Keiichi Yokogawa

Osaka City University, Ōsaka, Ōsaka, Japan

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Publications (24)61.82 Total impact

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    ABSTRACT: We studied the electronic transport properties of the charge transfer salt β-(BDA-TTP)2I3 [BDA-TTP: 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene] by applying uniaxial strains along the three crystallographic axes, and obtained three corresponding temperature-pressure phase diagrams. Three phase diagrams were quite dependent on the direction of compression. Following the preceding paper by Kikuchi et al., we speculate that the insulating states are of 1/2-filled Mott insulators for the a- and b-axes compressions, and of 1/4-filled charge ordered states for the c-axis compression as well as hydrostatic pressure. The superconducting phase under uniaxial strain was realized with Tc = 5 K at 1.9 GPa along the a-axis and with Tc = 5.6 K at 1.75 GPa along the b-axis. Superconductivity was also reproduced with a Tc of 9.5 K at 1.0 GPa for the c-axis compressions in the range of 0.85 to 1.53 GPa as previously reported. We studied tentative measurement on upper critical fields, Bc2's of these superconductivities and found that the extrapolated values, Bc2(0)'s, exceeded Pauli-limit by about 2--3 times. However, at least in terms of Bc2, the difference in superconductivity associated with two different insulating states was not clear.
    Journal of the Physical Society of Japan 12/2012; 81(12):4703-. · 1.48 Impact Factor
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    ABSTRACT: Field-induced successive transitions were observed in magnetoresistance in the quasi-one-dimensional organic conductor, HMTSF-TCNQ, hexamethylene-tetraselena-fulvalene-tetracyanoquinodimethane. The magnetoresistance exhibits kink transitions accompanying hysteresis at pressures around 1 GPa, but neither at p=0 nor at 2 GPa. At p=0 and below 30 K, this material undergoes charge density wave (CDW) state, which is suppressed around 1 GPa, where successive transitions are observed. Since these behavior is only observed at the boundary in pressure between insulator and metal, and the nature of the insulating state is CDW at p=0, it is strongly suggested that the successive transitions observed in magnetoresistance might be associated with CDW, accordingly a field-induced CDW by the one-dimensionalization by strong magnetic field ranging from 10 to 30 T. The behaviors are compared with previously claimed FICDW and the established FISDW (field-induced-spin-density-wave).
    Physica B Condensed Matter 06/2012; 407(11):1927–1929. · 1.28 Impact Factor
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    ABSTRACT: HMTSF-TCNQ is a quasi-one-dimensional organic conductor which undergoes CDW(charge density wave) transition at 30 K at ambient pressure, where HMTSF-TCNQ is hexamethylenetetraselena fulvalene-tetracyano quino dimethane. This CDW is suppressed by the pressure of 1 GPa. At this pressure, we found field-induced successive hysteretic transitions in magnetoresistance. This reminds us of the successive field-induced SDW (spin density wave) phases in TMTSF2X salts. However, the field range of interest is 2–3 times higher than that of TMTSF2X salts. Therefore, we need really high field to examine these properties. It is very likely that the field induced phases are of field induced CDW (FICDW), where quantum Hall effect and many interesting phenomena are expected like in the case of FISDW. Together with the magnetoresistance study up to the field of 31 Tesla and at temperatures down to 0.4 K in various magnetic field angles respective to the crystal axes, we examined the angular dependence of magnetoresistance oscillations(AMRO). It turned out that AMRO demonstrates clearly the occurrence of field-induced phase rather than the magneto-resistance by field sweep. Since the Hall resistance, R xy in the field-induced phases showed stepwise plateau structure against the field sweep, and its strength was in the order of magnitude of h/e 2 per molecular sheet, the Hall effect is very suggestive of quantum Hall effect.
    Journal of Low Temperature Physics 03/2012; 170(5-6). · 1.04 Impact Factor
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    ABSTRACT: Dependence of the superconducting transition temperature (T(c)) and critial superconducting pressure (P(c)) of the pressure-induced superconductor β-(BDA-TTP)(2)I(3) [BDA-TTP = 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene] on the orientation of uniaxial strain has been investigated. On the basis of the overlap between the upper and lower bands in the energy dispersion curve, the pressure orientation is thought to change the half-filled band to the quarter-filled one. The observed variations in T(c) and P(c) are explained by considering the degree of application of the pressure and the degree of contribution of the effective electronic correlation at uniaxial strains with different orientations parallel to the conducting donor layer.
    Journal of the American Chemical Society 11/2011; 133(49):19590-3. · 11.44 Impact Factor
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    ABSTRACT: The magnetoresistance of quasi-1D two-chain organic conductor HMTSF-TCNQ (hexamethylenetetraselenafulvalene-tetracyanoquinodimethane), which shows charge density wave transition at ambient pressure, was studied under pressure up to 27 T and 31 T, in two high field facilities. We found a kink structure in the magnetoresistance reminiscent of field-induced spin density wave at an intermediate pressure of 1.5 GPa between 0 and 2 GPa. We speculate that these are successive quantum phases induced by magnetic field.
