Reizo Kato

RIKEN, Вако, Saitama, Japan

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Publications (571)1424.52 Total impact

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    Full-text · Dataset · Jan 2016
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    ABSTRACT: Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.
    No preview · Article · Dec 2015 · Science
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    Full-text · Dataset · Aug 2015
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    ABSTRACT: Thermodynamic properties of EtMe3Sb[Pd(dmit)2]2 where dmit is 1,3-dithiole-2-thione-4,5-dithiolate are investigated based on heat capacity measurements. This compound is a two-dimensional (2D) dimer-Mott insulating system with a triangular lattice structure and known to have spin-liquid ground state at low temperatures similar to the case of κ-(BEDT-TTF)2Cu2(CN)3. Comparative discussion of two salts of which EtMe3Sb+ cations are consisting of methyl groups and deuterated methyl groups respectively is given through the analyses of electronic and lattice heat capacities. The gapless features in the spin excitations were observed in both salts, although the electronic heat capacity coefficient, γ of the latter is nearly twice larger than that of former salt. In addition, the deuterated salt shows a gradual increase of the electronic heat capacity divided by temperature, C elT–1 with the decrease of temperature below about 2 K. This temperature dependence is in contrast with the typical feature of the Schottky type heat capacity due to paramagnetic impurities or molecular motions such as the rotation of methyl groups. We also observed that the hump structure of C pT–1 exists in the deuterated salt. The peak temperature and the magnitude of the hump are almost the same with the hydrogenated (pristine) salt. This fact supports that this hump is related to the lattice or charge degrees of freedoms as was discussed in κ-(BEDT-TTF)2Cu2(CN)3.
    No preview · Article · Aug 2015
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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. In these crystals, two H(Cat-EDT-TTF/ST) units share a hydrogen atom with a short O--H--O hydrogen bond. The calculated 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 in contrast with conventional molecular conductors. We discuss effective low-energy models based on H(Cat-EDT-TTF/ST) units and its dimers, respectively, where the microscopic character of the orbitals composing them are analyzed. Furthermore, we find a stable structure which is different from the experimentally determined structure, where the shared hydrogen atom becomes localized to one of the oxygen atoms, in which charge disproportionation between the two types of H(Cat-EDT-TTF) units is associated. The calculated 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.
    Full-text · Article · Jul 2015 · Physical Review B
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    ABSTRACT: Pressure dependence of the conductivity and thermoelectric power is measured through the Mott transition in the layer organic conductor EtMe_{3}P[Pd(dmit)_{2}]_{2}. The critical behavior of the thermoelectric effect provides a clear and objective determination of the Mott-Hubbard transition during the isothermal pressure sweep. Above the critical end point, the metal-insulator crossing, determined by the thermoelectric effect minimum value, is not found to coincide with the maximum of the derivative of the conductivity as a function of pressure. We show that the critical exponents of the Mott-Hubbard transition fall within the Ising universality class regardless of the dimensionality of the system.
    Full-text · Article · Mar 2015 · Physical Review Letters
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    ABSTRACT: Electric double layers (EDLs) of ionic liquids have been used in superconducting field-effect transistors as nanogap capacitors. Because of the freezing of the ionic motion below ~200 kelvin, modulations of the carrier density have been limited to the high-temperature regime. Here we observe carrier-doping-induced superconductivity in an organic Mott insulator with a photoinduced EDL based on a photochromic spiropyran monolayer. Because the spiropyran can isomerize reversibly between nonionic and zwitterionic isomers through photochemical processes, two distinct built-in electric fields can modulate the carrier density even at cryogenic conditions. Copyright © 2015, American Association for the Advancement of Science.
    No preview · Article · Feb 2015 · Science
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    ABSTRACT: A many-body quantum system on the verge of instability between two competing ground states may exhibit quantum-critical phenomena, as has been intensively studied for magnetic systems. The Mott metal-insulator transition, a phenomenon that is central to many investigations of strongly correlated electrons, is also supposed to be quantum critical, although this has so far not been demonstrated experimentally. Here, we report experimental evidence for the quantum-critical nature of the Mott instability, obtained by investigating the electron transport of three organic systems with different ground states under continuously controlled pressure. The resistivity obeys the material-independent quantum-critical scaling relation bifurcating into a Fermi liquid or Mott insulator, irrespective of the ground states. Electrons on the verge of becoming delocalized behave like a strange quantum-critical fluid before becoming a Fermi liquid..
    No preview · Article · Feb 2015 · Nature Physics
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    ABSTRACT: In this article, a modified technique using diamond anvil cell (DAC) for four-probe resistivity measurement is introduced. The full detail of sample mounting technique in a DAC is described. We performed high-pressure transport measurements for a Mott insulator βʹ-ET2ICl2 up to 12 GPa. By comparing the result obtained by the cubic anvil cell measurement, we proved that a choice of small culet size, small samples, and encapsulating the sample provides high quality hydrostatic pressure in DAC. © 2015, Japan Society of High Pressure Science and Technology. All rights reserved.
    Preview · Article · Jan 2015 · The Review of High Pressure Science and Technology
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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.
    Full-text · Article · Dec 2014 · Journal of the Physical Society of Japan
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    ABSTRACT: The back cover picture shows a diamond anvil cell and a mounted sample in the cell, which is the setup used for measuring the electrical conductivity of a small single-component molecular crystal of a metal–dithiolene complex, [Ni(dmit)2], under very high pressure (up to 25.8 GPa). The pressure is comparable to the lower mantle pressure (more than 600 km deep in the earth). This very high pressure changes the insulating crystal to a metal with a three-dimensional Fermi surface. Details are discussed in the Short Communication by H. Cui, R. Kato et al. on p. 3837 ff.
    No preview · Article · Aug 2014 · European Journal of Inorganic Chemistry
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    ABSTRACT: We have measured the electrical resistivity and magnetoresistance of [Ni1-xCux(tmdt)(2)] (x = 0 and x approximate to 0.10) single crystals to investigate the role of the pi-d exchange interaction, J(pi-d), where tmdt stands for trimethylenetetrathiafulvalenedithiolate. At 0 T, the difference in the resistivity between x approximate to 0.10 and x = 0 as a function of temperature, rho(mag)(T), increases logarithmically with decreasing temperature expected for a Kondo alloy. However, it does not show a logarithmic divergence but tends to saturate at a constant value. For H > 0, rho(mag)(T, H) is quite field-dependent, which is very similar to that observed in other Kondo compounds. Assuming that the effect of magnetic field on rho(mag)(T) is attributed to the suppression of the Kondo effect, the Kondo magnetic field H-K similar to 7 T and the Kondo temperature T-K similar to 5 K are found, which is consistent with the previous report on the magnetic susceptibility measurements. The pi-d interaction of J(pi-d) approximate to 30 meV is obtained using the relation k(B)T(K) approximate to E-F expd(-1/vertical bar J(pi-d)vertical bar N(E-F)), where N(E-F) is the density of states at the Fermi energy E-F.
    No preview · Article · Jul 2014 · Journal of the Physical Society of Japan
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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.
    Full-text · Article · Jun 2014 · Advanced Materials
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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.
    Full-text · Article · May 2014 · Crystal Growth & Design
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    ABSTRACT: We have investigated the property of the non-magnetic insulating state of a pressure-induced molecular superconductor with a quasi-triangular lattice consisting of tight dimer units applying vibrational and reflectance spectroscopy to monoclinic-phase of C2H5(CH3)(3)P[Pd(dimt)(2)](2) (dmit = 1,3-dithiole-2-thione-4,5-dithiolate). From the analyses of the spectra, it is founded that two asymmetric dimers form a weakly bounded tetramer and the 2D layer is a slightly anisotropic. Theses results indicate that the cooperation between nearest Coulomb repulsions and valence bond order, which is operative for the anisotropic system of triclinic-phase of C2H5(CH3)(3)P[Pd(dimt)(2)](2), inhibits spin frustration in the monoclinic-phase. Our result also suggests that a half-filled picture does not always hold for the molecular superconductors consisting of tight dimer units.
    No preview · Article · May 2014 · Journal of the Physical Society of Japan
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    ABSTRACT: We performed 13C NMR measurements of a selectively 13C isotope-labeled single-crystal sample of a frustrated spin system, β′A-Et2Me2P[Pd(dmit)2]2. A long-range antiferromagnetic (AF) ordering below 17K was confirmed by the observation of NMR spectrum broadening and well split resonance lines at lower temperatures. NMR spectra in the AF state can be well explained by a two sublattice model. From the analysis of the angular dependence of the NMR spectrum, we clarified the magnetic structure in the AF state, where the easy and hard axes are the crystallographic c∗- and b-axes, respectively, and the effective localized moments are quite small, ∼0.28 μB/dimer. This suggests a strong quantum fluctuation effect due to magnetic frustrations in a quasi-triangular spin-1/2 system.
    No preview · Article · May 2014 · Journal of the Physical Society of Japan
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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.
    No preview · Article · May 2014 · Journal of the American Chemical Society
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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.
    No preview · Article · May 2014 · Berichte der deutschen chemischen Gesellschaft
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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.
    No preview · Article · Apr 2014 · Low Temperature Physics

