15 Fakultätsübergreifend / Sonstige Einrichtung

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1989 - 198911990 - 199421

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Institute: search.filters.UBSInstitut.3. Physikalisches Institut

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    Zero bias anomalies in point-contact characteristics of αt-(BEDT-TTF)2I3
    (1994) Ernst, Gabriele; Nowack, Andreas; Weger, Meir; Schweitzer, Dieter
    The 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.
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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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    Resonant Raman scattering from superconducting single crystals of (BEDT-TTF)2I3
    (1993) Pokhodnia, Konstantin I.; Graja, Andrzej; Weger, Meir; Schweitzer, Dieter
    A 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.
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    FT-IR investigations of BEDO-TTF and radical salts of BEDO-TTF
    (1993) Moldenhauer, Jörg; Pokhodnia, Konstantin I.; Schweitzer, Dieter; Heinen, Ilsabe; Keller, Heimo J.
    We investigated powder absorption spectra of the donor BEDO-TTF and some radical salts of BEDO-TTF, which are metals even at low temperatures. After an assignment of the bands in the neutral donor we found in the radical salts different vibrational frequencies of bands correlated with C-O vibrations. These frequencies are directly related to the average charge on the donor molecule. Additionally frequencies of CH2-stretching vibrations exhibit differences, which can be ascribed to a varying strength of donor-anion interaction depending on the respective anion.
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    Temperature dependent resistivity under pressure and magnetoresistance data of the organic superconductor (BEDO-TTF)2ReO4(H2O)
    (1993) Kahlich, Siegfried; Schweitzer, Dieter; Auban-Senzier, Pascale; Jerome, Denis; Keller, Heimo J.
    The temperature dependence of the resistivity under pressure (up to 6 kbar) of the organic metal (BEDO-TTF)2ReO4(H20) is reported. An increase of the resistivity below 35 K, which is observed at ambient pressure, is already suppressed at 1 kbar. In addition at this pressure the superconducting transition sharpens and the onset temperature of 2.3 K is nearly the same as at ambient pressure. Magnetoresistance data, observed at 1.3 K and 6.7 Tesla show a strong angle dependence. In addition at 1.3 K first SdH -oscillations in the magnetoresistance are found already at such low fields as 5 Tesla.
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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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    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, Dieter
    We 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.
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    Characterisation of the Fermi surface and phase transitions of (BEDO-TTF)2 ReO4·(H2O) by physical property measurements and electronic band structure calculations
    (1994) Kahlich, Siegfried; Schweitzer, Dieter; Rovira, Concepcio; Paradis, Jeffrey A.; Whangbo, Myung-Hwan; Heinen, Ilsabe; Keller, Heimo J.; Nuber, Bernhard; Bele, Petra; Brunner, Hermann; Shibaeva, Rimma P.
    The electronic properties of the organic superconductor (BEDO-TTF)2 ReO4·(H2O) were investigated by temperature dependent resistivity, ESR, Hall effect and magnetoresistance measurements. Shubnikov-de Haas (SdH) oscillations were observed in magnetic fields up to 24 T in the temperature range 0.5 K to 4.2 K. The electronic band structure of (BEDO-TTF)2 ReO4·(H2O) was calculated by employing the extended Hückel tight binding method on the basis of its room temperature crystal structure. The two observed SdH frequencies of 75 T and 37 T correspond very well with two cross-sectional areas of the hole and electron Fermi surface pockets obtained from the tight binding calculation. From the temperature dependence of the SdH oscillation amplitudes, the cyclotron effective mass (mc) belonging to the larger and smaller pockets were found to be 0.9 m0 and mc=1.15 m0 respectively. Measurements of the angular dependence of the SdH frequencies show no deviation from that expected for a cylindrical Fermi surface. In terms of our tight binding calculations and experimental measurements, probable causes for the 213 K and ∼35 K phase transitions are discussed. The calculations show that (BEDO-TTF)2 ReO4·(H2O) is a two dimensional semimetal but possesses a hidden nesting. The latter is likely to cause an SDW instability leading to the ∼35 K transition. The resistivity drop associated with the 213 K transition is likely to be induced by an abrupt increase in the relaxation time. The excellent agreement between the calculated and experimentally observed Fermi surface implies that, with decreasing temperature below 35 K, (BEDO-TTF)2 ReO4·(H2O) gradually gets out of the SDW state and re-enters the "original" metallic state, in which it becomes superconducting below 2.4 K.
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    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.
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    Zero-field splittings of the two lowest excited electronic states in crystalline [Ru(bpy)3]X2 with X=PF6, ClO4
    (1989) Yersin, Hartmut; Gallhuber, Erich; Hensler, Gerold; Schweitzer, Dieter
    For the title compounds zero-magnetic field ODMR signals have been observed on the zero-phonon lines which correspond to the two lowest excited states and which result from Ru4d→bpyπ* charge-transfer transitions. The splitting into sublevels are of the order of 0.1 cm−1 and the corresponding electronic states may be classified as doubly degenerate on the basis of the usual resolution of the optical emission spectra. Several microwave resonances are detected and assigned to different sites of the [Ru(bpy)3]2+ chromophores. Small relaxation rates between the sublevels (compared to other excited-state deactivation rates) are found to be important for a description of the spectroscopic properties at low temperatures.