Browsing by Author "Weger, Meir"
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Item Open Access Band structure calculation and tunneling measurements in (BEDT-TTF)2X (X=I3, IAuI)(1990) Weger, Meir; Nowack, Andreas; Schweitzer, Dieter; Kübler, Jürgen; Bentum, Jan M. van; Sommers, Charles S.At about the same time when little proposed room-temperature superconductivity in organic polymers, it was suggested that the high-Tc of the more conventional A-15's is associated with their one-dimensional electronic band structure. When TTF-TCNQ was discovered in 1973, it was suggested that the electron-phonon coupling in this 1-D organic molecular crystal is responsible for the metal-to-insulator (Peierls) transition at 52 K2, and reducing λ will cause a crossover to a superconducting state. Since then, the electronic structure, the conduction mechanism, and the superconducting mechanism were subject to controversy. Therefore, it is of some importance to establish whether the electronic band structure, and conduction mechanism, are similar to those in more conventional metals, and whether the superconductivity mechanism is the normal BCS phonon-mediated interaction.Item Open Access Determination of the electron phonon coupling and the superconducting gap in β-(BEDT-TTF)2X crystals (X=I3, IAuI)(1987) Nowack, Andreas; Poppe, Ulrich; Weger, Meir; Schweitzer, Dieter; Schwenk, HelmutWe performed point-contact and tunneling measurements on the organic superconductors beta-(BEDT-TTF)2I3 and beta-(BEDT-TTF)2IAuI, in the normal and superconducting states. The point-contact measurement in the normal state provides the Eliashberg function alpha2(omega)F (omega). This function has maxima at 4 and 15 mV, as well as a sharp peak at 1 mV which seems to indicate a very soft phonon strongly coupled to the electrons. The measurements in the superconducting state provide the superconducting gap, which for the tunneling measurements has a value 2Delta/kB Tc=4, just slightly above the BCS value.Item Open Access Electronic band structure and point contact spectroscopy of the organic superconductor β-(BEDT-TTF)2I3(1987) Weger, Meir; Kübler, Jürgen; Schweitzer, Dieter-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.Item Open Access Magnetic susceptibility of α and β phases of di[bis(ethylenediothiolo)tetrathiafulvalene] tri-iodide [(BEDT-TTF)2I3] under pressure(1986) Rothaemel, Bernd; Forró, Laszlo; Cooper, John R.; Schilling, James S.; Weger, Meir; Bele, Petra; Brunner, Hermann; Schweitzer, Dieter; Keller, Heimo J.We report static magnetic susceptibility data, χ(T), for the α and β phases of the organic conductor di[bis(ethylenediothiolo)tetrathiafulvalene] tri-iodide [(BEDT-TTF)2I3] both at ambient pressure and under applied pressures of up to 10 kbar. Ambient pressure ESR data for both phases below 300 K are also presented. The temperature dependence of the spin susceptibility χsβ(T) for the superconductor β-(BEDT-TTF)2I3 is unusually small for organic conductors; the pressure derivative ∂lnχsβ/∂P is found to be only moderate in magnitude (-3.3% per kbar) and independent of temperature below 300 K. For the αq phase, ∂lnχsα/∂P≃-2.0%/Kbar at ambient temperature.Item Open Access Magnetic susceptibility under pressure and anisotropy of electrical conductivity in quasi-one-dimensional (perylene)2(AsF 6)0.75(PF6)0.35 × 0.85CH2Cl2(1983) Moritz, I.; Shaheen, Salem A.; Hardebusch, U.; Schilling, James S.; Weger, Meir; Bender, Klaus; Schweitzer, Dieter; Keller, Heimo J.The magnetic susceptibility and electrical conductivity of the quasi-one-dimensional organic metal (Perylene)2(AsF6)0.75(PF6)0.35 × 0.85CH2Cl2 was studied in the temperature range 3–300 K. The measured susceptibility can be separated into its defect χd, core χc and enhanced paramagnetic spin susceptibility χp components. χp is found to decrease upon lowering the temperature or applying pressure, in analogy with results on charge transfer compounds such as TTF-TCNQ. The conductivity ratio σdouble vertical bar/σperpendicular also decreases with falling temperature. The present results are discussed within the context of a model which takes into account band-narrowing (electron localization) due to strong electron-phonon coupling.Item Open Access Point contact spectroscopy measurements of αt-(BEDT-TTF)2I3 : an alternative interpretation(1992) Weger, Meir; Nowack, Andreas; Schweitzer, DieterWe report on point contact measurements on the organic superconductor αt-(BEDT-TTF)2I3 (Tc~8K). The dV/dI vs V characteristics of homocontacts of this material