Reizo Kato

RIKEN, Вако, Saitama, Japan

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Publications (518)1213.81 Total impact

  • Nature Physics 02/2015; · 20.60 Impact Factor
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    ABSTRACT: Electronic properties of quasi-two-dimensional molecular conductors $X$[Pd(dmit)$_2$]$_2$ are studied theoretically. We construct an effective model based on the fragment molecular orbital scheme developed recently, which can describe the multi-orbital degree of freedom in this system. The tight-binding parameters for a series of $\beta'$-type compounds with different cations $X$ are evaluated by fitting to first-principles band calculations. We find that the transfer integrals within the dimers of Pd(dmit)$_2$ molecules, along the intramolecular and intermolecular bonds including the diagonal ones, are the same order, leading to hybridization between different molecular orbitals. This results in charge disproportionation within each molecule, as seen in our previous ab initio study [T. Tsumuraya et al, J. Phys. Soc. Jpn. 82, 033709 (2013)], and also to a revised picture of an effective dimer model. Furthermore, we discuss broken-symmetry insulating states triggered by interaction effects, which show characteristic features owing to the multi-orbital nature. The on-site Coulomb interaction induces antiferromagnetic states with intramolecular antiparallel spin pattern, while electron-lattice couplings stabilize non-magnetic charge-lattice ordered states where two kinds of dimers with different charge occupation arrange periodically. These states showing different spatial patterns compete with each other as well as with the paramagnetic metallic state.
    12/2014;
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    ABSTRACT: We theoretically study hydrogen-bonded molecular conductors synthesized recently, $\kappa$-H$_3$(Cat-EDT-TTF)$_2$ and its diselena analog, $\kappa$-H$_3$(Cat-EDT-ST)$_2$, by first-principles density-functional theory calculations. The band structure near the Fermi level shows a quasi-two-dimensional character, with a rather large interlayer dispersion due to the absence of insulating layers as in charge transfer salts. In these crystals, two H(Cat-EDT-TTF/ST) units share a hydrogen atom with a short O--H--O hydrogen bond. We show that an effective half-filled model on an anisotropic triangular lattice can be constructed based on dimers of H(Cat-EDT-TTF/ST) units which are face-to-face to each other but, not connected by the hydrogen bond. Furthermore, we find a a stable structure where the shared H atom becomes localized to one of the O atoms; there, charge disproportionation between the two types of H(Cat-EDT-TTF) units is associated. Nevertheless, the calculated adiabatic potential energy surface for the H atom is very shallow near the minimum points, therefore the probability of the H atom can be delocalized between the two O atoms.
    08/2014;
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    ABSTRACT: We report on a crystalline rotor that undergoes a reversible phase transition at 145 K. Variable-temperature X-ray and 1H spin−lattice relaxation experiments, and calculations of rotational barriers, provide a description (i) of the way in which the rotators' dynamics changes back and forth at the onset of the phase transition and (ii) of the mechanism responsible for the abrupt switching of the crystalline rotors from a very low-energy 4-fold degenerate equilibrium state, in which the rotation is ultrafast (9.6 GHz at 145 K), to a single higher-energy state associated with a slower motion (2.3 GHz at 145 K). Our results provide evidence that the reversible change observed in the rotational barriers at the transition is due to a cooperative modulation of the C−H rotator ···I stator hydrogen bond cloud across a C−I stator ···I stator −C halogen bond-mediated phase transition. In addition, we report evidence for second-harmonic generation from this material, thereby confirming with a second example the benefit of using polarized light to probe the torsional degree of freedom of chiral helix blades, as well as symmetry and dimensionality of large collections of chiral rotors in the solid state.
    Crystal Growth & Design 05/2014; 14(7). · 4.56 Impact Factor
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    ABSTRACT: The pressure dependence of the resistivities of a single-component molecular conductor, [Ni(hfdt)2] (hfdt = bis(trifluoromethyl)tetrathiafulvalenedithiolate) with semiconducting properties at ambient pressure was examined. The four-probe resistivity measurements were performed up to ~10 GPa using a diamond anvil cell (DAC). The low-temperature insulating phase was suppressed above 7.5 GPa and the resistivity dropped, indicating the superconducting transition occurred around 7.5-8.7 GPa with a maximum Tc (onset temperature) of 5.5 K. The high-pressure crystal and electronic band structures were derived by the first-principle calculations at 6-11 GPa. The crystal was found to retain the semiconducting band structure up to 6 GPa. But the electron and hole Fermi surfaces appear at 8 GPa. These results of the calculations agree well with the observation that the pressure-induced superconducting phase of [Ni(hfdt)2] appeared just above the critical pressure where the low-temperature insulating phase was suppressed.
    Journal of the American Chemical Society 05/2014; · 11.44 Impact Factor
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    ABSTRACT: A high-quality field-effect transistor (FET) with an organic Mott insulating channel was fabricated, and its low-temperature transport properties were measured at various gate voltages (VG). The resistance of the FET showed a clear ambipolar field effect as well as a sudden drop in both the p-type and n-type regions, the areas of which merged into one at lower temperatures. These drops in the resistance were attributed to Mott transitions that were induced by electrostatic doping into the FET interface. The n-type transition started to appear at higher temperatures but showed a relatively narrow VG range relative to that of the p-type transition. These results are suggestive of electron–hole asymmetry of the Mott-insulator-to-metal or Mott-insulator-to-superconductor transitions in the doped organic correlated materials. A strain on the device was also evaluated by X-ray diffraction.
