Details of a Researcher - MIYAMOTO Tatsuya

Intrinsic carrier scattering mechanism in anatase Ti O2 investigated by ultraviolet-pump terahertz-probe spectroscopy Reviewed

Y. Matsui, T. Terashige, R. Uchida, T. Miyamoto, H. Yada, H. Matsuzaki, B. S. Li, A. Sawa, H. Okamoto

Physical Review B 94 ( 4 ) 2016.07

Publishing type：Research paper (scientific journal)

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.

DOI： 10.1103/PhysRevB.94.041113

Terahertz Radiation by Subpicosecond Magnetization Modulation in the Ferrimagnet LiFe5O8 Reviewed

Yuto Kinoshita, Noriaki Kida, Masato Sotome, Tatsuya Miyamoto, Yusuke Iguchi, Yoshinori Onose, Hiroshi Okamoto

ACS PHOTONICS 3 ( 7 ) 1170 - 1175 2016.07

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

DOI： 10.1021/acsphotonics.6b00272

Novel electronic ferroelectricity in an organic charge-order insulator investigated with terahertz-pump optical-probe spectroscopy Reviewed

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 2016.02

Publishing type：Research paper (scientific journal)

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

Electrical Transport in the Quasi-Two-Dimensional Ionic Mott Insulator M2P-TCNQF(4) under High Pressures Reviewed

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 2015.10

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

DOI： 10.7566/JPSJ.84.104702

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> Reviewed

B. S. Li, Y. Matsui, T. Miyamoto, H. Yada, A. Sawa, H. Okamoto

Journal of Crystal Growth 425 230 - 233 2015.07

Publishing type：Research paper (scientific journal)

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.

DOI： 10.1016/j.jcrysgro.2015.04.005

Temperature and carrier-density dependence of electron-hole scattering in silicon investigated by optical-pump terahertz-probe spectroscopy Reviewed

T. Terashige, H. Yada, Y. Matsui, T. Miyamoto, N. Kida, H. Okamoto

Physical Review B 91 ( 24 ) 2015.06

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

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.91.241201/fulltext

Photoinduced complete melting of spin-Peierls phase in Na-tetracyanoquinodimethane revealed by frequency doubling of coherent molecular oscillations Reviewed

H. Uemura, K. Iwasawa, H. Yamakawa, T. Miyamoto, H. Yada, H. Okamoto

Physical Review B 91 ( 12 ) 2015.03

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

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.91.125111/fulltext

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 2015.03

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

DOI： 10.1016/j.physb.2014.11.045

Excitation-Photon-Energy Selectivity of Photoconversions in Halogen-Bridged Pd-Chain Compounds: Mott Insulator to Metal or Charge-Density-Wave State Reviewed

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 ) 2014.08

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

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.113.096403/fulltext

Ultrafast modulation of polarization amplitude by terahertz fields in electronic-type organic ferroelectrics Reviewed

Tatsuya Miyamoto, Hiroyuki Yada, Hiromichi Yamakawa, Hiroshi Okamoto

Nature Communications 4 ( 1 ) 2013.12

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Other Link： http://www.nature.com/articles/ncomms3586

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 Reviewed

Tatsuya Miyamoto, Kazuki Kimura, Takanori Hamamoto, Hirotaka Uemura, Hiroyuki Yada, Hiroyuki Matsuzaki, Sachio Horiuchi, Hiroshi Okamoto

Physical Review Letters 111 ( 18 ) 2013.10

Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：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

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.111.187801/fulltext

Terahertz-field-driven sub-picosecond optical switching enabled by large third-order optical nonlinearity in a one-dimensional Mott insulator Reviewed

Hiroyuki Yada, Tatsuya Miyamoto, Hiroshi Okamoto

Applied Physics Letters 102 ( 9 ) 091104 - 091104 2013.03

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

Relaxation Dynamics of Photoexcited Excitons in Rubrene Single Crystals Using Femtosecond Absorption Spectroscopy Reviewed

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 ) 2012.08

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

Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.109.097403/fulltext

Role of Coulomb Interactions and Spin-Peierls Dimerizations in Neutral-to-Ionic Phase Transition Investigated by Femtosecond Reflection Spectroscopy Reviewed

Tatsuya Miyamoto, Hirotaka Uemura, Hiroshi Okamoto

Journal of the Physical Society of Japan 81 ( 7 ) 073703 - 073703 2012.07

Authorship：Lead author Language：English Publisher：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.

DOI： 10.1143/jpsj.81.073703