generally difficult to solve. In the present study, we apply the terahertz-radiation imaging method to an organic molecular ferroelectric, alpha-type bis(ethylenedithio)tetrathiafulvalene iodide salt, and investigate the nature of its negative resistance phenomenon. When the negative resistance state is produced, the ferroelectric order is melted in an elongated region with the width of similar to 100 mu m and that region grows along the direction inclined by about 40 degrees from the b axis with the increase of nonlinear current. A comparison of the terahertz radiation intensity with the current magnitude revealed that the melted region forms a conducting path. We interpreted the diagonal growth of the conduction path by taking into account the anisotropy of the intermolecular transfer integrals.
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Multiple-Hydrogen-Bond Approach to Uncommon Pd(III) Oxidation State: A Pd–Br Chain with High Conductivity and Thermal Stability 査読あり
Mohammad Rasel Mian, Hiroaki Iguchi, Shinya Takaishi, Hideaki Murasugi, Tatsuya Miyamoto, Hiroshi Okamoto, Hisaaki Tanaka, Shin-ichi Kuroda, Brian K. Breedlove, Masahiro Yamashita
Journal of the American Chemical Society 139 ( 19 ) 6562 - 6565 2017年05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：American Chemical Society (ACS)
A Br-bridged Pd chain complex with the Pd ion in an uncommon +3 oxidation state, [Pd(dabdOH)(2)Br]Br-2 (3), was prepared using a new method involving multiple hydrogen bonds. The PdBr chain complex exhibited superior electrical conductivity and thermal stability. An in-plane ligand with an additional hydrogen donor group (hydroxy group), (2S,3S)-2,3-diaminobutane-1,4-diol (dabdOH), was used to create a multiple-hydrogen-bond network, which effectively shrinks the Pd-Br-Pd distance, stabilizing the Pd(III) state up to its decomposition temperature (443 K). 3 shows semiconducting behavior with quite high electrical conductivity (3-38 S cm(-1) at room temperature), which is 10(6) times larger than the previous record for analogous PdBr chains. Indeed, 3 is the most conductive MX-type chain complex reported so far. The precise positional control of ions via a multiple-hydrogen-bond network is a useful method for controlling the electronic states, thermal stability and conductivity of linear coordination polymers.
DOI： 10.1021/jacs.7b02558
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Ultrafast Electron and Molecular Dynamics in Photoinduced and Electric-Field-Induced Neutral-Ionic Transitions 査読あり
Takeshi Morimoto, Tatsuya Miyamoto, Hiroshi Okamoto
CRYSTALS 7 ( 5 ) 2017年05月
記述言語：英語 出版者・発行元：MDPI
Mixed-stacked organic molecular compounds near the neutral-ionic phase boundary, represented by tetrathiafulvalene-p-chloranil (TTF-CA), show a unique phase transition from a paraelectric neutral (N) phase to a ferroelectric ionic (I) phase when subjected to decreasing temperature or applied pressure, which is called an NI transition. This NI transition can also be induced by photoirradiation, in which case it is known as a prototypical 'photoinduced phase transition'. In this paper, we focus on the ultrafast electron and molecular dynamics in the transition between the N and I states induced by irradiation by a femtosecond laser pulse and a terahertz electric-field pulse in TTF-CA. In the first half of the paper, we review the photoinduced N-to-I transition in TTF-CA studied by femtosecond-pump-probe reflection spectroscopy. We show that in the early stage of the transition, collective charge transfers occur within 20 fs after the photoirradiation, and microscopic one-dimensional (1D) I domains are produced. These ultrafast I-domain formations are followed by molecular deformations and displacements, which play important roles in the stabilization of photogenerated I domains. In the photoinduced I-to-N transition, microscopic 1D N domains are also produced and stabilized by molecular deformations and displacements. However, the time characteristics of the photoinduced N-to-I and I-to-N transitions in the picosecond time domain are considerably different from each other. In the second half of this paper, we review two phenomena induced by a strong terahertz electric-field pulse in TTF-CA: the modulation of a ferroelectric polarization in the I phase and the generation of a large macroscopic polarization in the N phase.
