Thomas Klutz

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.

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.

Temperature and pressure dependence of the resistivity of β-(BEDT-TTF)2X (X=I3, I2Au) and αt-(BEDT-TTF)2I3

Abstract The temperature dependence of the resistivity of the organic metals and superconductors β-(BEDT-TTF) 2 I 3 , β-(BEDT-TTF) 2 I 2 Au and α t (BEDT-TTF) 2 I 3 was measured at ambient and at several isotropic pressures (up to 2.5 kbar). By the application of a relatively low pressure, it is possible to measure the term in the resistivity that is linear in temperature. This term is masked by the larger T 2 term at ambient pressure. It is shown that in the temperature range below 120 K, the external modes are mainly responsible for the resistivity, while above this temperature the flipping of the CH 2 groups becomes important. The linear electron-phonon coupling corresponds to a value of λ between 1 and 1.5.

Proton spin-lattice relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2. Evidence for relaxation by localized paramagnetic centres

The spin-lattice relaxation of the protons in the compound was investigated at nu L=13.5 MHz and 270 MHz for 4.2 K 10 K and becomes non-exponential for T<10 K. At nu L=13.5 MHz the transition from exponential to noticeably non-exponential relaxation occurs already at T approximately=25 K. The cause for the non-exponential proton relaxation in the coarse grains is finite penetration of the RF-field (skin-effect) into the electrically conducting crystals. Powdering the crystals suppresses the skin-effect; this procedure, however, generates relaxation sinks in the form of localized paramagnetic centres. There is reason to believe that the skin-effect in coarse grains and generation of relaxation sinks by a powdering procedure complicate proton relaxation studies as well in other organic superconductors.

Proton relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2

The origin of the nonexponentiallity of the proton spin-lattice relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2 is clarified. In fine powders the nonexponentiallity results from localized paramagnetic centers whose concentration is nonuniform over the powder sample. In coarse grains at low temperatures it is due to finite penetration of the rf-field into the conducting grains.

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.