    Journal of the Physical Society of Japan 10/2010; 79(10):3702-. · 1.48 Impact Factor
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    ABSTRACT: We studied superconductivity and spin density wave by measuring temperature dependence of resistivity of (TMTTF)2Br under uniaxial strain up to 3.7GPa (room temperature). The superconducting phase of (TMTTF)2Br under uniaxial strain was observed below 0.8K at 3.7GPa for the first time, which was realized by the compression along the a-axis but not along other directions. Superconductivity was found below a slight SDW-hump in resistance with the a-axis strain; however, with the b′- and c*-axis strain both superconductivity and SDW was suppressed. The anomaly in dlnρc*/d(1/T) was observed at T*∼0.22TSDW under uniaxial strain along the a-axis. This behavior suggests the existence of the sub-phase structure in incommensurate SDW phase under the a-axis compression. This behavior reminds us of the same tendency in (TMTSF)2PF6 in the previous reports of Kondo et al. where only the a-axis uniaxial strain favors superconductivity, even though the SDW transition is not suppressed sufficiently in both samples.
    Physica B Condensed Matter 06/2010; 405(11). · 1.28 Impact Factor
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    ABSTRACT: With the interest of ground state near CDW at low temperature, the electronic properties under high pressure, at low temperature and in high magnetic field of HMTSF-TCNQ are examined. At 1.5–1.6GPa, but not at P=0, a novel hysteretic behavior irrespective of magnetic field orientation and probable quantum oscillations in a specific orientation of magnetic field in magnetoresistance are found.
    Physica B Condensed Matter 06/2010; 405(11). · 1.28 Impact Factor
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    ABSTRACT: With the interest of ground state near CDW at low temperature, the electronic properties under high pressure, at low temperature and in high magnetic field of HMTSF-TCNQ are examined. Up to 8 GPa, the overall resistivity-temperature behaviour are unchanged, i.e. broad resistance minimum around 100–150 K and subtle resistance decrease below around 30 K. The pressure insensitive nature is not consistent with previous data. At 1.5–1.6 GPa, but not at P = 0, a novel hysteretic behaviour and probable quantum oscillations in magnetoresistance are found in a clear form.
    Journal of Physics Conference Series 04/2010; 215(1):012064.
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    ABSTRACT: A pressure transmitting medium named Daphne 7474, which solidifies at P(s)=3.7 GPa at room temperature, is presented. The value of P(s) increases almost linearly with temperature up to 6.7 GPa at 100 degrees C. The high pressure realized by a medium at the liquid state allows a higher limit of pressurization, which assures an ideal hydrostatic pressure. We show a volume change against pressure, pressure reduction from room to liquid helium temperature in a clamped piston cylinder cell, pressure distribution and its standard deviation in a diamond anvil cell, and infrared properties, which might be useful for experimental applications.
    The Review of scientific instruments 09/2008; 79(8):085101. · 1.58 Impact Factor
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    ABSTRACT: Electronic properties of the pi-d system beta-(EDT-DSDTFVSDS)2FeBr4 are investigated, where EDT-DSDTFVSDS = ethylenedithiodiselenadithiafulvalenothioquinone-1,3-diselenolemethide. Clear contrast between the FeBr4 salt and the nonmagnetic anion analogue salt, beta-(EDT-DSDTFVSDS)2GaBr4, was observed in the electrical resistance and magnetoresistance (MR). The FeBr4 salt exhibits a giant negative MR of 85% at 0.7 K and 14 T. Furthermore, MR of the FeBr4 salt has a deep and sharp dip anomaly associated with the spin flop of the d spin at around 1.8 T below Néel temperature TNd = 3.3 K. A model calculation, based on the assumption that antiferromagnetic arrangement of the d spins enhances an insulating state, qualitatively reproduced the experimental result. These results demonstrate the strong pi-d interaction in the FeBr4 salt. Further, it is remarkable that in the higher angle of magnetic field from the easy axis, two anomalies associated with spin flop were observed. This behavior is neither explained by the conventional three-dimensional Ising nor XY models. The magnetic susceptibility of the FeBr4 salt obeys the Curie-Weiss law without a peak, which is characteristic of the usual low-dimensional antiferromagnet at higher temperature than Néel temperature.