DOI： 10.3390/cryst7050132
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Ultrafast Photoinduced Electric-Polarization Switching in a Hydrogen-Bonded Ferroelectric Crystal 査読あり
K. Iwano, Y. Shimoi, T. Miyamoto, D. Hata, M. Sotome, N. Kida, S. Horiuchi, H. Okamoto
Physical Review Letters 118 ( 10 ) 2017年03月
掲載種別：研究論文（学術雑誌） 出版者・発行元：American Physical Society (APS)
Croconic acid crystals show proton displacive-type ferroelectricity with a large spontaneous polarization reaching 20 μC/cm2, which originates from the strong coupling of proton and π-electron degrees of freedom. Such a coupling makes us expect a large polarization change by photoirradiations. Optical-pump second-harmonic-generation-probe experiments reveal that a photoexcited croconic-acid crystal loses the ferroelectricity substantially with a maximum quantum efficiency of more than 30 molecules per one absorbed photon. Based on density functional calculations, we theoretically discuss possible pathways toward the formation of a one-dimensional domain with polarization inversion and its recovery process to the ground state by referring to the dynamics of experimentally obtained polarization changes.
DOI： 10.1103/physrevlett.118.107404
その他リンク： https://link.aps.org/article/10.1103/PhysRevLett.118.107404
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Terahertz-Field-Induced Large Macroscopic Polarization and Domain-Wall Dynamics in an Organic Molecular Dielectric 査読あり
T. Morimoto, T. Miyamoto, H. Yamakawa, T. Terashige, T. Ono, N. Kida, H. Okamoto
PHYSICAL REVIEW LETTERS 118 ( 10 ) 2017年03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 PHYSICAL SOC
A rapid polarization control in paraelectric materials is important for an ultrafast optical switching useful in the future optical communication. In this study, we applied terahertz-pump second-harmonic-generation-probe and optical-reflectivity-probe spectroscopies to the paraelectric neutral phase of an organic molecular dielectric, tetrathiafulvalene-p-chloranil and revealed that a terahertz pulse with the electric-field amplitude of similar to 400 kV/cm produces in the subpicosecond time scale a large macroscopic polarization whose magnitude reaches similar to 20% of that in the ferroelectric ionic phase. Such a large polarization generation is attributed to the intermolecular charge transfers and breathing motions of domain walls between microscopic neutral and ionic domains induced by the terahertz electric field.
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Mott transition by an impulsive dielectric breakdown 査読あり
Hiroshi M. Yamamoto, Yamakawa, H., Miyamoto, T., Morimoto, T., Terashige, T., Yada, H., Kida, N., Suda, M., Yamamoto, H. M., Kato, R., Miyagawa, K., Kanoda, K., Okamoto, H.
Nature Materials 16 ( 11 ) 2017年
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Intrinsic carrier scattering mechanism in anatase Ti O2 investigated by ultraviolet-pump terahertz-probe spectroscopy 査読あり
Y. Matsui, T. Terashige, R. Uchida, T. Miyamoto, H. Yada, H. Matsuzaki, B. S. Li, A. Sawa, H. Okamoto
Physical Review B 94 ( 4 ) 2016年07月
掲載種別：研究論文（学術雑誌）
We measured the optical conductivity σ (ω) spectra caused by photocarriers in anatase TiO2 by applying ultraviolet-pump terahertz-probe spectroscopy on epitaxially grown thin films. Drude model analyses of σ (ω) spectra reveal that, in the terahertz region, the reduced mass (m) of the carriers is greater and their damping constant (γ) is smaller than those in the infrared region. With regard to the extended Drude model, these differences can be attributed to the frequency dependence of m and γ originating from carrier-phonon scatterings. Furthermore, the interaction between carriers and Ti-O stretching modes in the 500-1000-cm-1 region strongly influences the intrinsic carrier scattering mechanism in anatase TiO2.