show strong nonlinearities which, interpreted as a gap-structure, lead to extremly high values of Δ/κTc. Among other possibilities, we discuss the anomaly in terms of a strongly enhanced McMillan-Rowell structure Δ(ε) at ε = Δ BCS + α ph. The enhancement is caused by deviations from Eliashberg equations due to strong coupling (λ = 1-2) together with an Einstein phonon spectrum.Item Open Access Point contact spectroscopy measurements on αt-(BEDT-TTF)2I3(1991) Weger, Meir; Nowack, Andreas; Schweitzer, DieterWe report point-contact spectroscopy 1-V data on superconducting αt-(BEDT-TTF)2I3. The I-V curves show structure in 3 regions: for energies of order 0.25 meV, of about 4,5 meV and for energies of about 10 meV. No structure is observed near the BCS gap (1.3-1.4 meV). We attribute the structure at 4.5 meV to interaction with the phonon possessing this frequency which was observed independently by Raman spectroscopy, and the structure near 0.25 meV to an excitation energy considerably smaller than the BCS gap.Item Open Access Point-contact spectra of the organic metal β-(BEDT-TTF)2I3(1986) Nowack, Andreas; Weger, Meir; Schweitzer, Dieter; Keller, Heimo J.The d2U/dI2-characteristics of point-contacts between the organic superconductor β-(BEDT-TTF)2I3 in the normal metallic state and a normal metal (copper) are measured and found to be quite similar to those of point-contacts between normal metals which are well known to be correlated with the electron phonon coupling and the phonon density of states.Item Open Access Pressure effect on the magnetic susceptibility of low dimensional organic conductors α-, β-(BEDT-TTF)2I3 and (TMTSF)2X (X=ClO4, PF6, ReO4)(1987) Forró, Laszlo; Cooper, John R.; Rothaemel, Bernd; Schilling, James S.; Weger, Meir; Schweitzer, Dieter; Keller, Heimo J.; Bechgaard, KlausThe pressure dependence of the static susceptibility of the α and β phases of (BEDT-TTF)2I3 and of (TMTSF)2X for X = CI04, PF6, Re04 was measured using a Faraday balance up to a pressure of 10 kbar. The fractional pressure dependence σ InXs/σP is -2.0%/kbar and -3.3%/kbar for the α- and β-(BEDT-TTF)2I3 respectively and it is temperature independent. For (TMTSF)2X σ1nXs/σP is about -3.0%/kbar at room temperature and increases to -4.8%/kbar on cooling. Both the magnitude and temperature dependence of the fractional pressure dependence in those two families are in contrast with the behaviour in TTF-TCNQ, where the magnitude is -8.%kbar and decreases with decreasing temperature.Item Open Access Radical cation salts of BEDT-TTF and an unsymmetrical BEDT-TTF derivative(1986) Schweitzer, Dieter; Bender, Klaus; Endres, Helmut; Gärtner, Stephan; Heid, Rolf; Heinen, Ilsabe; Keller, Heimo J.; Weber, Angelika; Weger, MeirInvestigations of several BEDT-TTF and DIMET radical salts are presented. Measurements on β-(BEDT-TTF)2IAuI crystals show that their conducting and superconducting properties might vary strongly depending on the decomposition of (IAuI)− anions during the electrochemical process.Item Open Access Resonant Raman scattering from superconducting gap excitations in single crystals of (BEDT-TTF)2I3(1993) Graja, Andrzej; Pokhodnia, Konstantin I.; Weger, Meir; Schweitzer, DieterA study of low-energetic resonant Raman scattering of αt- and βH-(BEDT-TTF)2I3 superconductor was performed. The softening and weakening of the low frequency optical phonons at about 30 cm-1 in the superconducting state of both αt- and βH-phases were observed below Tc.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 Temperature and pressure dependence of the resistivity of β-(BEDT-TTF)2X (X=I3, I2Au) and αt-(BEDT-TTF)2I3(1988) Weger, Meir; Bender, Klaus; Klutz, Thomas; Schweitzer, Dieter; Gross, Frieder; Heidmann, Claus Peter; Probst, Christian; Andres, KlausThe temperature dependence of the resistivity of the organic metals and superconductors {β-(BEDT-TTF)2I3, β-{3-(BEDT-TTF)2I2Au and αt(BEDT-TTF)2I3 was measured at ambient and at several isotropic pressures (up to 2.5 kbar). By the application of a relatively low pressure, it is possible to measure the term in the resistivity that is linear in temperature. This term is masked by the larger T2 term at ambient pressure. It is shown that in the temperature range below 120 K, the external modes are mainly responsible for the resistivity, while above this temperature the flipping of the CH2 groups becomes important. The linear electron-phonon coupling corresponds to a value of λ between 1 and 1.5.Item Unknown 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.