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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.3. Physikalisches InstitutStart date: 1990End date: 1991

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Now showing 1 - 10 of 26
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    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.
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    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, Dieter
    We 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.
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    (BEDT-TTF)2X(TCE)0,5[X:BF4, FSO3, IO4] new organic metals
    (1991) Armbruster, Kurt; Bele, Petra; Brunner, Hermann; Chen, Benming; Dahm, Stefan; Geiger, Rolf; Heinen, Ilsabe; Keller, Heimo J.; Schweitzer, Dieter
    The structure, the temperature dependence of the resistivity, thermopower, ESR-linewidth and the spin susceptibility of a new series of organic metals (BEDT-TTF)2X·(TCE)0,5 [X = BF4, FS03 and IO4] is presented and discussed.
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    Proton spin-lattice relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2 : evidence for relaxation by localized paramagnetic centres
    (1990) Klutz, Thomas; Haeberlen, Ulrich; Schweitzer, Dieter
    The spin-lattice relaxation of the protons in the compound was investigated at nu L=13.5 MHz and 270 MHz for 4.2 K10 K and becomes non-exponential for T<10 K. At nu L=13.5 MHz the transition from exponential to noticeably non-exponential relaxation occurs already at T approximately=25 K. The cause for the non-exponential proton relaxation in the coarse grains is finite penetration of the RF-field (skin-effect) into the electrically conducting crystals. Powdering the crystals suppresses the skin-effect; this procedure, however, generates relaxation sinks in the form of localized paramagnetic centres. There is reason to believe that the skin-effect in coarse grains and generation of relaxation sinks by a powdering procedure complicate proton relaxation studies as well in other organic superconductors.
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    Photochromic thiophenefulgides : photokinetics of two isopropyl derivatives
    (1991) Ulrich, Karl; Port, Helmut; Wolf, Hans Christoph; Wonner, Johann; Effenberger, Franz; Ilge, Hans-Dieter
    For two photochromic isopropyl-thiophenefulgides a complete photokinetic analysis is given. For one compound (5b), the partial quantum yield for the colouring process (ΦEC = 54%) is the highest value found for thiophenefulgides. EZ-isomerisations do not play an important role. The colouring quantum yield is temperature independent down to 20 K, whereas bleaching requires little thermal activation. In case of an adamantylidene substituted isopropyl-thiophenefulgide (5c), the bleaching efficiency (ΦCE = 31%) is very high. Upon heating of the cyclic products, thermal bleaching occurs with an activation energy of 1 eV.
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    Proton relaxation in the superconducting organic solid (BEDT-TTF)2Cu(NCS)2: evidence for relaxation by localized paramagnetic centers
    (1990) Klutz, Thomas; Haeberlen, Ulrich; Schweitzer, Dieter
    The nonexponential character of the build-up of the nuclear magnetization usually increases on lowering the temperature. Superconducting fluctuations have been offered in a speculative way as an explanation for the nonexponential spin relaxation. We have observed the same relaxation behaviour in our investigation of the proton spin relaxation in (BEDT-TTF)2Cu(NCS)2 which is an organic conductor at room temperature and becomes a superconductor at Tc=10.4 K at ambient pressure. As we noticed during preliminary measurements that the degree of deviation from exponential relaxation depends on the sample under study we decided to do experiments under controlled conditions of sample preparation.
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    Intramolecular energy transfer through linear polyenes : effect of spacers
    (1990) Blessing, Gerd; Holl, Norbert; Port, Helmut; Wolf, Hans Christoph; Effenberger, Franz; Kesmarszky, Thomas; Schlosser, Hubert
    The optical properties of linear polyenes terminally substituted with different molecules (mainly anthryl- and TPP-substituents) have been studied. Bicyclic spacers within the polyene chain affect the quantum yields but do not interrupt intramolecular energy transfer between the endgroups.
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    Microwave hall mobility and conductivity in crystals of various (BEDT-TTF) radical salts
    (1991) Dressel, Martin; Helberg, Hans Wilhelm; Schweitzer, Dieter
    We report investigations of the microwave transport properties carried out on the organic conductors and superconductors of (BEDT-TTF)-salts: α-, αt- and β-(BEDT-TTF)2I3, α-(BEDT-TTF)3(NO3)2 and x-(BEDT-TTF)2Cu(NCS)2. The anisotropy and temperature dependence (300 K to 4 K) of the high-frequency conductivity was measured in a microwave cavity at 10.3 GHz. There is low anisotropy in the high conducting plane of the quasi two-dimensional crystals of the (BEDT-TTF)-family, e.g. the (a,b)-plane in β-(BEDT-TTF)2I3; perpendicular to this the microwave conductivity is one order of magnitude lower. At room temperature the microwave Hall mobility at 9.5 GHz of the different phases of (BEDT-TTF)2I3 and (BEDT-TTF)3(NO3)2 is 100 to 200 cm2/Vs in the high conducting planes of the crystals.
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    Knight shift tensors and π-spin densities in the organic metals αt-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(NCS)2
    (1991) Klutz, Thomas; Hennig, Ingolf; Haeberlen, Ulrich; Schweitzer, Dieter
    13C-MASS spectra of pure BEDT-TTF and of the organic metals αt-(BEDT-TTF) 2I 3 and (BEDT-TTF) 2Cu(NCS)2 were recorded at νL - 68 MHz. Isotropic shifts and the principal components of the shift tensors were determined, respectively, from the center and spinning side bands. For pure BEDT-TTF which is a diamagnetic insolator, the measured shifts are chemica/ shifts, while for the organic metals they are the sum of chemical and Knight shifts. In each of the compounds the shifts are assigned in groups to the inner, middle and outer carbons of the BEDT-TTF molecule. For the organic metals the separation of the experimental shifts into chemical and Knight shifts is discussed. From the anisotropic part of the Knight shift tensors the π-spin densities at the carbon and sulphur positions of the BEDT-TTF molecule are inferred. The result is that the π-spin density of the unpaired hole is concentrated on the center part of the BEDT-TTF molecuIe, i.e. on the inner and middle carbons, and on the inner sulphurs. It is argued that the current density is concentrated on this part of the BEDT-TTF molecule as well.
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    Bulk superconductivity at ambient pressure in polycrystalline pressed samples of organic metals
    (1990) Schweitzer, Dieter; Kahlich, Siegfried; Gärtner, Stephan; Gogu, Emil; Grimm, Hans; Zamboni, Roberto; Keller, Heimo J.
    Bulk superconductivity in polycrystalline pressed samples of αt-(BEDT-TTF)2I3 and βp-(BEDT-TTF)2I3 is reported. This finding shows that organic superconductors can be used in principle for the preparation of electronic devices and superconducting cables.