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Terahertz Radiation by Subpicosecond Magnetization Modulation in the Ferrimagnet LiFe5O8 査読あり
Yuto Kinoshita, Noriaki Kida, Masato Sotome, Tatsuya Miyamoto, Yusuke Iguchi, Yoshinori Onose, Hiroshi Okamoto
ACS PHOTONICS 3 ( 7 ) 1170 - 1175 2016年07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 CHEMICAL SOC
We report the terahertz radiation in the polar and ferrimagnetic insulating oxide LiFe5O8 upon irradiation of a femtosecond laser pulse at room temperature. By the resonant optical excitation of an originally spin-forbidden d-d transition in Fe3+ ions, we observed the emission of terahertz waves, which is attributable to the magnetic-dipole radiation originating from the light-induced subpicosecond magnetization modulation. By mapping out the emission of the terahertz waves, we successfully visualized the vector images of in-plane magnetic domains in magnetic fields. The terahertzradiation imaging presented here is useful as a new tool to observe ferromagnetic or ferrimagnetic domains.
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Novel electronic ferroelectricity in an organic charge-order insulator investigated with terahertz-pump optical-probe spectroscopy 査読あり
H. Yamakawa, T. Miyamoto, T. Morimoto, H. Yada, Y. Kinoshita, M. Sotome, N. Kida, K. Yamamoto, K. Iwano, Y. Matsumoto, S. Watanabe, Y. Shimoi, M. Suda, H. M. Yamamoto, H. Mori, H. Okamoto
Scientific Reports 6 2016年02月
掲載種別：研究論文（学術雑誌）
In electronic-type ferroelectrics, where dipole moments produced by the variations of electron configurations are aligned, the polarization is expected to be rapidly controlled by electric fields. Such a feature can be used for high-speed electric-switching and memory devices. Electronic-type ferroelectrics include charge degrees of freedom, so that they are sometimes conductive, complicating dielectric measurements. This makes difficult the exploration of electronic-type ferroelectrics and the understanding of their ferroelectric nature. Here, we show unambiguous evidence for electronic ferroelectricity in the charge-order (CO) phase of a prototypical ET-based molecular compound, α-(ET)2I3 (ET:bis(ethylenedithio)tetrathiafulvalene), using a terahertz pulse as an external electric field. Terahertz-pump second-harmonic-generation(SHG)-probe and optical-reflectivity-probe spectroscopy reveal that the ferroelectric polarization originates from intermolecular charge transfers and is inclined 27° from the horizontal CO stripe. These features are qualitatively reproduced by the density-functional-theory calculation. After sub-picosecond polarization modulation by terahertz fields, prominent oscillations appear in the reflectivity but not in the SHG-probe results, suggesting that the CO is coupled with molecular displacements, while the ferroelectricity is electronic in nature. The results presented here demonstrate that terahertz-pump optical-probe spectroscopy is a powerful tool not only for rapidly controlling polarizations, but also for clarifying the mechanisms of ferroelectricity.
DOI： 10.1038/srep20571
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Electrical Transport in the Quasi-Two-Dimensional Ionic Mott Insulator M2P-TCNQF(4) under High Pressures 査読あり
Ryosuke Takehara, Kazuya Miyagawa, Tatsuya Miyamoto, Hiroshi Okamoto, Hiromi Taniguchi, Kazuyuki Matsubayashi, Yoshiya Uwatoko, Kazushi Kanoda
JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 84 ( 10 ) 104702 - 104702 2015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 SOC JAPAN
We performed electrical resistivity measurements for the quasi-two-dimensional ionic Mott insulator M2P-TCNQF(4) (M2P: 5,10-dihydro-5,10-dimethylphenazine, TCNQF(4): 2,3,5-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) under high pressures up to 80 kbar, to search for a metallic state emerging from an ionic insulator. At room temperature, the resistivity is markedly decreased by five orders of magnitude with increasing pressure up to 80 kbar. Although the temperature dependence of resistivity shows insulating behavior at 80 kbar, the activation energy of 0.36 eV at ambient pressure is reduced to 0.077 eV at 80 kbar, indicating that the system is brought close to the metallic state. At 60 and 80 kbar, low-temperature resistivity is described by variable range hopping rather than by thermal activation over a gap, which suggests that some disorder is induced in the system at high pressures.
