Reizo Kato

RIKEN, Вако, Saitama, Japan

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Publications (516)859.28 Total impact

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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).
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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; · 10.68 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; · 2.94 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
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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; · 14.83 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; · 2.94 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: 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; · 4.59 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. · 4.69 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).
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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). · 1.80 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
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    ABSTRACT: Photoinduced effects caused by intramolecular excitation were investigated by simultaneous optical and transport measurement in two charge-ordered organic salts, (BEDT-TTF)_{3}X_{2} (X=ReO_{4}, ClO_{4}) [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene]. Although the two salts have the same molecular (average) charge and arrangement, they showed different photoinduced effects. A photoinduced insulator-to-metal phase transition with a metastable charge order-melting state was observed in the ReO_{4} salt where the charge ordered state is associated with the lattice distortion. On the other hand, no photoinduced insulator-to-metal phase transition was noted in the ClO_{4} salt where the charge ordered state is not accompanied by the lattice distortion. This comparative study suggested that the lattice distortion plays a key role in the stabilization of the photoinduced phase.
    Physical Review Letters 05/2013; 110(22):227401. · 7.73 Impact Factor
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    ABSTRACT: We accomplished simultaneous measurement of electrical conductance and optical transmittance changes induced by intramoleculer excitation using nanosecond pulse laser in a thin crystal of the organic conductor α-(BEDT-TTF)2I3α-(BEDT-TTF)2I3 [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] in the charge-ordered (CO) state aiming at observing the photo-induced state quantitatively. We evaluated both conductance and transmittance for the charge-transfer band in the photo-induced state. Experimental results suggest that a metastable CO melting phase grew over the entire area of the crystal. In addition, different conductance and transmittance behaviors were observed in terms of threshold and decay time, suggesting that the photo-induced phase grew inhomogeneously.
    Journal of Luminescence 05/2013; 137:237–240. · 2.14 Impact Factor
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    ABSTRACT: A novel bilayer Mott system, (Et-4BrT)[Ni(dmit)2]2 (Et-4BrT = ethyl-4-bromothiazolium; dmit = 1,3-dithiole-2-thione-4,5-dithiolate), contains two nonequivalent Ni(dmit)2 anion layers, where both layers form Mott insulating states. Supramolecular Brcation···Sanion and Scation···Sanion interactions play a crucial role in constructing the bilayer structure. The ferro- and antiferromagnetic short-range-ordering layers coexist in the crystal, which achieves large negative magnetoresistance (Δρ/ρ0 ≈ -75% at 70 kOe) at 5 K under 1 GPa.
    Inorganic Chemistry 04/2013; · 4.59 Impact Factor
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    ABSTRACT: Halopyridinium cations with multiple halogen and hydrogen bonding donor sites have been used as the counter ions for the Ni(dmit)2 anion (dmit = 1,3-dithiole-2-thione-4,5-dithiolate). Because of the competing supramolecular interactions, such as halogen and hydrogen bondings, some of the Ni(dmit)2 salts showed complicated structures with multiple functionality that originates from the intriguing molecular arrangements. In these multifunctional salts, Ni(dmit)2 molecules play two roles, conducting and magnetic, depending on the molecular arrangement in the layers that they belong to. The physical properties of these salts have been examined by conductivity measurements, with or without hydrostatic/uniaxial pressure, as well as magnetic susceptibility and band calculations. By analysing the low temperature conductivity, it can be concluded that the itinerant electrons in the conducting layer have magnetic coupling with the localized spins in the magnetic layers to result in a Kondo singlet formation.
    CrystEngComm 03/2013; 15(16):3200-3211. · 3.88 Impact Factor
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    ABSTRACT: A quantum spin liquid state (QSL) with a magnetic gapless excitation has been found in the organic Mott insulator EtMe3Sb[Pd(dmit)2]2 with nearly identical 2D triangular lattices of S = 1/2 [1]. To examine the nature of the QSL, it is essential to determine the phase diagram, especially how the gapless QSL evolves when the degree of frustration is changed. Although the gapless QSL is shown to be robust against deuteration of the cation EtMe3Sb [1], the difference of frustration caused by the deuteration is not clear. We study the frustration dependence of the elementary excitation in the mixed-cation materials (Me4Sb)1-x(EtMe3Sb)x[Pd(dmit)2]2 in which the degree of frustration is directly reduced by mixing the smaller cation. Magnetic torque measurements showed that spin susceptibilities of the mixed cation (x = 0.32 and 0.35) were temperature independent down to 30 mK and were almost the same with that of x = 1, indicating that the QSL exists as a quantum critical phase, rather than a point, when the frustration is varied. We will also present magnetic torque and thermal transport measurements of mixed-cation materials with different x.[4pt] [1] D. Watanabe et al., Nat. commun. 3, 1090 (2012).
    03/2013;
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    ABSTRACT: The family of molecular conductors, β'-type X[Pd(dmit)2]2 (X: monovalent cation) salts, show a variety of electronic states: dimer-type Mott insulator, magnetic order, spin-liquid behavior, metallic/superconducting states, and a peculiar charge ordering involving multi-orbitals[1]. In this work, we construct an effective low-energy model which takes into account the multi-orbital degree of freedom. We consider fragments of molecular orbital as a basis set, nearly localized on either one of the dmit ligands. The transfer integrals are obtained for a series of salts by fitting to the first-principles band calculations[2]. We find that all the intra-dimer transfer integrals including the diagonal ones are of the same order; this results in a modification of the orbital scheme in strongly dimerized [Pd(dmit)2]2 discussed in the literatures, then to the effective one-band model. We calculate possible spin and charge ordering based on mean-field approximation to the extended Hubbard model incorporating the fitted parameters. [1] R. Kato, Chem. Rev. 104 (2004) 5319; K. Kanoda and R. Kato, Annu. Rev. Condens. Matter Phys. 2 (2011) 167. [2] T. Miyazaki and T. Ohno, Phys. Rev. B 59 (1999) 5269; T. Tsumuraya, H. Seo, M. Tsuchiizu, R. Kato, and T. Miyazaki, in preparation.
    03/2013;

Publication Stats

2k Citations
859.28 Total Impact Points

Institutions

  • 2000–2014
    • RIKEN
      • • Condensed Molecular Materials Laboratory
      • • Nishina Center for Accelerator-Based Science (RNC)
      Вако, Saitama, Japan
  • 1984–2013
    • The University of Tokyo
      • • Department of Chemistry
      • • Institute for Solid State Physics
      Edo, Tōkyō, Japan
  • 2012
    • Nihon University
      • Department of Chemistry
      Tokyo, Tokyo-to, Japan
  • 2011
    • Osaka University
      • Department of Chemistry
      Ōsaka-shi, Osaka-fu, Japan
  • 2007–2011
    • Saitama University
      • Department of Chemistry
      Saitama, Saitama, Japan
    • Gakushuin University
      • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 1999–2011
    • Tohoku University
      • Department of Chemistry
      Sendai-shi, Miyagi-ken, Japan
  • 2006
    • Institute for Molecular Science
      Okazaki, Aichi, Japan
    • High Energy Accelerator Research Organization
      • Institute of Materials Structure Science
      Tsukuba, Ibaraki, Japan
  • 2003
    • Tokyo Metropolitan University
      • Department of Physics
      Tokyo, Tokyo-to, 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