Hiroyuki Anzai

Spin-density-wave and superconducting states in thermally quenched (TMTSF)2ClO4

Abstract Successive transitions from metal to the spin-density-wave (SDW) state and the three-dimensional superconducting (3D-SC) state were found in thermally quenched bis-tetramethyltetraselenafulvalenium perchlorate, (TMTSF)2ClO4. The SDW state was suppressed after the sample was warmed above ∼25 K and cooled slowly. The appearance of the SDW state in addition to the 3D-SC state in the quenched sample is interpreted by the decrease in interchain coupling which could result from the thermal quenching of a high temperature configuration with disordered (ClO4)-anions. The effect of thermal quenching on quantum oscillations in transverse magnetoresistance was also studied.

Superconducting Transition of (TMTSF)2CLO4 in Magnetic Fields

Abstract Superconducting transition of (TMTSF)2ClO4 was studied by conductivity measurements along the most conductive a-axis in magnetic fields applied along three different principal crystallographic axes. The GL coherence lengths at OK along the a-, b-, and c*-axes, ϵa(0)≈ 600A, ϵ(0) ≈ 540A, and ϵc*(0) ≈ 60A were obtained from the measurements of the temperature dependence of the upper critical field HC2 near the transition temperature. The anisotropy is discussed in terms of the dimensionality arising from the crystal structure and of the conductivity anisotropy in the normal state.

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.

Superconductivity in BEDT-TTF Based Organic Metals: Role of Uniaxial Pressure and Inverse Isotope Effect

Abstract The superconducting properties characteristic to the two most extensively studied BEDT-TTF based organic metals are compared. A whole family of β-(BEDT-TTF)2X salts, including its mixed-anion crystals, with T c varying from 8 K to below 1 K enables us to study various factors governing T c in a systematic way. A new member, κ-(BEDT-TTF)2Cu(NCS)2, with the highest Tc among organic superconductors, obviously constitutes another case. One of the important feature common to both β-(BEDT-TTF)2X and κ-(BEDT-TTF)2Cu(NCS)2 is seen in the effect of hydrostatic pressure on Tc. In order to clarify its origin, the role of uniaxial pressure is discussed with some recent experimental results. Finally, an estimation of the isotope effect on the superconductivity in TTF-analog based organic metals is made, which leads us to an inverse isotope effect.

Kink Deformation of (TMTSF)2ClO4 Single Crystal and Its Influence on Electrical Resistivity

Mechanical kinks are formed in a needle crystal of bis (tetramethyltetraselenafulvalenium) perchlorate, (TMTSF) 2 ClO 4 , by applying stresses. The kink which has definite angle of 18.4° accompanies antikink with the same angle in the opposite direction. By the X-ray precession photograph, we show that the kink is ascribed to the mechanical twinning with the boundary on the (210) plane. The electrical resistivity measurements in a deformed part lying between the kink and antikink and in a region including the kink were carried out, but they did not show substantial difference compared with those in an underformed part. However, the kink which accompanies distortion in a molecular stack is considered to be one of causes of the resistance jump (stepwise increase in resistance), which is frequently observed during cooling.

X-Ray Scattering Study of 2kF and 4kF CDW in TMTTF-TCNQ

In the organic one-dimensional conductor TMTTF-TCNQ, the 2 k F and the 4 k F CDW were found by the diffuse X-ray photographic method. The 2 k F CDW condenses at 36 K, at which the metal-insulator transition occurs, and forms three-dimensionally ordered structures indicating the occurence of the Peierls transition. A different and unusual temperature dependence is shown by the 4 k F CDW. The transverse ordering of CDW is easily destroyed by the X-ray irradiation. Observed wave number of the transverse ordering and the strong irradiation effect suggest that the 2 k F CDW grows on both TCNQ and TMTTF stacks. However the 4 k F CDW is expected to be present mostly on the TMTTF stack.

Metal-Insulator Transition in (TMTTF)2BF4 under Pressure

The pressure-temperature phase diagram of bis-tetramethyltetrathiafulvalene-tetrafluoroborate, (TMTTF) 2 BF 4 , is determined through the electrical conductivity measurement along the a -axis. Two distinct metal-insulator transitions are observed at T 1 =200 K ( P 5 kbar). We conjecture that the T 1 transition is associated with an anion-orientational ordering, while T 2 is with the spin-Peierls ordering. The large difference in the transition temperatures for the resistivity at 200 K and for both the magnetic susceptibility and the specific heat at 41 K in the early reports is reconciled by taking account of the two types of transitions.

Nonlinear Effect above the Superconducting Critical Current in (TMTSF)2ClO4

A novel nonlinear characteristics of the resistivity above the superconducting critical current was observed along the a -axis of the di-(tetramethyltetraselenafulvalene)-perchlorate [(TMTSF) 2 ClO 4 ] by measuring the voltage-current characteristics. The nonlinearity has close relations with the appearance of the semiconducting phase just above the superconducting phase.

Synthesis and properties of the ttf donors with a five-membered cyclic o,o-, s,s-, or o, s-acetal

Abstract Synthesis of the TTF (tetrathiafulvalene) donors with a 1,3-dioxolane, 1,3-dithiolane, or 1,3-oxathiolane ring ( 1–3 ) has been accomplished via (MeO) 3 P-promoted cross-coupling reaction of the corresponding new ketones ( 6–8 ) and thiones ( 9a-c ). Although an X-ray diffraction analysis of the neutral 2a revealed that its molecular structure is nonplanar, its radical-cation salts with AuI 2 − , BF 4 − , and AsF 6 − exhibited metallic conducting behavior.