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Epitaxial growth of high quality La<inf>2</inf>CuO<inf>4</inf> film on LaSrAlO<inf>4</inf> substrate with introduction of ultrathin amorphous layer of La<inf>2</inf>CuO<inf>4</inf> 査読あり
B. S. Li, Y. Matsui, T. Miyamoto, H. Yada, A. Sawa, H. Okamoto
Journal of Crystal Growth 425 230 - 233 2015年07月
掲載種別：研究論文（学術雑誌）
The authors present the apparent improvement of quality of La2CuO4 (LCO) thin films with the introduction of an amorphous layer (AL) of LCO. The samples were grown on LaSrAlO4 substrates by pulsed laser deposition. X-ray diffraction (XRD) measurements demonstrate that the introduction of the AL of LCO significantly improves the structural quality of LCO thin films. The linewidth of XRD peaks is significantly narrowed down to 129 arcsec. Atomic force microscopy (AFM) shows that the surface precipitates are sufficiently suppressed for the LCO films with ALs. The spectra of optical absorption indicate that the ALs can improve the stoichiometry of the LCO films. The mechanism for the improved quality of LCO films with the ALs is discussed based on the results of XRD and AFM measurements.
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Temperature and carrier-density dependence of electron-hole scattering in silicon investigated by optical-pump terahertz-probe spectroscopy 査読あり
T. Terashige, H. Yada, Y. Matsui, T. Miyamoto, N. Kida, H. Okamoto
Physical Review B 91 ( 24 ) 2015年06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：American Physical Society (APS)
We measured the optical conductivity σ(ω) spectra of photodoped silicon by optical-pump terahertz-probe spectroscopy and analyzed them with a two-carrier Drude model. Taking into account the values of electron (hole)-phonon scattering rates previously reported in chemically doped silicon, we evaluated the electron-hole scattering rates γe-h. From 293 to 90K, the magnitudes and temperature dependence of γe-h were successfully reproduced by a theoretical model including the effects of Rutherford scattering, Coulomb screening, and Pauli exclusion. This suggests that these three factors dominate electron-hole scattering processes in silicon. Below 90K, γe-h becomes larger than that of the theoretical curve, which is attributable to a prolongation of the relaxation time of hot carriers.
DOI： 10.1103/physrevb.91.241201
その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.91.241201/fulltext
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Photoinduced complete melting of spin-Peierls phase in Na-tetracyanoquinodimethane revealed by frequency doubling of coherent molecular oscillations 査読あり
H. Uemura, K. Iwasawa, H. Yamakawa, T. Miyamoto, H. Yada, H. Okamoto
Physical Review B 91 ( 12 ) 2015年03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：American Physical Society (APS)
Photoinduced melting of the spin-Peierls phase was investigated in Na-tetracyanoquinodimethane by pump-probe reflection spectroscopy with a time resolution of 49 fs. Photoirradiation generates coherent molecular oscillation, which indicates that the spin-Peierls dimerization decreased. Increasing the excitation photon density to >0.02 photon/molecule doubles the oscillation frequency attributable to recovery of the original symmetry, that is, complete melting of the spin-Peierls phase. We observed a complete photoinduced transition with symmetry recovery revealed by frequency doubling of coherent oscillations.
DOI： 10.1103/physrevb.91.125111
その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.91.125111/fulltext
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Electron transport in TTF-CA under High pressures
Ryosuke Takehara, Kazuya Miyagawa, Kazushi Kanoda, Tatsuya Miyamoto, Hiroyuki Matsuzaki, Hiroshi Okamoto, Hiromi Taniguchi, Kazuyuki Matsubayashi, Yoshiya Uwatoko
PHYSICA B-CONDENSED MATTER 460 83 - 87 2015年03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：ELSEVIER
We investigate the electron transport in the neutral-ionic transition material, tetrathialulvalene-p-chloranil (TTF-CA), under extremely high pressures up to 82.5 kbar, using a cubic anvil system. At room temperature, as pressure is applied, the resistivity steeply decreases in the neutral phase and exhibits a minimum followed by an increase, which reproduces the previously reported results. However, resistivity is found to become insensitive to pressure above 20 kbar, where the ionic phase is stabilized. The robust transport characteristics of the ionic phase is in contrast with the pressure-sensitive transport in the neutral phase. The temperature dependence of resistivity is insulating in the whole pressure range up to 82.5 kbar but shows breaks in the activation plot at certain temperatures, suggesting that some phase transition occurs in the ionic phase. (C) 2014 Elsevier B.V. All rights reserved.
