[en] The layered superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl in the magnetic field applied strictly parallel to the superconducting plane shows the upper critical field exhibiting non-saturating behavior toward 0 K. The relation to the spatially modulated order-parameter state predicted by Fulde-Ferrell and Larkin-Ovchinnikov is argued

[en] This paper describes part of the present status of the research in Japan on the crystal structures and physical properties of organic superconductors κ-(BEDT-TTF)₂[Cu(NCS)₂], κ-(BETD-TTF)₂Cu[N(CN₂]Br, α-(BEDT-TTF)₂[NH₄Hg(SCN)₄] and other related materials. The Cu(NCS)₂ salt shows an inverse isotope effect. The Cu[N(CN)₂]Br salt shows Tc of 11.7 K. H_c2 value and lattice parameters of these κ-type salts are compared. (orig.)

[en] The multi-instability of the electronic structure of (EDO-TTF)₂PF₆, where EDO-TTF means ethylene-dioxytetrathiafulvalene, is reviewed. This complex showed the metal-insulator transition at 280 K associated with distinct molecular deformations. The mechanism is interpreted as the cooperation of Peierls transition, charge ordering, and the order-disorder transition of the countercomponent. The charge ordering pattern in the low-temperature phase is of the novel [0, 0, 1, 1] type. The sensitivity of the electronic state to external perturbations is demonstrated applying not only static but also instantaneous stimuli. In the latter case, the photo-induced phase transition is ultrafast and highly efficient. One photon causes the transition of several hundreds of donor molecules in the low-temperature phase to relax into a highly conducting metastable state within about 1.5 ps. In the early stage of the transient state, the charge ordering of the [1, 0, 1, 0] type occurs. As for the chemical modifications of this material, the partial deuteration of this complex increases the metal-insulator transition temperature. The introduction of a methyl group greatly modulates the electronic structure of the complex, i.e. (methyl-EDO-TTF)₂X (X=BF₄, ClO₄) shows a two-dimensional electronic structure. The working hypotheses for developing the systems with multi-instability are described. (topical review)

[en] Transport properties of a strongly correlated organic conductor (CHC⁺)TCNQ^- were examined under high pressures up to about 7 GPa using a diamond anvil cell, where CHC⁺ denotes hemiprotonated cytosine pair and TCNQ represents 7,7,8,8-tetracyanoquinodimethane. This salt is a highly conductive semiconductor as a fully ionic TCNQ salt with the activation energy E_a of 0.14 eV at ambient pressure. Application of high pressure gradually increased the conductivity. The E_a decreased monotonically by a rate of 0.013 eV/GPa.

[en] We studied the superconducting transition temperature T_C of β-(BEDT-TTF)₂I₃ in the high-T_C phase under uniaxial strain. By embedding the crystal into epoxy resin, 8 K superconducting phase is stabilized. By applying uniaxial compression, further T_C increase taking a maximum at a piston pressure of 3-4 kbar is observed for both directions parallel and perpendicular to the donor stack. The behavior is understood as a competition between the electron correlation (U_dimer/W) and spin frustration (t^'/t) in terms of the spin-fluctuation mediated superconductivity on the dimerized triangular lattice Hubbard model.

[en] An organic conductor θ-(BTM-TTP)₂SbF₆, where BTM-TTP denotes 2-[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-5-(1,3-dithiol-2-ylidene) -1,3,4,6-tetrathiapentalene, is semiconducting and shows Curie-Weiss-like magnetism, whereas the X-ray structure shows that all BTM-TTP molecules are crystallographically equivalent and the tight-binding band calculation affords the closed Fermi surface. We have studied the electrical properties and Raman spectra of θ-(BTM-TTP)₂SbF₆ in order to clarify the origin of the carrier localization. From the electrical resistivity measurement, the maximum of activation energy appeared at ca. 240 K. Raman spectroscopy reveals the charge disproportionation in θ-(BTM-TTP)₂SbF₆ from room temperature, and the degree of charge disproportionation is estimated to be +0.0₃∼0.1₃:+0.9₃ at 6 K.

[en] (EDO-TTF)₂PF₆ is a quasi one-dimensional molecular conductor and undergoes a peculiar metal-insulator (MI) transition. The phase transition mechanism is cooperative among Peierls, charge ordering, and anion ordering transitions, which is accompanied with a distinct molecular deformation. (EDO-TTF)₂XF₆ (X = P (1h), As (2h), Sb (3h)) showed that the transition temperature (T_MI) and the width of thermal hysteresis (ΔT_MI) were systematically changed depending on the anion size, i.e., the T_MI decreased in order of 279.0 K (1h), 270.8 K (2h), and 242 K (3h) and the ΔT_MI increased in order of 1.0 K (1h), 5.5 K (2h), and 14 K (3h). The deuterated salts, (EDO-TTF-d₂)₂XF₆ (X = P (1d) and As (2d)), showed higher T_MI of 282.0 (1d) and 274.0 K (2d) and narrower ΔT_MI of 0.0 (1d) and 5.0 K (2d) than the undeuterated 1h and 2h, respectively. X-ray analysis revealed the anisotropic thermal expansion of unit cell. The linear relationship between ΔT_MI and volume change at T_MI was found.

[en] We have investigated the nature of the photo-induced state and coherent phonon in the conducting charge transfer complex (EDO-TTF)₂PF₆ by measuring the ultrafast reflectivity change over a wide photon energy range from 0.069 eV (18 μm) to 2.1 eV (580 nm). The photo-induced spectra just after photo-excitation indicate that the photo-induced phase is similar to but clearly different from that in a thermally induced metal phase though they are the same for general photo-induced phase transition. The temporal profiles at each probe photon energy are accompanied by extraordinarily large amplitude oscillation originating from coherent phonon generation. The period of the coherent phonon depends discretely on the probe photon energy, while the generation efficiency of both the photo-induced phase and coherent photon reaches a maximum when the peak top of a charge transfer band is excited. These results are probably attributable to the strong electron-vibration coupling in this complex

[en] Radical cation salts, (TOT)PF₆, (TOT)C1O₄, and (TOT)ReO₄, were prepared by means of galvanostatic electrocrystallization, where TOT is 2,2':6',2'':6'',6-trioxytriphenylamine, and the crystal structures were solved. Calculated overlap integrals between highest occupied molecular orbitals of TOT molecules indicated very weak intermolecular interactions in these salts. Virtually isolated electronic structure of TOT molecule in the salts was also confirmed by UV-vis-IR spectral study.

[en] We report here the first observation of the photoinduced insulating-to-metal phase transition in the (EDO-TTF)₂SbF₆ salt, which occurs on the picosecond time-scale. The time-resolved optical experiments performed with 80 fs time-resolution demonstrate that the dynamical process involves several low-frequency phonons, as the crystalline structure is destabilized upon laser excitation.