    Berichte der deutschen chemischen Gesellschaft 05/2014; · 2.97 Impact Factor
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    ABSTRACT: Motivated by an interest to see if the field-induced (FI) phase in the charge-density wave (CDW) system is similar to the field-induced-SDW (FISDW) in (TMTSF)2X, (TMTSF: tetramethyltetraselenafulvalene), we examined the magnetic-field-induced phases in a quasi-one-dimensional (Q1D) organic conductor HMTSF–TCNQ (hexamethylene- tetraselenafulvalene- tetracyanoquinodimethane) under a pressure of 1.1 GPa, where the CDW occurring at 30 K is suppressed. The work was carried out by measurements of angular-dependent magnetoresistance oscillations and exploratory work on the Hall effect. It turned out that the FI-phase, most likely a FICDW for B > 0.1 T, accompany a quantum Hall effect, and the FI-phase transitions are controlled by the field component along the least conducting axis. Above 10 T, the lowest Landau level of the small 2D Fermi pocket (due to incomplete nesting of Fermi surface) exceeds the Fermi level, reaching the quantum limit. Although there are many differences between the CDW (HMTSF–TCNQ) and SDW ((TMTSF)2X) systems, a similar scenario for field-induced phases seems to hold.
    Low Temperature Physics 04/2014; 40(4):371-376. · 0.88 Impact Factor
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    ABSTRACT: λ-BETS2FeCl4 (BETS = bis (ethylenedithio) tetraselenafulvalene) system exhibits a mysterious paramagnetic metal (PM)–antiferromagnetic insulator (AFI) phase transition. Just after AF ordering, the large entropy of the Fe 3d localized spin still remains. To investigate why the mysterious free 3d spin exists in the π spin AF ordered state, we measured the specific heat in the vicinity of the PM–AFI transition. The formation process of this AF ordering is discussed in the context of a low-dimensional spin network. It is proposed that the FeCl4 crystal field plays a crucial role in this magnetic ordering via a π–d interaction.
    Proceedings of the 12th Asia Pacific Physics Conference (APPC12); 03/2014
  • Proceedings of the 12th Asia Pacific Physics Conference (APPC12); 03/2014
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    ABSTRACT: A novel type of flexible organic field-effect transistor in which strain effects can be finely tuned continuously has been fabricated. In this novel device structure, electronic phases can be controlled both by "band-filling" and by "band-width" continuously. Finally, co-regulation of "band-filling" and "band-width" in the strongly-correlated organic material realize field-induced emergence of superconducting fractions at low temperature.
    Advanced Materials 03/2014; · 15.41 Impact Factor
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    ABSTRACT: Given that a molecular system has a soft lattice, high-pressure applications are effective ways to discover new single-component molecular metals and superconductors. In this study, we measured the high-pressure electrical resistivity of a single-component molecule crystal of [Ni(dmit)2] (dmit = 1,3-dithiole-2-thione-4,5-dithiolate) up to 25.5 GPa by using a newly developed diamond anvil cell technique that generates high-quality hydrostatic pressures. We successfully observed the metallic state over a wide temperature range above 15.9 GPa. Two different band calculation methods, tight-binding calculations coupled with the interatomic repulsion model and ab initio DFT calculations, indicated that 2D and 3D Fermi surfaces appear under high pressures.
    Berichte der deutschen chemischen Gesellschaft 03/2014; · 2.97 Impact Factor
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    ABSTRACT: We investigated a photoexcited state in the molecular conductor (BEDT -TTF)3(ClO4)2 (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) with charge localization due to the electron-electron Coulomb interaction. Photocurrent induced by intramolecular excitation was observed in a charge-ordered insulating state. As a result, nonlinear photocurrent with a threshold of excitation light density was experimentally obtained. The threshold decreased as the temperature increased. This nonlinear photocurrent indicates a transition from an excitonic state to a free excited electronic state. The excitonic state below the threshold is formed by the long-range electron-electron Coulomb interaction. In the free excited electronic state above the threshold, high-density photoexcitation induces Coulomb screening, which results in exciton dissociation and a free electronic state.
    Journal of Physics Condensed Matter 02/2014; 26(5):055603. · 2.22 Impact Factor
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    ABSTRACT: Femtosecond electron diffraction has been used to probe the photoinduced phase transition in the strongly-correlated system, Me4P[Pt(dmit)2]2, revealing molecular motions involved in this process and breaking new ground in terms of chemical complexity.
    International Conference on Ultrafast Phenomena; 01/2014
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    ABSTRACT: Ferrocene (Fc) and tetrathiafulvalene (TTF) moieties were incorporated into novel hybrid molecules of structure FcS4TTF(R)2 (R = CF3 and SMe). [FcS4TTF(R)2](•+) exhibited R-dependent spin-density distribution, and [FcS4TTF(CF3)2](••2+) showed the coexistence of Fc-centered Fe 3d and TTF-centered π spins. The solid-state molecular structures in different oxidation states reflect their characteristic spin states.
    Inorganic Chemistry 11/2013; · 4.79 Impact Factor
