"Pudding Mold"-type Band as an Origin of Large Thermopower in tau-type Organic Conductors

We study the origin of the large thermopower in quasi-two-dimensional a $\tau$-type organic conductor, $\tau-(EDO-S,S-DMEDT-TTF)_2(AuBr_2)_{1+y}$ ($y \le 0.875$), from the view point of a "pudding mold"-type band structure. We calculate the electronic band structure using an \textit{ab initio} band calculation package, and obtain a tight binding model fit to the \textit{ab initio} band structure. Using the model and the Boltzmann's equation approach, we calculate the temperature dependence of the Seebeck coefficient. We conclude that the peculiar band structure is the origin of the large Seebeck coefficient and the appearance of the maximum value at a certain temperature.Comment: proceedings of ISCOM 2009 (to be published in Physica B

Extremely Large Magnetoresistance in the Nonmagnetic Metal PdCoO2

Extremely large magnetoresistance is realized in the nonmagnetic layered metal PdCoO2. In spite of a highly conducting metallic behavior with a simple quasi-two-dimensional hexagonal Fermi surface, the interlayer resistance reaches up to 35000% for the field along the [1-10] direction. Furthermore, the temperature dependence of the resistance becomes nonmetallic for this field direction, while it remains metallic for fields along the [110] direction. Such severe and anisotropic destruction of the interlayer coherence by a magnetic field on a simple Fermi surface is ascribable to orbital motion of carriers on the Fermi surface driven by the Lorentz force, but seems to have been largely overlooked until now.Comment: Phys. Rev. Lett. 111, 056601 (2013

The electrical properties of DMET and DIMET radical salts

東京都立大学199

Multidimensional Nature of Molecular Organic Conductors Revealed by Angular Magnetoresistance Oscillations

Angle dependent magnetoresistance experiments on organic conductors exhibit a wide range of angular oscillations associated with the dimensionality and symmetry of the crystal structure and electron energy dispersion. In particular, characteristics associated with 1, 2, and 3 dimensional electronic motion are separately revealed when a sample is rotated through different crystal planes in a magnetic field. Originally discovered in the TMTSF based conductors, these effects are particularly pronounced in the related system (DMET)2I3. Here, experimental and computational results for magnetoresistance oscillations in this material, over a wide range of magnetic field orientations, are presented in such a manner as to uniquely highlight this multidimensional behavior.The calculations employ the Boltzmann transport equation that incorporates the systems triclinic crystal structure, which allows for accurate estimates of the transfer integrals along the crystallographic axes, verifying the 1d, 2d and 3d nature of (DMET)2I3, as well as crossovers between dimensions in the electronic behavior

Disorder-Enhanced Dimensionless Thermoelectric Figure of Merit zT of Non-stoichiometric Organic Conductor (TTT)2I3+δ (δ ≤ 0.1)

Sample dependence of dimensionless thermoelectric figure of merit (zT) and power factor (PF) were determined for the non-stoichiometric organic conductor (TTT)2I3+δ (TTT = tetrathiatetracene, δ ≤ 0.1) with the simultaneous measurement of the electrical resistivity (ρ), thermopower (S) and thermal conductivity on small single crystals. Both the zT and PF show large sample dependence between 10 and 310 K, even though all the samples have nearly stoichiometric composition of TTT : I3- ~ 2 : 1 (δ ∼ 0). It was found that both the electrical conductivity (σ = 1/ρ) and S increase at room temperature as disorder — that is phase mismatch among the iodine chains — becomes more pronounced. This behavior contrasts the usual tendency that the S decreases as the σ increases in conventional conductors; and suggests a new strategy to improve the zT and PF by introducing an appropriate type of disorder

Superconductivity and physical properties of Ba24Si100 determined from electric transport, specific-heat capacity, and magnetic susceptibility measurements

Both Ba24Si100 and Ba24Ge100 with crystallographically identical structure are found to be superconducting at 1.4 and 0.27 K, respectively. Physical properties of this superconductor Ba24Si100 are studied by electric transport, specific heat capacity, and magnetic susceptibility measurements. The density of states at the Fermi level NEF=0.148 states eV-1(Siatom)-1 and a distinct jump of Cp at the superconducting transition temperature ΔCp=0.272JK-1mol-1 are obtained. An exponential fit of Cp below the superconducting states gives an energy gap 2Δ=0.423meV and shows that this is a superconductor having s-wave character or isotropic energy gap. On the basis of our experimental data other important physical parameters are also derived

DMET型ドナーからなる低次元有機伝導体における輸送現象

DMET, dimethyl(ethylenedithio)diselenadithiafulvalene is the first unsymmetrical donor that gives an organic superconductor. Among the conducting DMET salts, (DMET)_2Au(CN)_2, (DMET)_2AuI_2, (DMET)_2AuC1_2, (DMET)_2I_3, (DMET)_2IBr_2, (DMET)_2AuBr_2 and (DMET)_2SCN are quasi-one-dimensional metals at ambient pressure. Although the crystal structures of the salts are isostructural, the salts have different low temperature phases. Some of them undergo metal to superconductor or semiconductor transition at low temperature, while the others remain in the normal metallic state. Thermopower was measured for the salts along the highest conducting direction. Transfer integral, t(4t=bandwidth) of the salts was determined from the temperature dependence of the thermopower. Dimensionality of the electronic system in the salts was estimated from the transfer integral and their crystallographic data. It was found that the difference in the low temperature phases can be explained by the relatively high dimensionality of the salts that undergo the superconducting transition. To estimate the dimensionality by the fully experimental method, the anisotropy of magnetoresistance was measured for (DMET)_2Aut_2, (DMET)_2I_3 and (DMET)_2AuBr_2 along the least conducting direction for the magnetic field perpendicular to the electrical current. The measurement was alto carried out for the (DIMET)_2I_3 for the comparison with the DMET salts, where DIMET is an abbreviation of dimethyl(ethylenedithio)tetrathiafulvalene and (DIMET)_2I_3 is isostructural to the DMET salts. The phase transition induced by the magnetic field was suggested for (DMET)_2I_3 and (DMET)_2AuBr_2 by the change in the angular dependence of the magnetoresistance at low temperature. It was found that the estimation of the dimensionality from the magnetoresistance is difficult due to the possible phase transition, at least, among the DMET salts. (DMET)_2BF_4, (DMET)_2ClO_4 and (DIMET)_2BF_4 have the novel crystal structure, where two types of quasi-one-dimensional donor columns are almost perpendicular to each other. To characterize their physical properties, resistivity by Montgomery method and thermopower along the each column were measured for the salts. Plural phase transition were found in the semiconducting sate below the metal-semiconductor transition temperature for all the salts. To know about the nature of the phase transitions, the angular dependence of the magnetoresistance along the direction perpendicular both to the two kinds of columns was measured for (DMET)_2ClO_4. The mechanism of the phase transitions was discussed. The transport properties of organic conductors based on DMET analogs, DIMET and ETDM, ethylenedithio(dimethy1)diselenadithiafulvalene were investigated by the measurement of the resistivity and thermopower at ambient pressure and under hydrostatic pressure. For the sake of a guidance to get a new organic superconductors the comparison among the physical properties of the salts based on DMET and its analogs was carried out.東京都立大学, 1995-03-25, 博士(理学), 甲第357