    Journal of the Physical Society of Japan 01/2008; 77. · 1.48 Impact Factor
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    ABSTRACT: We have measured the electrical resistivities and magnetoresistances (MR) of (EDT-DSDTFVO)2X (X=FeCl4, GaCl4), where EDT-DSDTFVO stands for ethylenedithiodiselenadithiafulvalenothioquinone-1,3-dithiolemethide. These materials undergo gradual metal-insulator transitions at Tmin=52 K for FeCl4-salt and Tmin=30 K for GaCl4-salt, respectively. In spite of the similarity of the temperature dependence of the resistivity and its pressure effect, MR of both salts exhibit a clear contrast, i.e. FeCl4-salt shows negative and GaCl4, positive. Origin of the difference in the sign of MR between these salts are discussed in terms of the existence of π-d interaction.
    Journal of Low Temperature Physics 06/2007; 142:473-476. · 1.04 Impact Factor
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    ABSTRACT: The solidification pressure of Daphne 7373, which is widely used as a pressure medium in high pressure studies, was examined at room temperature. Using a new generation clamp-type pressure cell, we found that Daphne 7373 solidifies at 2.2 GPa at room temperature. This is exactly on the natural extrapolation of the melting curve obtained at lower pressures and temperatures in our previous report. The solidification pressure of Daphne 7373 is twice as high as that of another well-known medium Fluorinert 77/70 (0.9 GPa). This allows us to hold hydrostatic pressure even in the newly developed BeCu-NiCrAl clamp-type pressure cell, which exceeds the limit of 1.5 GPa generated by a conventional BeCu cell.
    Japanese Journal of Applied Physics 06/2007; 46:3636-3639. · 1.06 Impact Factor
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    ABSTRACT: Crystals of the 2 : 1 salts of a new donor molecule, benzotetrathiafulvalenoquinone-1,3-dithiolemethide (4, Benzo-TTFVO) with magnetic FeX4− and non-magnetic GaX4− (X = Cl, Br) ions, 42·FeX4 and 42·GaX4, are isostructural to each other and showed a β-type packing of the donor molecules where they form a uniform-stacked structure with an interplanar distance of 3.50 Å. These salts exhibited metallic behavior down to 140–170 K, but at these temperatures (TM−I) an abrupt increase in the resistivities (ρ) occurred and thereafter semiconducting behavior with an activation energy of 40–100 meV was observed. A structural change in the donor column from uniform to tetramer-unit stacks was observed in the 42·FeBr4 crystal before and after TM−I. By application of pressures up to 1.0 GPa, the metallic behavior in the higher temperature region was gradually strengthened and TM−I gradually became lower with increasing pressure, but the transitions could not be suppressed at all. In response to the metal–semiconductor transition at TM−I, there was a sharp decrease in the paramagnetic susceptibility of the π electron system, where the transition from Pauli paramagnetism due to the metal-conducting behavior to the spin singlet state caused by tetramer formation of the donor molecules was observed. In addition, the FeX4− (X = Cl, Br) salts showed comparatively strong antiferromagnetic interactions between the Fe(III) d spins of the FeCl4− and FeBr4− ions (Weiss temperature: −11 K for 42·FeCl4 and −37 K for 42·FeBr4), giving rise to antiferromagnetic orderings at 1.6 K for 42·FeCl4 and 9.3 K for 42·FeBr4. The magnitudes of the d–d and π–d interactions in 42·FeBr4 are calculated to be Jdd = 2.06 K and Jπd = 2.32 K, respectively. The comparison of these J values with the other magnetic conductors based on our system suggests that the d–d interaction of 42·FeBr4 is stronger than the π–d interaction. Since the three-dimensional antiferromagnetic ordering appears at the comparatively high temperature of 9.3 K, there is an important contribution of the π electrons to the antiferromagnetic ordering of the Fe(III) d spins in order to mediate the magnetic interaction between two-dimensional magnetic anion layers.
    Journal of Materials Chemistry 04/2007; 17(17):1664-1673. · 6.63 Impact Factor
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    ABSTRACT: In order to investigate its electronic structure and the π–d interaction, we studied magnetoresistance (MR) of β″-(EDO-TTFVO)2FeCl4. In the course of experiment Shubnikov–de Haas (SdH) effect was found, which shows that this material is a 2D metal. Below 3.5K (close to TN), MR shows a dip anomaly near 8T. The anomaly becomes more prominent and shifts slightly to higher field with lowering temperature. The dip anomaly may be caused by the transition from the antiferromagnetic to the paramagnetic states. In addition, SdH oscillations are clearly seen at 0.5K, whose cross-sectional area corresponds to 13% of the first Brillouin zone. The mass of the π-electrons is m*≃3.3me, which seems heavy as compared with non-magnetic organic conductors with similar cross-sectional areas, suggesting the enhancement of the mass due to magnetic interaction with the Fe 3d spins.
