Heimo J. Keller

(BEDT-TTF)+ 2J- 3: A Two-Dimensional Organic Metal

Abstract Temperature - dependent thermopower-, dc- and microwave conductivity meaurements on the two-dimensional organic metal (BEDT-TTF)+ 2J- 3 are reported.

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.

Proton NMR Relaxation in the High-Tc Organic Superconductor β-(BEDT-TTF)2I3

We report the measurement of a striking enhancement of the proton spin-lattice relaxation rate occurring at the critical temperature in the high-Tc superconducting state of β-(BEDT-TTF)2I3. The T1-1 behaviour is attributed to a logarithmic critical singularity whose amplitude is enhanced by the low-dimensional character of these conductors.

The High-Tc Superconducting State of β-(BEDT-TTF)2I3 at Atmospheric Pressure: Bulk Superconductivity and Metastability

The AC susceptibility study of β-(BEDT-TTF)2I3 reveals that the high-Tc metastable superconducting state which can be stabilized at atmospheric pressure after a particular pressure-temperature cycling procedure, exhibits bulk superconductivity resembling very closely that of the high-Tc state, which is stabilized above 1 kbar. Annealing experiments show that the high-Tc state remains stable at low temperature as long as the annealing temperature does not exceed 125 K.

Overhauser effect experiments on the one-dimensional conductor (perylene)2+PF6-·2/3THF

Abstract We have measured the Overhauser enhancement of the protons and the 19F-nuclei of the one-dimensional conductor (perylene) + 2 PF - 6 · 2 3 THF in its metallic phase at room temperature. The enhancement factor V was found to be VP=+320±20 for the protons and VF=+175±40 for the 19F-nuclei indicating a predominantly scalar interaction with the conduction electrons in both cases.

Pressure effect on the magnetic susceptibility of low dimensional organic conductors ∝, β- (BEDT-TTF)2I3 and (TMTSF)2X (X = ClO4, PF6, ReO4

Abstract The pressure dependence of the static susceptibility of the α and β phases of (BEDT-TTF) 2 I 3 and of (TMTSF) 2 X for X = ClO 4 − , PF 6 − , ReO 4 − was measured using a Faraday balance up to a pressure of 10 kbar. The fractional pressure dependence δ 1n ξ s / δ P is -2.0%/kbar and −3.3%/kbar for the α- and β-(BEDT-TTF) 2 I 3 respectively and it is temperature independent. For (TMTSF) 2 X δ 1n ξ s / δ P is about −3.0%/kbar at room temperature and increases to −4.8%/kbar on cooling. Both the magnitude and temperature dependence of the fractional pressure dependence in those two families are in contrast with the behaviour in TTF-TCNQ, where the magnitude is −8.%/kbar and decreases with decreasing temperature.

Frequency-dependent conductivity in pure and iodine-doped α- (BEDT-TTF)2I3

Abstract Measurements of the microwave conductivity at 10 GHz show a plateau below the metal-insulator transition at 135 K in contrast to the further dropping dc conductivity. In pure material the plateau is independent of frequency (4 GHz, 10 GHz, and 22 GHz), but in the temperature range above 135 K strong frequency dependence is observed. Iodine doping causes frequency dependence also in the plateau range. Particular doping rates give rise to several conductivity behaviors for different crystal directions. The results are discussed referring to CDW depinning and relaxation processes.

Thermal expansion of the organic conductor β-(BEDT-TTF)2I3

Abstract We report hiogh resolution thermal expansion measurements between 1.2 and 300 K along the c∗-axis in the organic conductor β-(BEDT-TTF)2I3 where the superconducting properties are strongly dependent on the way the system is prepared in the pressure-temperature diagram. We clearly observed a strong anomaly at 172 K related to the structural phase transition and the resistivity anomalies reported by several groups. The temperature analysis in the low temperature regime is coherent with a phonon dominant contribution. Finally, the presence of small hysteresis in a large region around the transitions seem to indicate why both the pressure and the temperature cycling may stabilize small regions of the high-Tc phase of the system, as observed.

RADICAL CATION SALTS OF BEDT-TTF AND AN UNSYMMETRICAL BEDT-TTF DERIVATIVE

Investigations of several BEDT-TTF and DIMET radical salts are presented. Measurements on β-(BEDT-TTF)2IAuI crystals show that their conducting and superconducting properties might vary strongly depending on the decomposition of (IAuI)− anions during the electrochemical process.

The effect of disorder on the conduction electron spin resonance linewidth of the α and β phases of (BEDT-TTF)2I3

We present the electron spin resonance linewidth of α and β phases of the organic conductor di-bis (ethylenedithiolo) tetrathiafulvalene driiodide [(BEDT-TTF)2I3] disordered by fast electron irradiation. In the high temperature metallic phase the linewidth decreases in both phases with defect concentration in contrast to the predictions of the Elliott mechanism for spin relaxation in metals.