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    ABSTRACT: We prepared novel Ni(dmit)2 anion radical salts with ethyl-4-halothiazolium cations (Et-4XT, with X denoting the halogen: I, Br, or Cl), (Et-4IT)[Ni(dmit)2]2 (1), (Et-4BrT)[Ni(dmit)2]2 (2), and (Et-4ClT)2[Ni(dmit)2]5 (3). Single-crystal X-ray diffraction analysis of 1–3 indicates that, unlike the halogen atoms that have only one σ-hole each, the cations’ sulfur atoms each have two σ-holes that lie approximately along the extensions of the C–S bonds. The presence of the σ-holes is supported by electrostatic potential of the cations calculated based on the density functional theory method. In the crystals of 1–3, these σ-holes interact with lone pairs on the terminal thioketone moieties in the Ni(dmit)2 anion radicals to form electrostatic σ-hole bonds (halogen bonds and chalcogen bonds). This results in supramolecular cation···anion networks. Crystal and electronic structure analyses, and electrical and magnetic measurements reveal that the salts 1 and 2 are isostructural bilayer Mott systems, in which two crystallographically different Mott-insulating anion layers coexist in one crystal. The unusual magnetic properties, including the ferromagnetic anomalies of 1 and 2, are consistent with one of the anion layers forming an antiferromagnetic short-range ordering (SRO) state and the other layer forming a ferromagnetic SRO state. The spin-polarization of the Ni(dmit)2 anion radical was shown to influence significantly the observed ferromagnetic interactions, while the antiferromagnetic interactions resulted from π–π overlapping in the anions. The competition between these two interactions dominates the low-temperature magnetic properties of the present bilayer Mott systems. This study reveals that noncovalent intermolecular interactions mediated by σ-holes are influential in preparing novel crystal and electronic structures and that they have the potential to allow the development of materials with unusual physical properties.
    Crystal Growth & Design 09/2013; 13(10):4533–4541. · 4.56 Impact Factor
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    ABSTRACT: We study the temperature and frequency dependence of the dielectric constant and the ac conductivity in the dimer Mott insulator family β′-type Pd(dmit)2 (where dmit denotes 1,3-dithiole–2-thione–4,5-dithiolate). We find that all compounds within this family have relaxorlike dielectric relaxation with a Curie-Weiss increase in the dielectric constant with decreasing temperature. An analysis of the dielectric data shows a clear relation between electronic correlation and the dipole-dipole interaction; also, geometrical frustration appears to weaken this interaction. Furthermore, the dc and ac conductivities are shown to be closely correlated and follow the same universal ac conductivity as disordered solids.
    Physical Review B 08/2013; 88(7). · 3.66 Impact Factor
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    ABSTRACT: We study the temperature and frequency dependence of the dielectric constant and the ac conductivity in the dimer Mott insulator family β′-type Pd(dmit)2 (where dmit denotes 1,3-dithiole–2-thione–4,5-dithiolate). We find that all compounds within this family have relaxorlike dielectric relaxation with a Curie-Weiss increase in the dielectric constant with decreasing temperature. An analysis of the dielectric data shows a clear relation between electronic correlation and the dipole-dipole interaction; also, geometrical frustration appears to weaken this interaction. Furthermore, the dc and ac conductivities are shown to be closely correlated and follow the same universal ac conductivity as disordered solids.
    Physical Review B 08/2013; 88(7). · 3.66 Impact Factor
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    ABSTRACT: In state-of-the-art silicon devices, mobility of the carrier is enhanced by the lattice strain from the back substrate. Such an extra control of device performance is significant in realizing high-performance computing and should be valid for electric-field-induced superconducting (SC) devices, too. However, so far, the carrier density is the sole parameter for field-induced SC interfaces. Here we show an active organic SC field-effect transistor whose lattice is modulated by the strain from the substrate. The soft organic lattice allows tuning of the strain by a choice of the back substrate to make an induced SC state accessible at low temperature with a paraelectric solid gate. An active three-terminal Josephson junction device thus realized is useful both in advanced computing and in elucidating a direct connection between filling-controlled and bandwidth-controlled SC phases in correlated materials.
    Nature Communications 08/2013; 4:2379. · 10.74 Impact Factor
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    ABSTRACT: An organic Mott-insulator κ-(BEDT-TTF)2Cu[N(CN)2] Cl(κ-Cl) on SiO2/Si substrate showed an ambipolar field-effect-transistor (FET) characteristics without any hysteresis, which means a continuous Mott-transition at filling-controlled regime (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene). In order to extract the critical exponent in the vicinity of the Mott-insulating phase, an analysis based on Efros-Shklovskii approximation is performed. The model fitting well reproduces the device characteristics over wide range of temperature and gate voltage. In this analysis, Coulomb gap temperature that characterizes the ground state of doped Mott-insulator shows inversely proportional critical behavior against the doping concentration.
    The European Physical Journal Special Topics 07/2013; 222(5). · 1.76 Impact Factor
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    ABSTRACT: To clarify the mechanism of the later process of photoinduced phase transition (PIPT) in organic charge-transfer complexes, we examined by time-resolved infrared vibrational spectroscopy two dimeric anion radical salts, Et2Me2Sb[Pd(dmit)2]2 (Et2Me2Sb salt) and Cs[Pd(dmit)2]2 (Cs salt) (Et, Me, and dmit are C2H5, CH3, and 1,3-dithiol-2-thione-4,5-dithiolate, respectively), having similar characteristics except for the order of their phase transitions at thermal equilibrium. The phase transition is first order for the Et2Me2Sb salt and second order for the Cs salt at thermal equilibrium. Although both salts exhibit a high-temperature phase at later delay times (>100 ps) after the photoexcitation of the low-temperature phase, the time required for the emergence of the high-temperature phase was significantly different: 70 ps for the Et2Me2Sb salt and <0.1 ps for the Cs salt. The slow emergence of the high-temperature phase in the PIPT of the Et2Me2Sb salt presumably has an origin similar to that recognized for the first-order thermal phase transition, that is, steric effects of the Et2Me2Sb cation when the phase transitions occur.
    The Journal of Physical Chemistry C 06/2013; 117(25):13187–13196. · 4.84 Impact Factor