    Journal of Magnetism and Magnetic Materials 03/2007; 310(2):1093-1095. · 2.00 Impact Factor
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    ABSTRACT: The 2:1 salts of a new donor molecule, EDT-DSDTFVO with MX4- (M = Fe, Ga; X = Cl, Br) ions, were prepared. The crystal structures of the donor molecules had a beta-type packing motif. All the salts essentially exhibited metallic behaviors despite the small upturns in the resistances below 30-70 K. A large negative magnetoresistance (MR) effect [-14.7% (rho(perpendicular)) at 4.0 K and 5 T] was observed in the FeCl4- salt, while a positive MR effect [+4.0% (rho(perpendicular)) at 4.0 K and 5 T] was observed in the GaCl4- salt, suggesting that there is a pi-d interaction in the FeCl4- salt. The pressure application suppressed the resistivity upturns, increased the negative MR effect (-17.7% at 9.5 kbar) in the FeCl4- salt, and decreased the positive MR effect (+3.3% at 15 kbar) in the GaCl4- salt.
    Inorganic Chemistry 08/2006; 45(15):5712-4. · 4.79 Impact Factor
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    ABSTRACT: To explore novel physical phenomena related to strong π-d interaction, we measured the resistivity (ρ) and magnetoresistance of the first organic ferrimagnetic π-d system, (EDT-TTFVO)2FeBr4 under high pressures up to 8 GPa, where EDT-TTFVO denotes ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide. At ambient pressure, ρ(T) exhibits resistivity minimum near Tmin ∼ 170 K followed by a gradual increase below it. With increasing pressure, Tmin abruptly decreases till 4 GPa, beyond which it slightly increases. The increase ofT min above 4 GPa is discussed in terms of the enhancement of the π-d interaction by applying pressure.
    Journal of Low Temperature Physics 06/2006; 142:613-616. · 1.04 Impact Factor
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    ABSTRACT: We have measured the electrical resistivities and magnetoresistances (MR) of (EDT-DSDTFVO)2X (X=FeCl4, GaCl4), where EDT-DSDTFVO stands for ethylenedithiodiselenadithiafulvalenothioquinone-1,3-dithiolemethide. These materials undergo gradual metal-insulator transitions at Tmin=52 K for FeCl4-salt and Tmin=30 K for GaCl4-salt, respectively. In spite of the similarity of the temperature dependence of the resistivity and its pressure effect, MR of both salts exhibit a clear contrast, i.e. FeCl4-salt shows negative and GaCl4, positive. Origin of the difference in the sign of MR between these salts are discussed in terms of the existence of π-d interaction.
    Journal of Low Temperature Physics 02/2006; 142(3):469-472. · 1.04 Impact Factor
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    ABSTRACT: The magnetoresistance (MR) for a new organic conductor, β″-(EDO-TTFVO)2FeCl4 which exhibits metallic temperature dependence of the resistivity down to 0.3 K, was studied. Remarkable and unusual features were found: isotropic MR below 3 T, dip anomaly below 3 K, and SdH effect above 17 T. These features are discussed in terms of an interaction between the π-electrons on the EDO-TTFVO donors and the d-spins on the magnetic FeCl 4 − ions.
    Journal of Low Temperature Physics 02/2006; 142(3):485-489. · 1.04 Impact Factor
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    ABSTRACT: A new TTP donor, Me-DH-TTP (2-methyl-5-(1,3-dithiolan-2-yliden)-1,3,4,6-tetrathiapentalene), was designed to realize a system with large on-site Coulomb repulsion as compared with the previously known bis-fused type TTP donors. Probably as a consequence, (Me-DH-TTP)2AsF6 exhibits semiconducting behavior from room temperature to liquid helium temperature. By increasing pressure, metallic behavior appears below 300 K, and with distinct metal-insulator (M-I) transition up to 2.2 GPa. This M-I transition suddenly disappears beyond 2.5 GPa, and metallic state is stabilized down to 2.6 K. We discuss the possibility of quantum critical point around 2.4 GPa.
    Journal of Low Temperature Physics 01/2006; 142(3):323-326. · 1.04 Impact Factor
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    ABSTRACT: A new TTP donor, Me-DH-TTP (2-methyl-5-(1,3-dithiolan-2-yliden)-1,3,4,6-tetrathiapentalene), was designed to realize a system with large on-site Coulomb repulsion as compared with the previously known bis-fused type TTP donors. Probably as a consequence, (Me-DH-TTP)2AsF6 exhibits semiconducting behavior from room temperature to liquid helium temperature. By increasing pressure, metallic behavior appears below 300 K, and with distinct metal-insulator (M-I) transition up to 2.2 GPa. This M-I transition suddenly disappears beyond 2.5 GPa, and metallic state is stabilized down to 2.6 K. We discuss the possibility of quantum critical point around 2.4 GPa.
    Journal of the Physical Society of Japan 01/2006; 142(3):327-330. · 1.48 Impact Factor