  • No preview · Conference Paper · Mar 2014

Publication Stats

9k Citations
1,424.52 Total Impact Points

Institutions

  • 1999-2015
    • RIKEN
      • Condensed Molecular Materials Laboratory
      Вако, Saitama, Japan
  • 2006-2011
    • Saitama University
      • Department of Chemistry
      Saitama, Saitama, Japan
  • 1995-2010
    • Gakushuin University
      • Department of Physics
      Edo, Tokyo, Japan
    • Osaka University
      • Department of Electronics and Materials Physics
      Suika, Ōsaka, Japan
  • 2009
    • Japan Research Institute
      • Institute of Physical and Chemical Research (RIKEN)
      Saitama, Saitama, Japan
  • 1984-2008
    • Toho University
      • • Department of Physics
      • • Department of Chemistry
      Edo, Tōkyō, Japan
  • 2000-2003
    • Toyota Physical and Chemical Institute
      Seto, Aichi, Japan
    • Saitama Institute of Technology
      Saitama, Saitama, Japan
  • 1983-2002
    • The University of Tokyo
      • • Institute for Solid State Physics
      • • Department of Chemistry
      Tōkyō, Japan
  • 1994-2001
    • Okayama University
      • Department of Physics
      Okayama, Okayama, Japan
    • Toyama Medical and Pharmaceutical University
      Тояма, Toyama, Japan
  • 1991
    • Institute for Molecular Science
      Okazaki, Aichi, Japan