Universität Stuttgart
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Item Open Access 1,3,5-Tris(dimethylamino)benzol mit Iod und 1,3,5-Tricyanobenzol : eine ungewöhnliche Dimerisierung und ein "normaler" Donor/Akzeptor-Komplex(1988) Keller, Heimo J.; Niebl, Roland; Renner, Gerd; Ruhr, Dorothea von der; Schweitzer, DieterWir sind auf der Suche nach "organischen" Ferromagneten. Als Zielsubstanz haben wir kristallisierte Radikal-Ionen mit bahnentarteten Grundzuständen ausgewählt. Molekulare Festkörper aus planaren organischen Radikalionen zeigen ein breites Spektrum teilweise recht interessanter physikalischer Eigenschaften, die durch kollektives Elektronenverhalten zu erklären sind. In den letzten Jahren wurde vor allem das elektrische Verhalten dieser Substanzen untersucht. Dies führte zur Entdeckung einer Vielzahl "organischer Metalle", von denen einige bei tiefer Temperatur auch supraleitend werden.Item Open Access Metallic coordination polymers using CS2 as starting material(1987) Keller, Heimo J.; Klutz, Thomas; Münstedt, Helmut; Renner, Gerd; Schweitzer, DieterOrganic polymers with "metallic" properties have found widespread interest during the last few years. Acetylene and aniline as well as different nitrogen and sulfur heterocycles have been used as starting materials. One main problem hampering the technical application of these solids up to now is their environmental and thermal instability. Therefore, we introduced metal ions to stabilize polymeric backbones with high electrical conductivity. Because of the enormous coordination ability of sulfur to many transition metal ions we decided to use a polymeric carbon-sulfur backbone.Item Open Access New charge tranfer salts containing BEDT-TTF and structurally related transition metal complexes(1987) Keller, Heimo J.; Niebl, Roland; Nöthe, Dietrich; Schweitzer, Dieter; Weber, AngelikaThe two-dimensional behaviour of BEDT-TTF salts is normally explained with the typical geometry of this donor and its arrangement in the crystal lattice. In order to extend the range of physical properties produced by BEDT-TIF like compounds, we synthesized sulfur containing transition metal complexes with a molecular structure very similar to BEDT-TTF. The bis(5,6-dihydro-1,4-dithiin-2,3-dithiolate)metallates of the nickel triad can be prepared in different oxidation states as anions and in a fully oxidized neutral form.Item Open Access The effect of disorder on the conduction electron spin resonance linewidth of the α and β phases of (BEDT-TTF)2I3(1987) Forró, Laszlo; Schweitzer, Dieter; Keller, Heimo J.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.Item Open Access Frequency-dependent conductivity in pure and iodine-doped α-(BEDT-TTF)2I3(1987) Przybylski, Michael; Helberg, Hans W.; Schweitzer, Dieter; Keller, Heimo J.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.Item Open Access FT-IR absorption spectra of polycrystalline pressed samples of the organic metals and superconductors α-, β-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(NCS)2(1989) Zamboni, Roberto; Schweitzer, Dieter; Keller, Heimo J.; Taliani, CarloIR absorption spectra of polycrystalline pressed samples of the organic metals α-(BEDT- TTF)2I3, β-(BEDT-TTF)2I3 and (BEDT-TTF)Cu(NCS) are presented and discussed. Normal prononated as well as deuterated materials were studied. The spectra range from 400 cm-1 to 5000 cm-1 and are similar to the IR reflectance spectra reported earlier. The data are of special interest in view of the fact that recently bulk superconductivity was observed in such polycrystalline samples of organic metals, and therefore the application of organic superconductors for the preparation of electronic devices seems possible.Item Open Access A stable superconducting state at 8K and ambient pressure in αt-(BEDT-TTF)2I3(1987) Schweitzer, Dieter; Bele, Petra; Brunner, Hermann; Gogu, Emil; Haeberlen, Ulrich; Hennig, Ingolf; Klutz, Thomas; Swietlik, Roman; Keller, Heimo J.We report bulk superconductivity at 8 K and ambient pressure in crystals of α t (BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α t -crystals after tempering.Item Open Access Infrared spectra of the BEDT-TTF and DIMET salts with NO3- and ClO4- anions(1988) Swietlik, Roman; Schweitzer, Dieter; Keller, Heimo J.The infrared absorption spectra of the salts α-(BEDT-TTF)3(N03)2, (BEDT-TTF)3(Cl04)2, (DIMET)2Cl04xTHF and (DIMET)xx(N03)yx(TRF)z have been investigated in the temperature range 80-300 K. The vibrational bands in the BEDT-TTF crystals result from the coupling between electrons and a g modes of the BEDT-TTF molecule. The infrared spectra of the DIMET g salts suggest that the electron-molecular-vibration coupling is not limited to totally symmetric modes.Item Open Access Superconductivity in polycrystalline pressed samples of organic metals(1989) Schweitzer, Dieter; Gärtner, Stephan; Grimm, Hans; Gogu, Emil; Keller, Heimo J.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.Item Open Access Pulsed ESR investigation of spin transport properties in the organic conductor (perylenyl)2(AsF6)0.75(PF6)0.35 · 0.85CH2Cl2(1987) Dobbert, Olaf; Prisner, Thomas; Dinse, Klaus Peter; Schweitzer, Dieter; Keller, Heimo J.Spin transport properties in the organic conductor (perylenyl2)+ X- were studies in the temperature range of 4-300 K. Being constant in the "metallic" range (Dmid = 1.8(2)cm2s-1, the diffusion constant Dmid exhibits a temperature activated behaviour in the low temperature range, at variance with the prediction invoking the model of an intrinsic semiconductor. A low-temperature divergence of the homogenous ESR line width, indicative of one-dimensional fluctuations, was also observed.