Hans Grimm

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.

Raman studies of the organic superconductor β-(BEDT-TTF)2IAuI

The resonance Raman spectra of (IAuI)--anions in the organic superconductor β-(BEDTTTF)2IAuI as well as in the electrolyte salt n-Bu4AuI2 are investigated. In contrast to β-(BEDT-TTF)2I3 in the crystals of β-BEDT-TTF)2IAuI at low temperature only one Raman line at 120 cm-1 corresponding to the symmetrical stretching mode of the (IAuI)--anion is observed. No distinct change in the Raman spectra is observed at the superconducting transition.

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.

ODMR of triplet states of organic electron donors

Energies of the first excited triplet states and triplet zero field splitting parameters /D/ and /E/ of several organic donors - usually used for the preparation of organic metals and superconductors - are reported.