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Excitation-Photon-Energy Selectivity of Photoconversions in Halogen-Bridged Pd-Chain Compounds: Mott Insulator to Metal or Charge-Density-Wave State 査読あり
H. Matsuzaki, M. Iwata, T. Miyamoto, T. Terashige, K. Iwano, S. Takaishi, M. Takamura, S. Kumagai, M. Yamashita, R. Takahashi, Y. Wakabayashi, H. Okamoto
Physical Review Letters 113 ( 9 ) 2014年08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：American Physical Society (APS)
Ultrafast photoinduced transitions of a one-dimensional Mott insulator into two distinct electronic phases, metal and charge-density-wave (CDW) state, were achieved in a bromine-bridged Pd-chain compound [Pd(en)2Br](C5-Y)2H2O (en=ethylenediamine and C5-Y=dialkylsulfosuccinate), by selecting the photon energy of a femtosecond excitation pulse. For the resonant excitation of the Mott-gap transition, excitonic states are generated and converted to one-dimensional CDW domains. For the higher-energy excitation, free electron and hole carriers are produced, giving rise to a transition of the Mott insulator to a metal. Such selectivity in photoconversions by the choice of initial photoexcited states opens a new possibility for the developments of advanced optical switching and memory functions. © 2014 American Physical Society.
DOI： 10.1103/physrevlett.113.096403
その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.113.096403/fulltext
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Ultrafast modulation of polarization amplitude by terahertz fields in electronic-type organic ferroelectrics 査読あり
Tatsuya Miyamoto, Hiroyuki Yada, Hiromichi Yamakawa, Hiroshi Okamoto
Nature Communications 4 ( 1 ) 2013年12月
担当区分：筆頭著者 記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：Springer Science and Business Media LLC
Ferroelectrics sometimes show large electro-optical and non-linear optical effects, available for polarization rotation and frequency conversion of light, respectively. If the amplitude of ferroelectric polarization is modulated in the picosecond time domain, terahertz repetition of optical switching via electro-optical and non-linear optical effects would be achieved. Here we show that polarization amplitude can be rapidly modulated by a terahertz electric field in an organic ferroelectric, tetrathiafulvalene-p-chloranil (TTF-CA). In this compound, alternately stacked donor (TTF) and acceptor (CA) molecules are dimerized via the spin-Peierls mechanism, and charge transfer within each dimer results in a new type of ferroelectricity called electronic-type ferroelectricity. Using a terahertz field, the intradimer charge transfer is strongly modulated, producing a subpicosecond change in the macroscopic polarization, which is demonstrated by transient reflectivity and second-harmonic generation measurements. Subsequently, coherent oscillation of the dimeric molecular displacements occur, which is explained by the modulation of the spin moment of each molecule.
DOI： 10.1038/ncomms3586
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Measurement of a Photoinduced Transition from a Nonordered Phase to a Transient Ordered Phase in the Organic Quantum-Paraelectric Compound dimethyltetrathiafulvalene-dibromodichloro-p-benzoquinone Using Femtosecond Laser Irradiation 査読あり
Tatsuya Miyamoto, Kazuki Kimura, Takanori Hamamoto, Hirotaka Uemura, Hiroyuki Yada, Hiroyuki Matsuzaki, Sachio Horiuchi, Hiroshi Okamoto
Physical Review Letters 111 ( 18 ) 2013年10月
担当区分：筆頭著者 記述言語：英語 掲載種別：研究論文（学術雑誌） 出版者・発行元：American Physical Society (APS)
We report a new photoinduced transition from a nonordered phase to a transient ordered phase with symmetry breaking in an organic charge-transfer compound, dimethyltetrathiafulvalene (DMTTF)-dibromodichloro-p-benzoquinone (2, 6QBr(2)Cl(2)), which is a neutral compound located near the neutral-ionic phase boundary and shows quantum paraelectricity at low temperatures. By an irradiation of a femtosecond laser pulse, an ionic domain consisting of similar to 40 molecules is introduced into the neutral lattice per photon, giving rise to coherent molecular oscillations with fractional charge modulations over similar to 400 molecules. This response is due to the recovery of ferroelectric nature from the quantum paraelectricity by a photoinjection of an ionic domain with a large dipole moment.
