08 Fakultät Mathematik und Physik
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Item Open Access Organic metals from chiral BEDT-TTF donors(1991) Chen, Ben-ming; Deilacher, Frank; Hoch, Matthias; Keller, Heimo J.; Wu, Pei-ji; Gärtner, Stephan; Kahlich, Siegfried; Schweitzer, DieterWe have shown that is is possible to obtain organic metals from chiral molecules. The X-ray results prove an oxidation number of + 1.5 for the radical cations in at least two cases. To our best knowledge. these are: the first examples of BEDT-TTF-related radical cation salts with this oxidation number. As expected, we obtained statistically disordered crystals, containing the two different enantiomers in a "racemic" mixture. The broad smeared-out phase transitions are probably due to this disorder, or may be caused by the three-dimensional interactions which have been observed, so far, only once in a BEDT-TTF radical salt. We will crystallize the metallic compounds using "optically pure" isomers, in the hope to be able to isolate chiral metals.Item Open Access Investigations of the transport properties of the organic superconductors β-(BEDT-TTF)2I3 and βt-(BEDT-TTF)2I3(1990) Kahlich, Siegfried; Schweitzer, Dieter; Keller, Heimo J.The temperature dependence of the resistivity, thermopower and ac-susceptibility of the organic superconductors β-(BEDT-TTF )2I3 and βt-(BEDT-TTF)2I3 were investigated. A metal-metal phase transition in β-(BEDT-TTF)2I3 at around 125K, which was found earlier already in thermopower measurements, was observed in resistivity measurements. βt-(BEDT-TTF)2I3 crystals were prepared by annealing β-(BEDT-TTF)2I3 at 85 K for several weeks. The superconducting transition temperatures of the βt-(BEDT-TTF)2I3 crystals were found between 4.5K and 7K, that means at much higher temperatures than the observed Tc = 1.3K of β-(BEDT-TTF)2I3 crystals. Ac-susceptibility measurements indicate that the superconducting state is a bulk effect in βt-(BEDT-TTF)2I3.Item Open Access Pressure dependence of the resistivity of (BEDO-TTF)2ReO4·(H2O)(1992) Kahlich, Siegfried; Schweitzer, Dieter; Auban-Senzier, Pascale; Jérome, Denis; Keller, Heimo J.The temperature dependence of the resistivity under pressure (up to 6 kbar) of the organic metal (BEDO-TTF)2ReO4(H2O) was investigated. The increase of the resistivity below 35 K, which is observed at ambient pressure, is already totally suppressed at 1 kbar. In addition at this pressure the superconducting transition sharpens drastically and the onset temperature of 2.3 K is nearly the same as at ambient pressure.Item Open Access Superconductivity at 7.5 K and ambient pressure in polycrystalline pressed samples of βp-(BEDT-TTF)2I3(1989) Schweitzer, Dieter; Gogu, Emil; Grimm, Hans; Kahlich, Siegfried; Keller, Heimo J.Recently, bulk superconductivity at ambient pressure in polycrystalline pressed samples of an organic metal - α-(BEDT-TTF)2I3 - was observed. This is a remarkable fact because this finding shows that organic superconductors can in principle be used for the production of electronic devices, such as squids, and might even be suitable for the preparation of superconducting cables similar to the high temperature superconductors of the copper oxides.Item Open Access Bulk superconductivity in polycrystalline pressed samples of αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3(1990) Schweitzer, Dieter; Kahlich, Siegfried; Gärtner, Stephan; Gogu, Emil; Grimm, Hans; Zamboni, Roberto; Keller, Heimo J.Bulk superconductivity at ambient pressure in pressed samples of polycrystalline αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3 is reported. This finding is remarkable because it demonstrates that organic superconductors can be used in principal for applications such as the preparation of electronic devices and superconducting cables. In addition the observation of bulk superconductivity in large pressed samples of crystallites of a typical diameter of 1μm of organic metals indicates that superconductivity in organic polymers should be observable as well.Item Open Access Superconductivity in polycrystalline pressed pellets of k-(BEDT-TTF)2I3(1993) Balthes, Eduard; Breining, Angela; Kahlich, Siegfried; Moldenhauer, Jörg; Schweitzer, Dieter; Bele, Petra; Brunner, Hermann; Heinen, Ilsabe; Nuber, Bernhard; Keller, Heimo J.Investigations of the transport properties of single crystals and polycrystalline pressed samples of K-( BEDT-TTF)2I3 are presented. In contrast to polycrystalline pressed samples of K-(BEDT-TTF)2Cu(NCS)2 and K-(BEDT-TTF)2Cu [N(CN)2]Br those of K-(BEDT-TTF)2I3 show bulk superconductivity indicating that the loss of superconductivity in the first two types of samples is not due to the special structure of the K-phase, but probably due to a disproportionation of Cu1+ under the anisotropic pressure during the preparation of the samples. Annealing the single crystals as well as the polycrystalline pressed samples of K-(BEDT-TTF)2I3 at 75°C for a few days results in a structural transfomation with an increase in the transition temperature for superconductivity (about 8 K for the crystals). IR-spectra indicate a transfomatlon into a structure which is identical with αt-(BED-TTF)2I3 which seems to be the thermodynamically most stable phase of the several (BEDT-TTF)2I3 phases.Item Open Access New radical salts of BEDO-TTF: structures and electronic properties of organic metals and superconductors(1993) Schweitzer, Dieter; Kahlich, Siegfried; Heinen, Ilsabe; Song, En Lan; Nuber, Bernhard; Keller, Heimo J.; Winzer, Klaus; Helberg, Hans W.Structural data as well as some measurements on the electronic properties of a few organic metals of the donor BEDO-TTF are reported.