Universität Stuttgart
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Item Open Access Resonant Raman scattering from superconducting single crystals of (BEDT-TTF)2I3(1993) Pokhodnia, Konstantin I.; Graja, Andrzej; Weger, Meir; Schweitzer, DieterA study of low energetic resonant Raman scattering of (BEDT-TTF)2I3 superconducors was performed. The vanishing of phonon bands accompanied by a decrease of the electronic background was observed below Tc. We propose a theoretical explanation for this novel effect in terms of the Balseiro-Falicov model of phonon-superconducting amplitude mode interaction.Item Open Access Zero bias anomalies in point-contact characteristics of αt-(BEDT-TTF)2I3(1994) Ernst, Gabriele; Nowack, Andreas; Weger, Meir; Schweitzer, DieterThe zero-bias anomaly in point-contact characteristics of the organic superconductor α-(BEDT-TTF)2I3 is investigated as a function of temperature and magnetic field. It is found that the zero-bias anomaly is insensitive to magnetic fields up to 5 T. In contrast, a structure at 5 meV, conventionally designated as the superconducting gap - but which is 4 times larger than the expected BCS gap - is strongly affected by magnetic fields above 1 T.Item Open Access Excited electronic states of flavin-containing coenzyme models(1988) Gückel, Friedemann; Schweitzer, Dieter; Becker, Katja; Schirmer, Rolf Heiner; Zipplies, Matthias F.; Staab, Heinz A.In order to gain further insight into the physical basis of flavin-catalyzed reactions, the interactions of a flavin with a second flavin and with other aromatic ring systems were studied. For this purpose compounds through 1 were synthesized These compounds contain the interacting units in defined geometric orientation. A monomeric flavin and glutathione reductase a flavoenzyme of known active-site chemistry, were included as reference molecules. The present report deals with excited triplet states of the flavin compounds as studied by optical spectroscopy and optically detected magnetic resonances (ODMR) at 1.3K.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 Microwave conductivity in polycrystalline (BEDT-TTF)2I3 material(1991) Müller, Gerhard; Helberg, Hans W.; Schweitzer, Dieter; Keller, Heimo J.Polycrystalline material of the α-phase of (BEDT-TTF)2I3 was compressed to small samples (4-mm x 1mm, thickness 0.3 mm typically) at a pressure of 10 kbar. Annealing at 70°C yields the superconducting αt-phase. Microwaves (10,2 GHz) enable the measurements of the conductivity for stepwise annealing after every annealing step in always the same sample. For annealing times 10 min all conductivity versus temperature curves are intersecting in an isosbestlc point at 190 K. This behaviour can be described by a conductivity relation for a two component system, from which was determined the volume fraction of the new grown αt-phase in dependence of the annealing time. Starting annealing (annealing times < 10 min) shows another unexpected phase transformation. After 2 min annealing the conductivity at 200 K increases by more than one order of magnitude, but then decreases of further annealing (5-10 min) down to the value for the unannealed sample.Item Open Access Electrical transport in the organic superconductor β-(BEDT-TTF)2AuI2 [with BEDT-TTF≡bis(ethylenedithio)tetrathiafulvalene] : influence of x-ray-induced defects on the normal phase and superconducting ground state(1992) Dolanski Babic, Sanja; Biskup, Neven; Tomic, Silvia; Schweitzer, DieterWe have measured the in-plane and out-of-plane resistivities of the organic superconductor β-(BEDT-TTF)2AuI2 with disorder introduced by x-ray irradiation. [Here BEDT-TTF is bis(ethylenedithio)- tetrathiafulvalene]. For defect concentrations less than 0.15% per mole, Matthiessen’s rule is obeyed at all temperatures and a T2 term in the resistivity is found in the temperature region between 5 and 20 K. The long-range superconducting order is destroyed by 0.1% per mole and the transition is completely smeared out above 0.2% per mole of defects. The overall temperature behavior