Takehiko Ishiguro

Transport properties of κ-(BEDT-TTF)2Cu(NCS)2; HC2, Its anisotropy and their pressure dependence

Electronic properties of κ-(BEDT-TTF)2Cu(NCS)2 are surveyed through measurements of the upper critical fields (Hc2), its anisotropy and their pressure dependence. The value of Hc2 parallel to the plane (Hc2//) at 1.8 K is ∼ 20 T, which is almost the same as the Pauli limit value. The anisotropy of Hc2///Hc2⊥ is 19:1 at ambient pressure which reproduced Oshimas result. However, the anisostropy of Hc2///Hc2⊥ increased with increasing pressure. These properties are discussed comparing the Fermi surface study and the pressure dependence on Tc.

Superconductivity of BEDT-TTF salts: (I)Effect, of pressure and alloying and (II) Shubnikov de Haas effect

The superconductivity of s-(BEDT-TTF)2trihalides is discussed based on the experimental results on the pressure effect on resistivity and on Fermi surface. We examine anion-alloyed s-(BEDT-TTF)2trihalides how the temperature dependence of resistivity changes with pressure. We find that the temperature dependence of resistivity is distinctively classified between those that exhibit superconductivity and those that do not. The low temperature resistivity of the superconducting samples decreases with pressure with almost the same rate as that at room temperature, while the low temperature resistivity of the non-superconducting samples does not change with pressure. This is also true with the recent 10 K-superconductor, κ-(BEDT-TTF)2Cu(NCS)2. Second topic of this paper is of the Shubnikov de Haas effect on s-(BEDT-TTF)2I3 and s-(BEDT-TTF)2IBr2. The area of the Fermi surface are found to be 14 of the first Brillouin zone contrary to the expected value of 12. We discuss the structure of the Fermi surface of these materials and other isostructural salts.

Polarized reflectance spectra of organic metals α- and κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTFd8)2Cu(NCS)2

Polarized reflectance spectra of two different phases (α- and κ-) of (BEDT-TTF)2Cu(NCS)2 as well as κ-(BEDT-TTFd8)2Cu(NCS)2 were measured over the spectral range from 800 cm−1 to 9000 cm−1. A clear reflectance drop with the plasma-edge like dispersion was observed for polarization parallel to any directions in the conducting b-c plane in both κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTFd8)2Cu(NCS)2. Small in-plane anisotropy reflects the characteristics stacking configuration of dimerized BEDT-TTF molecules in the crystal structure similar to κ-(BEDT-TTF)2I3, rather than the highly two-dimensional electronic structure within the conducting plane. In contrast, optical properties of α-(BEDT-TTF)2Cu(NCS)2, having a herring-bone type stacking of donors, was found to show higher reflectivity when the polarization was perpendicular to the stacking direction, similar to the case of α-(BEDT-TTF)2I3.

Theoretical studies of polarized xanes of Br doped in trans-polyacetylene

Abstract In this work, we have analysed the Br K-edge XANES of (CHBr y ) x excited by polarized X-ray in //c (parallel to fibril axis) and c directions for three different doping region. In the case of Br doping in the intermediate doping region (y=0.0145), the XANES analyses also confirm the model previously proposed by Oyanagi et al. based on EXAFS analyses, in which polyanions Br 3 − and covalently bonded bromine atoms coexist. From the analysis of E//c XANES, it is concluded that Br 3 − has larger polarization approximately shown by Br −0.6 Br 0.2 Br −0.6 than I 3 − by I −0.5 I 0.0 I −0.5 .