Papers - MIYAMOTO Tatsuya
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Doublon-holon pairing mechanism via exchange interaction in two-dimensional cuprate Mott insulators Reviewed
T. Terashige, T. Ono, T. Miyamoto, T. Morimoto, H. Yamakawa, N. Kida, T. Ito, T. Sasagawa, T. Tohyama, H. Okamoto
SCIENCE ADVANCES 5 ( 6 ) 2019.06
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
T. Yamaguchi, K. Asada, H. Yamakawa, T. Miyamoto, K. Iwano, T. Nakamura, N. Kida, H. Okamoto
PHYSICAL REVIEW B 99 ( 24 ) 2019.06
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
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 2019.06
Language：English Publishing type：Research paper (scientific journal) Publisher：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) Reviewed
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 ) 2019.03
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
Tomoaki Ishii, Hiromichi Yamakawa, Toshiki Kanaki, Tatsuya Miyamoto, Noriaki Kida, Hiroshi Okamoto, Masaaki Tanaka, Shinobu Ohya
APPLIED PHYSICS LETTERS 114 ( 6 ) 2019.02
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
M. Rasel Mian, H. Iguchi, S. Takaishi, U. Afrin, T. Miyamoto, H. Okamoto, M. Yamashita
Inorganic Chemistry 58 ( 1 ) 114 - 120 2019.01
Publishing type：Research paper (scientific journal)
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 Reviewed
T. Miyamoto, D. Hata, T. Morimoto, H. Yamakawa, N. Kida, T. Terashige, K. Iwano, H. Kishida, S. Horiuchi, H. Okamoto
Scientific Reports 8 ( 1 ) 2018.12
Authorship：Lead author Publishing type：Research paper (scientific journal)
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 Reviewed
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 ) 2018.12
Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：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.
DOI： 10.1038/s41467-018-06312-z
Other Link： http://www.nature.com/articles/s41467-018-06312-z
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Evidence for solitonic spin excitations from a charge-lattice–coupled ferroelectric order Reviewed
K. Sunami, T. Nishikawa, K. Miyagawa, S. Horiuchi, R. Kato, T. Miyamoto, H. Okamoto, K. Kanoda
Science Advances 4 ( 11 ) 2018.11
Publishing type：Research paper (scientific journal)
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 Reviewed
Tatsuya Miyamoto, Hiromichi Yamakawa, Takeshi Morimoto, Hiroshi Okamoto
Journal of Physics B: Atomic, Molecular and Optical Physics 51 ( 16 ) 162001 - 162001 2018.08
Authorship：Lead author, Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：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.
Other Link： 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) Reviewed
Ryosuke Takehara, Keishi Sunami, Fumitatsu Iwase, Masayuki Hosoda, Kazuya Miyagawa, Tatsuya Miyamoto, Hiroshi Okamoto, Kazushi Kanoda
PHYSICAL REVIEW B 98 ( 5 ) 2018.08
Language：English Publishing type：Research paper (scientific journal) Publisher：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.
DOI： 10.1103/PhysRevB.98.054103
Other Link： http://arxiv.org/pdf/1802.00895v1
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Structural Study of Bromide-Bridged Pd Chain Complex with Weak CH center dot center dot center dot O Hydrogen Bonds Reviewed
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 2018.08
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
Tomoaki Ishii, Hiromichi Yamakawa, Toshiki Kanaki, Tatsuya Miyamoto, Noriaki Kida, Hiroshi Okamoto, Masaaki Tanaka, Shinobu Ohya
SCIENTIFIC REPORTS 8 ( 1 ) 2018.05
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
Yuto Kinoshita, Noriaki Kida, Tatsuya Miyamoto, Manabu Kanou, Takao Sasagawa, Hiroshi Okamoto
PHYSICAL REVIEW B 97 ( 16 ) 2018.04
Language：English Publishing type：Research paper (scientific journal) Publisher：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.
DOI： 10.1103/PhysRevB.97.161104
Other Link： http://arxiv.org/pdf/1803.06826v1
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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 Reviewed
Takeshi Morimoto, Naoki Sono, Tatsuya Miyamoto, Noriaki Kida, Hiroshi Okamoto
APPLIED PHYSICS EXPRESS 10 ( 12 ) 2017.12
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
M. Sotome, N. Kida, Y. Kinoshita, H. Yamakawa, T. Miyamoto, H. Mori, H. Okamoto
PHYSICAL REVIEW B 95 ( 24 ) 2017.06
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
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 2017.05
Language：English Publishing type：Research paper (scientific journal) Publisher：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 Reviewed
Takeshi Morimoto, Tatsuya Miyamoto, Hiroshi Okamoto
CRYSTALS 7 ( 5 ) 2017.05
Language：English Publisher：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 Reviewed
K. Iwano, Y. Shimoi, T. Miyamoto, D. Hata, M. Sotome, N. Kida, S. Horiuchi, H. Okamoto
Physical Review Letters 118 ( 10 ) 2017.03
Publishing type：Research paper (scientific journal) Publisher：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
Other Link： 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 Reviewed
T. Morimoto, T. Miyamoto, H. Yamakawa, T. Terashige, T. Ono, N. Kida, H. Okamoto
PHYSICAL REVIEW LETTERS 118 ( 10 ) 2017.03
Language：English Publishing type：Research paper (scientific journal) Publisher：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.