Publication Stats

5k Citations
1,213.81 Total Impact Points

Institutions

  • 2000–2014
    • RIKEN
      • Condensed Molecular Materials Laboratory
      Вако, Saitama, Japan
    • Saitama Institute of Technology
      Saitama, Saitama, Japan
  • 1984–2013
    • The University of Tokyo
      • • Department of Chemistry
      • • Institute for Solid State Physics
      Edo, Tōkyō, Japan
  • 2011
    • Osaka University
      • Department of Chemistry
      Ōsaka-shi, Osaka-fu, Japan
  • 2007–2011
    • Saitama University
      • Department of Chemistry
      Saitama, Saitama, Japan
  • 1999–2011
    • Tohoku University
      • Department of Chemistry
      Sendai-shi, Miyagi-ken, Japan
  • 2010
    • Tokyo Denki University
      Edo, Tōkyō, Japan
  • 2006
    • High Energy Accelerator Research Organization
      • Institute of Materials Structure Science
      Tsukuba, Ibaraki, Japan
  • 2000–2003
    • Toyota Physical and Chemical Institute
      Seto, Aichi, Japan
  • 1985–1995
    • Toho University
      • • Department of Chemistry
      • • Department of Physics
      Edo, Tōkyō, Japan
  • 1993
    • Tokyo University of Science
      Edo, Tōkyō, Japan