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We study the Seebeck effect in the $��$-type organic conductors, $��$-(EDO-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$ and $��$-(P-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$, where EDO-$S$,$S$-DMEDT-TTF and P-$S$,$S$-DMEDT-TTF are abbreviated as OOSS and NNSS, respectively, both experimentally and theoretically. Theoretically in particular, we perform first-principles band calculation for the two materials and construct a two-orbital model, on the basis of which we calculate the Seebeck coefficient. We show that the calculated temperature dependence of the Seebeck coefficient $S$ is semi-quantitatively consistent with the experimental observation. In both materials, the absolute value of the Seebeck coefficient is maximum at a certain temperature, and this temperature is lower for NNSS than for OOSS. From a band structure viewpoint, we find that this can be traced back to the narrowness of the band gap between the upper and the lower pudding-mold type bands. On the other hand, the Seebeck coefficient of NNSS in the low temperature regime steeply increases with increasing temperature, which is due to the narrowness of the upper band. These differences in thermoelectric properties demonstrate the effectiveness of controlling the band structure through molecular modification., 6 pages, 7 figures. To be published in J. Phys. Soc. Jpn

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.

We report the temperature dependence of the upper critical magnetic field in the quasi-onedimensional molecular organic superconductor (DMET)2I3, for magnetic field applied along the intrachain, interchain, and interplane directions. The upper critical field tends to saturation at low temperature for field in all directions and does not exceed the Pauli paramagnetic limit. Superconductivity in (DMET)2I3 thus appears to be conventional spin singlet, in contrast to the status of the isostructural Bechgaard salts. We also discuss a magnetic field-induced dimensional crossover effect in the normal metallic state which had previously appeared to be associated with superconductivity.

Measurements and calculations of magnetotransport in the molecular organic conductor (DMET)2I3 detect and simulate all known angular magnetoresistance oscillation (AMRO) phenomena for quasi-one dimensional (Q1D) systems. Employing the true triclinic crystal structure in the calculations, these results address the mystery of the putative vanishing of the primary AMRO phenomenon, the Lebed magic angle effect, for orientations in which it was expected to be strongest. They also show a common origin for Lebed and so-called "LN" oscillations, and confirm the generalized nature of AMRO in Q1D systems., Comment: Accepted for the publication in Phys. Rev. Lett

Journal URL: http://www.ipap.jp/jpsj/index.htm

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号

We studied the electronic transport properties of the charge transfer salt β-(BDA-TTP) 2 I 3 [BDA-TTP: 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene] by applying uniaxial strains along ...

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.