Sachio Horiuchi

Reentrant superconductivity in κ-(BEDT-TTF)2Cu[N(CN)2]Cl and its pressure phase diagram

Abstract Electrical resistivity and magnetization measurements reveal that κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl exhibits reentrant superconductivity (reported here for the first time for an organic solid) in a wide pressure region. The behavior based on the subtle balance between superconductivity and resistive (magnetic) phase is affected dramatically by thermal history. The low temperature state below 300 bar is of inhomogeneous mixture of superconductive and non-metallic phases, while the phase in higher pressure region can be single: rich variety of phase are found in pressure phase diagram.

New transparent, colorless, metallically conductive polymer films and their electrochemical transformations

New surface conductive polymer composite films exhibiting extraordinary properties: high conductivity with metallic temperature dependence and exceptionally high transparency are presented. The conducting networks in these materials are formed of plate-like nanocrystals of bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) salts with iodine or bromine. The films with bromine salts are colorless and transparent while those with iodine are colored. It is demonstrated that colored films with (BEDO-TTF)/iodine networks can be electrochemically transformed into highly transparent, practically colorless materials without deterioration of the conducting network. The resulting metallically conducting polymer materials show the highest transparency among conductive composites reported so far.

Highly-Oxidized States of Organic Donor Bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF)

Abstract The monocation and dication salts of bis(ethylenedioxy)-tetrathiafulvalene (BEDO-TTF) were newly obtained by chemical oxidation by iodine. Their structural and physical properties are discussed.

Metallic polymer composites with bis(ethylenedioxy)-tetrathiafulvalene salts. Preparation–properties relationship

Abstract The relationship between preparation conditions and properties of surface-conductive composites obtained by crystallization of low molecular weight organic conductors in polymer matrices (reticulate doped polymers) is investigated. By optimization of the preparation procedure we have obtained the first organic conductive composites which show metallic surface conductivity without any additional treatment. Surface-conductive polycarbonate (PC) films with bis(ethylenedioxy)-tetrathiafulvalene (BEDO-TTF) salts with iodine and bromine as the additives were obtained by the two-step reticulate doping method. BEDO-TTF content in PC was 1 wt.%. Due to the strong tendency of BEDO-TTF to form organic metals the films obtained under optimum conditions show exceptionally high surface conductivity (even above 10−3 S/□ for both I and Br salts), and exhibit metal-like temperature dependence of conductivity in a broad temperature range (even down to at least 10 K). Depending on the preparation conditions (vapour composition, treatment time) it is also possible to obtain highly conductive films showing semiconductor-like temperature dependence or metal–insulator transition. Structure and morphology of the conductive network is characterized using scanning electron microscopy and X-ray diffraction. These results and optical absorption studies permit identification of the salts which are main components of the networks in most of the systems. When the conductive network is made of BEDO-TTF/Br salts the films show also exceptionally high transparency.

Strong tendency of BEDO-TTF to produce organic metals

Abstract Combining BEDO-TTF with 24 kinds of organic acceptors, 19 kinds of metallic complexes were obtained. The first redox potentials of the the acceptors which give the metallic complexes range over 0.82 V that is anomalously big among the organic metals of conventional 1:1 TTF-TCNQ system. The crystal structure of the (BEDO-TTF) 5 (HCTMM)(Phue5f8CN) 2 salt is regarded as one of the self-aggregated packing motif of the partially charged BEDO-TTF in which the donor interacts to each other through the side-by-side heteroatom network and through the face-to-face stacking. The semiconducting (BEDO-TTF) 5 (HCP)(Phue5f8CN) 0.2 salt preserves the common structural features to the metallic BEDO-TTF salts. The strong tendency of BEDO-TTF to produce organic metals is attributed both to its low ionization energy and to the ability of its self-aggregation.

Langmuir-blodgett films of charge transfer complexes of bisethylenedioxytetrathiafulvalene-alkyltetracyanoquinodimethane

Abstract The LB films of the charge transfer complexes of oxygen-containing BEDT-TTF type donor bisethylenedioxytetrathiafulvalene (BO) and long-chain derivatives of tetracyanoquinodimethane (C n TCNQ) are prepared with and without matrix molecules. The measurements of π-A isotherm for the complex-icosanoic acid mixed system indicated the formation of mixed monolayers at the air-water interface. This was confirmed by the AFM measurements showing a granular structure of the film with the grain size of 100 nm in diameter and 3–4 nm in height. These grains are probably attributable to the BO complex. The conductivity of the LB films were strongly affected by the film-deposition method, the spreading solvent and the matrix. The mixed LB films of BO-C 10 TCNQ and BO-C 14 TCNQ with icosanoic acid under suitable conditions showed conductivities as high as 10 and 3 S/cm, respectively. The former LB film showed a metallic temperature dependence of conductivity down to 250 K. The conduction mechanism is discussed in terms of the film granuality.

ESR study of the LB films containing metallic domains

Abstract ESR and IR absorption measurements have been performed on Langmuir-Blodgett (LB) films based on a charge-transfer complex of bis(ethylenedioxy)tetrathiafulvalene (BO) and decyltetracyanoquinodimethane. Analyses of the anisotropic ESR and IR spectra have provided quantitative structural information of the system, which is consistent with the reported high conductivity of the films. The temperature dependent ESR spectra have clearly indicated that the system contains metallic domains, whose spin susceptibility is dominated by the Pauli susceptibility due to the conduction electrons on the BO stacks.

Metallic transport properties of conducting Langmuir-Blodgett films

Abstract The conductivity of the Langmuir-Blodgett (LB) films was examined in terms of a granular structure. The conductivity of bis(ethylenedioxy)tetrathiafulvalene-decyltetracyanoquinodimethane (BO-C 10 TCNQ) LB film was well fitted by the formula, σ = AT − α exp(-E a /kT), where the behavior is determined by the semiconducting domain boundaries in the granular-structured film. The metallic nature of the domains was revealed by thermoelectric power measurements: the temperature dependence of the thermoelectric power was almost linear with T. The temperature dependence of the conductivity in tridecylmethylammonium-Au(dmit) 2 (3C10-Au) LB film is well explained by a linear combination of Shengs model and common metal, R = C 1 exp [ T 1 ( T+T 0 ) ] + C 2 T . The conduction was governed by the fluctuation-induced tunneling between metallic domains.

On superconductivity of the organic conductor α-(BEDT-TTF)2KHg(SCN)4

Abstract The resistivity decrease in α-(BEDT-TTF)2KHg(SCN)4 appearing in the temperature region below 300 mK was found to be suppressed by a magnetic fields of less than 0.1 T. We argue that the decrease in resistivity is due to the superconductivity occurring in a fibril-like structure. The observed behavior is in part due to the inhomogeneous molecular arrangement, but is also related to the subtle interplay between magnetism and superconductivity in this system.

Percolation conduction in BO-C10TCNQ conductive Langmuir-Blodgett films

Abstract The structure and electronic properties of the conductive Langmuir-Blodgett film consisting of bis(ethylenedioxy)tetrathiafluvalene-decyltetracyanoquinodimethane (BO-C 10 TCNQ) and icosanoic acid (IA) have been studied through the measurements of electrical conductivity, X-ray diffraction and electron spin resonance for samples with different mixing ratio of BO-C 10 TCNQ to IA. The X-ray diffraction and ESR measurements indicate the coexistence of metallic, semiconducting and insulating domains. The temperature dependences of the electrical conductivity are reproduced by the calculation based on a modified two-dimensional percolation model consisting of three kinds of sites and their characteristics are understood well by the related phase diagram.