of the resistivity is discussed and the possible importance of the electron-electron interaction and two-dimensional localization for the stabilization of the superconducting ground state is emphasized.Item Open Access Specific heat of the organic superconductor κ-(BEDT-TTF)2I3(1994) Wosnitza, Joachim; Liu, Xiao; Schweitzer, Dieter; Keller, Heimo J.We present high-resolution specific-heat, C, measurements on a large (4.47 mg) single crystal of κ-(BEDT-TTF)2I3 from 0.25 to 20 K in zero and different magnetic fields. The electronic specific-heat coefficient in the normal state is extracted to γ=(18.9±1.5) mJ K-2 mol-1. For the ratio ΔC/(γTc), where ΔC is the jump of C at Tc=3.4 K, a value of 1.6±0.2 consistent with the BCS prediction of 1.43 is found. The exact form of ΔC(T), the specific-heat difference between the superconducting and the normal state, however, deviates somewhat from the BCS dependence but might be explained by strong coupling. In magnetic fields applied perpendicular to the highly conducting b-c plane the height of the jump in C is strongly reduced and broadened with a concomitant reduction of Tc. In a field of B=0.5 T above Bc2 at low temperatures a hyperfine contribution to C is found which is larger than the value expected by nuclear hyperfine interaction alone.Item Open Access Magnetically modulated microwave absorption (MMMA) at low magnetic fields in (BEDT-TTF)-superconductors(1994) Bele, Petra; Brunner, Hermann; Schweitzer, Dieter; Keller, Heimo J.We report magnetically modulated microwave absorption (MMMA) at low magnetic fields in crystals of the organic superconductors αt-(BEDT-TTF)2I3 and κ-(BEDT-TTF)2Cu(NCS)2. In both cases a strong MMMA-signal can easily be found below the critical temperature Tc. In some selected crystals series of narrow, periodically spaced lines are observed comparable to corresponding spectra of the CuO-type high Tcsuperconductors. A model developed for the HTSC's single crystals is used to interpret the results of the organic superconductors.Item Open Access Static and ESR susceptibilities of the ambient pressure superconductors β- and αt-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(SCN)2(1988) Klotz, Stefan; Schilling, James S.; Gärtner, Stephan; Schweitzer, DieterWhereas the static spin-susceptibility χ(T) of β- and αt-(BEDT-TTF)2I3 at 5.7 tesla increases in a slow monotonic manner with temperature, χs(T) for the organic superconductor (BEDT-TTF)2Cu(SCN)2 displays a markedly structured temperature dependence. This gives evidence for two or more phase transitions in the metallic phase at temperatures near 100 K and 50 K, in addition to the onset of superconductivity at 5.7 tesla below not, vert, similar 8 K. Good agreement is found between the absolute magnitudes of static and ESR susceptibility results, thus confirming that orbital susceptibility contributions are of negligible importance here. Cooling in low magnetic fields reveals that superconductivity in (BEDT-TTF)2Cu(SCN)2 sets in at a temperature near 8.5 K, with a 7.6% Meissner effect in 14 G field.Item Open Access Freezeout of the electrical resistivity in (BEDT-TTF)2I3 below 20 K(1993) Weger, Meir; Tittelbach, Markus; Balthes, Eduard; Schweitzer, Dieter; Keller, Heimo J.We have measured the temperature (T) dependence of the resistivity rho of the beta and K phases of (BEDT-TTF)2I3 from 300 K down to Tc ( approximately 1.3 K and approximately 4 K for the beta and kappa phases, respectively). Between 100 K and 20 K, rho varies as T2. Below 20 K, the resistivity falls below the T2 law. We calculate the resistivity due to electron-electron scattering and find that this contribution is far too small to account for the measured resistivity, besides being inconsistent with the freezeout below 20 K. We suggest that the resistivity is due to electron-phonon scattering described by a novel mechanism that has been proposed for the high-Tc cuprates. We also suggest that this mechanism accounts for the T2 law observed in materials such as TiS2, Nb-doped SrTiO3, and intercalated graphite.