Emil Gogu

Superconductivity at 10 K and ambient pressure in the organic metal (BEDT-TTF)2Cu(SCN)2

We confirm the observation of superconductivity at ambient pressure above 10 K in the organic metal (BEDT-TTF)2Cu(SCN)2 as reported recently by Urayama et al [12]. In addition we have measured ESR, ac-susceptibility and thermopower in crystals of (BEDT-TTF)2Cu(SCN)2 and have shown that in contrast to other organic superconductors here a relatively sharp superconducting transition even in the ac-susceptibility can be observed which saturates already around 8 K. The thermopower measurements indicate a clear metal-metal phase transition at 100 K and a possible second phase transition at around 50 K, while from the temperature dependence of the resistivity and susceptibility (ESR) these phase transitions cannot be observed.

Electric conductivity and thermoelectric power of various polypyrroles

The dc-conductivity and thermoelectric pover of electrochemically prepared polypyrroles have been investigated in the temperature range between 300 K and 1.3 K on unaged. aged ( 2~-120 days at 80 0 c in air) and chemically modified samples. The linear temperature dependence of the thermopower of unaged polypyrroles demonstrates clearly the contribution of metallic regions to the electronic transport properties. The temperature dependence of the conductivity ot the polypyrroles investigated corresponds to that predicted by the variable range hopping model.

Superconductivity at ambient pressure in BEDT-TTF radical salts

Abstract Crystals of (BEDT-TTF)2Cu(NCS)2 were prepared by several methods and the super-conducting transitions investigated by resistivity and ac susceptibility measurements. Depending on the preparation of the crystals a variation of the temperature of the superconducting transition is observed. This variation is manifested in the upper critical fields Hc2 and proton NMR relaxation measurements at temperatures below TC show it as well. The upper critical fields HC2 of crystals of αt-(BEDT-TTF)2I3 were determined in dependence of the temperature and of the direction of the magnetic field with respect to the various crystal axes by measuring the mid transition of the resistivity and of the rf penetration depth. The data are analyzed with the anisotropic effective mass model in the picture of the Ginsburg Landau (GL) theory as well as in the picture of a layered superconductor. 13C Knight shifts by magic angle sample spinning and NMR cross polarisation methods support the picture of the layered superconductor.

Superconductivity in polycrystalline pressed samples of organic metals

We report bulk superconductivity at ambient pressure in polycrystalline pressed samples of αt-(BEDT-TTF)2I3. As in single crystals of αt-(BEDT-TTF)2I3 the onset for superconductivity in the polycrystalline pressed samples lies at about 9K but the transition is broader. Zero resistivity is observed at 2.2K and the middle of the transition is found at 5K. The measurement of the ac susceptibility shows that at 2K about 50% of the sample is superconducting, but at this temperature the transition is still not saturated. The observation of bulk superconductivity in this polycrystalline pressed samples is of special interest because it demonstrates that organic superconductors can be used in principle as well for the production of electronic devices such as squids and superconducting cables. In addition the finding of bulk superconductivity in large pressed samples of crystallites of the typical diameter of 1μm of an organic metal indicates that the observation of superconductivity in organic polymers should be possible as well.

Critical fields of the ambient pressure stable superconducting state at 8K of the organic metal αt-(BEDTTTF)2I3

Crystals of αt-(BEDT-TTF)2I3 are organic metals with a stable 8K superconducting transition at ambient pressure. The upper critical fields Hc2 of αt-(BEDT-TTF)2I3 were determined in dependence of the temperature and of the angle of the magnetic field with respect to the variuos cristal axes, by measuring the mid-transition of the resistivity as well as of the rf penetration depth (ac-susceptibility). The values obtained by both methodes agree quite well. The data were analised with the anisotropic effective mass model. The highly two dimensional nature of the metal is confirmed, but in the temperature dependence of the critical field with the magnetic field parallel to the ab-plane a crossover from a three- to a two-dimensional behaviour is observed.

Radical cation salts of an unsymmetrical BEDT-TTF derivative: DIMET☆

Abstract Crystal structures, temperature dependent conductivity measurements and thermopower data of several DIMET radical salts are presented and discussed.

Transport properties of single crystals and polycristalline pressed samples of (BEDT-TTF)2X salts and related coordination polymers

Ten years ago, in 1979, superconductivity was observed for the first time in an organic metal [1]. Today, about 30 different organic metals are known, which become superconducting under pressure or ambient pressure. The organic superconductors with the highest transition temperatures are all radical salts of the donor bis(ethylenedithiolo)-tetrathiafulvalene (BEDT-TTF), namely at ambient pressure (BEDT-TTF)2Cu(NCS)2 (Tc = 10.4 K) [2] and αt-(BEDT-TTF)2I3 (Tc = 8 K) [3] and under isotropic pressure βH - (BEDT-TTF)2I3 (0.5 kbar, Tc = 7.5 K) [4,5]. The latter βH-phase can even become superconducting at 8 K and ambient pressure, after a special pressure-temperature cycling procedure [6], i.e. pressurization up to 1 kbar at room temperature, and release of the helium gas pressure at temperatures below 125 K. Nevertheless, this superconducting state at 8 K in βH-(BEDT-TTF)2I3 is only metastable [6,7], since warming up the crystal above 125 K, and cooling down again under ambient pressure, results only in superconductivity at 1.3 K, the so-called βL - or β-phase.

Preparation, Structure and Investigations of BEDT-TTF Trihalides

Abstract Crystals of the β-phases of (BEDT-TTF)2Br-I-Br (1) and (BEDT-TTF)2I-I - Br (2) have been isolated by electrocrystallization. The solids can be obtained using different tetrabutylammonium trihalides containing iodine and bromine as supporting electrolytes. Cyclovoltam metric results show clearly that the trihalide anions are involved in the oxidation reactions near the anode which finally lead to the crystals. 1 C20H16Br2IS16, Mr = 1056.01, and 2 C20H16BrI2S16, Mt = 1103.01 are isom orphous, crystallizing in the triclinic space group P1̄, with Z = 1. Unit cell parameters for 1: a = 6.5979(6) A , b = 8.998(1) Å, c = 15.138(3) Å , a = 94.03(1)°, β = 95.13(1), γ = 110.28(1), V = 834.7 Å and for 2: α = 6.606(2) Å , b = 9.047(6) A , c = 15.229(6) Å, α = 94.28(2)°, β = 95.34(2), v = 109.92(2), V = 846.5 Å3. The different sizes of the Br2I- and Brl2- counterions lead to different intra- and interstack S -S contacts in the solids thus influencing the physical properties. An additional Br2I- phase with a modulation of the above-described structure could be found. Some relevant physical properties of 1 and 2 are reported