DOI： 10.1103/physrevlett.111.187801
その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.111.187801/fulltext
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Terahertz-field-driven sub-picosecond optical switching enabled by large third-order optical nonlinearity in a one-dimensional Mott insulator 査読あり
Hiroyuki Yada, Tatsuya Miyamoto, Hiroshi Okamoto
Applied Physics Letters 102 ( 9 ) 091104 - 091104 2013年03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
Sub-picosecond modulation of the optical reflectivity (R) using terahertz electric-field (E-THz) pulses was achieved in a typical one-dimensional Mott insulator, the bromine-bridged nickel compound, [Ni(chxn)(2)Br]Br-2 (chxn: cyclohexanediamine). The reflectivity change (Delta R/R) at around the Mott-gap transition peak (similar to 1.3 eV) was similar to 1% for E-THz similar to 45 kV/cm, and proportional to the square of E-THz. The relaxation time of Delta R/R was under 0.1 ps, enabling optical switching with a high repetition rate in the near-infrared region. The electric-field and probe-energy dependences of Delta R/R demonstrate that the modulation is due to large third-order optical nonlinearity of one-dimensional Mott insulators. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794413]
DOI： 10.1063/1.4794413
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Relaxation Dynamics of Photoexcited Excitons in Rubrene Single Crystals Using Femtosecond Absorption Spectroscopy 査読あり
S. Tao, N. Ohtani, R. Uchida, T. Miyamoto, Y. Matsui, H. Yada, H. Uemura, H. Matsuzaki, T. Uemura, J. Takeya, H. Okamoto
Physical Review Letters 109 ( 9 ) 2012年08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：American Physical Society (APS)
The relaxation dynamics of an exciton in rubrene was investigated by femtosecond absorption spectroscopy. Exciton relaxation to a self-trapped state occurs via the coherent oscillation with 78cm -1 due to a coupled mode of molecular deformations with phenyl-side-group motions and molecular displacements. From the temperature dependence of the decay time of excitons, the energy necessary for an exciton to escape from a self-trapped state is evaluated to be ∼35meV (∼400K). As a result, a self-trapped exciton is stable at low temperatures. At room temperature, excitons can escape from a self-trapped state and, subsequently, they are dissociated to charged species. The exciton dissociation mechanism is discussed on the basis of the results. © 2012 American Physical Society.
DOI： 10.1103/physrevlett.109.097403
その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.109.097403/fulltext
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Role of Coulomb Interactions and Spin-Peierls Dimerizations in Neutral-to-Ionic Phase Transition Investigated by Femtosecond Reflection Spectroscopy 査読あり
Tatsuya Miyamoto, Hirotaka Uemura, Hiroshi Okamoto
Journal of the Physical Society of Japan 81 ( 7 ) 073703 - 073703 2012年07月
担当区分：筆頭著者 記述言語：英語 出版者・発行元：Physical Society of Japan
To clarify hidden mechanisms of neutral-to-ionic (NI) transition in an organic molecular compound, tetrathiafulvalene-p-chloranil, we investigated temperature-and excitation-density dependences of photoinduced NI transition dynamics. With increasing temperature from 90 K just above the NI transition temperature (81 K), transient dimerizations due to the spin-Peierls (SP) mechanism monotonically decrease. The result suggests that a virtual SP transition temperature locates near 300 K. When photogenerated I states increase with increase of the excitation density, each I domain is further stabilized by interchain Coulomb interactions, but rather destabilized via the suppression of dimerizations. Roles of Coulomb interactions, SP dimerizations, and interchain interactions in the NI transition are discussed on the basis of the results.
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