Papers - MIYAMOTO Tatsuya
-
Phonon-dressed states in an organic Mott insulator Reviewed
Naoki Sono, Takashi Otaki, Takayuki Kitao, Takashi Yamakawa, Daiki Sakai, Takeshi Morimoto, Tatsuya Miyamoto, Hiroshi Okamoto
Communications Physics 5 ( 1 ) 2022.05
Authorship：Corresponding author Publishing type：Research paper (scientific journal) Publisher：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.DOI： 10.1038/s42005-022-00838-x
Other Link： https://www.nature.com/articles/s42005-022-00838-x
-
Band-filling effects in single-crystalline oligomer models for doped PEDOT: 3,4-ethylenedioxythiophene (EDOT) dimer salt with hydrogen-bonded infinite sulfate anion chains Reviewed
Ryohei Kameyama, Tomoko Fujino, Shun Dekura, Shusaku Imajo, Tatsuya Miyamoto, Hiroshi Okamoto, Hatsumi Mori
Journal of Materials Chemistry C 10 ( 19 ) 7543 - 7551 2022.04
Language：English Publishing type：Research paper (scientific journal) Publisher：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
-
Charge and Lattice Dynamics in Excitonic Insulator Ta2NiSe5 Investigated Using Ultrafast Reflection Spectroscopy Reviewed
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 ) 2022.02
Authorship：Lead author, Corresponding author Publishing type：Research paper (scientific journal) Publisher：Physical Society of Japan
-
Scattering mechanism of hole carriers in organic molecular semiconductors deduced from analyses of terahertz absorption spectra using Drude–Anderson model Reviewed
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 2022.01
Authorship：Corresponding author Publishing type：Research paper (scientific journal) Publisher：AIP Publishing
DOI： 10.1063/5.0073133
-
Ionic to neutral conversion induced by resonant excitation of molecular vibrations coupled to intermolecular charge transfer Reviewed
T. Morimoto, H. Suzuki, T. Otaki, N. Sono, N. Kida, T. Miyamoto, H. Okamoto
Physical Review Research 3 ( 4 ) 2021.12
Authorship：Corresponding author Publishing type：Research paper (scientific journal)
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.
-
Terahertz-field-induced polar charge order in electronic-type dielectrics Reviewed
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 ) 2021.12
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：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.
DOI： 10.1038/s41467-021-20925-x
Other Link： http://www.nature.com/articles/s41467-021-20925-x
-
Ultrafast control of electronic states by a terahertz electric field pulse in the quasi-one-dimensional organic ferroelectric (TMTTF)2PF6 Reviewed
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 ) 2021.11
Authorship：Corresponding author Publishing type：Research paper (scientific journal)
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.
-
Long Carbon-Carbon Bonding beyond 2 angstrom in Tris(9-fluorenylidene)methane Reviewed
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 2021.09
Language：English Publishing type：Research paper (scientific journal) Publisher：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
-
Fate of soliton matter upon symmetry-breaking ferroelectric order Reviewed
K. Sunami, R. Takehara, A. Katougi, K. Miyagawa, S. Horiuchi, R. Kato, T. Miyamoto, H. Okamoto, K. Kanoda
Physical Review B 103 ( 13 ) 2021.04
Language：English Publishing type：Research paper (scientific journal) Publisher：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
Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.103.134112/fulltext
-
Terahertz-Field-Induced Changes of Electronic States Associated with a Polarization Modulation in BiFeO3 Reviewed
Naoki Sono, Yuto Kinoshita, Noriaki Kida, Toshimitsu Ito, Hiroshi Okamoto, Tatsuya Miyamoto
Journal of the Physical Society of Japan 90 ( 3 ) 033703 - 033703 2021.03
Authorship：Last author, Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：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.
-
An unusual Pd(iii) oxidation state in the Pd-Cl chain complex with high thermal stability and electrical conductivity Reviewed
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 2021.02
Language：English Publishing type：Research paper (scientific journal) Publisher：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
-
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 Reviewed
T. Yamaguchi, K. Iwano, T. Miyamoto, N. Takamura, N. Kida, Y. Takahashi, T. Hasegawa, H. Okamoto
Physical Review B 103 ( 4 ) 2021.01
Publishing type：Research paper (scientific journal)
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.
-
Evaluations of nonlocal electron-phonon couplings in tetracene, rubrene, and C-10-DNBDT-NW based on density functional theory Reviewed
Takashi Otaki, Tsubasa Terashige, Junto Tsurumi, Tatsuya Miyamoto, Noriaki Kida, Shun Watanabe, Toshihiro Okamoto, Jun Takeya, Hiroshi Okamoto
PHYSICAL REVIEW B 102 ( 24 ) 2020.12
Language：English Publishing type：Research paper (scientific journal) Publisher：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.
-
Observation of the Three-Dimensional Polarization Vector in Films of Organic Molecular Ferroelectrics Using Terahertz Radiation Emission Reviewed
Yuto Kinoshita, Masato Sotome, Tatsuya Miyamoto, Yohei Uemura, Shunto Arai, Sachio Horiuchi, Tatsuo Hasegawa, Hiroshi Okamoto, Noriaki Kida
Physical Review Applied 14 ( 5 ) 2020.11
Language：English Publishing type：Research paper (scientific journal) Publisher：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
Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevApplied.14.054002/fulltext
-
Conductive zigzag Pd(iii)-Br chain complex realized by a multiple-hydrogen-bond approach Reviewed
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 2020.06
Language：English Publishing type：Research paper (scientific journal) Publisher：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
-
Long-term stabilization of carrier envelope phases of mid-infrared pulses for the precise detection of phase-sensitive responses to electromagnetic waves Reviewed
T. Yamakawa, N. Sono, T. Kitao, T. Morimoto, N. Kida, T. Miyamoto, H. Okamoto
AIP ADVANCES 10 ( 2 ) 2020.02
Language：English Publishing type：Research paper (scientific journal) Publisher：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
-
Strong Terahertz Radiation via Rapid Polarization Reduction in Photoinduced Ionic-To-Neutral Transition in Tetrathiafulvalene-p-Chloranil Reviewed
Yuto Kinoshita, Noriaki Kida, Yusuke Magasaki, Takeshi Morimoto, Tsubasa Terashige, Tatsuya Miyamoto, Hiroshi Okamoto
PHYSICAL REVIEW LETTERS 124 ( 5 ) 2020.02
Language：English Publishing type：Research paper (scientific journal) Publisher：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.
DOI： 10.1103/PhysRevLett.124.057402
Other Link： http://arxiv.org/pdf/1811.06202v1
-
Biexciton in one-dimensional Mott insulators Reviewed
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 ) 2019.12
Authorship：Lead author, Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：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.DOI： 10.1038/s42005-019-0223-8
Other Link： http://www.nature.com/articles/s42005-019-0223-8
-
Topological charge transport by mobile dielectric-ferroelectric domain walls Reviewed
R. Takehara, K. Sunami, K. Miyagawa, T. Miyamoto, H. Okamoto, S. Horiuchi, R. Kato, K. Kanoda
Science Advances 5 ( 11 ) 2019.11
Publishing type：Research paper (scientific journal)
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.
-
Evaluating intrinsic mobility from transient terahertz conductivity spectra of microcrystal samples of organic molecular semiconductors Reviewed
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 2019.09
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：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
