Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
68 results
Search Results
Item Open Access 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.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 Open Access 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, DieterThe 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.Item Open Access 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, DieterThe 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.Item Open Access 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.Item Open Access 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, Dieter13C-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.Item Open Access Far infrared response of α- and αt-(BEDT-TTF)2I3(1990) Zelezný, Vladimir; Petzelt, Jan; Swietlik, Roman; Gorshunov, Boris P.; Volkov, Alexander; Kozlov, Gennadij; Schweitzer, Dieter; Keller, Heimo J.The temperature dependence of the reflectivity of α- and αt-(BEDT-TTF)2I3 was determined in the 10-700 cm-1 range. Moreover the transmission spectra of α-phase between 10 and 33 cm-1 were studied. Using Kramers-Kronig analysis the IR conductivity and permittivity were calculated. The similarity of α t and β-phases was confirmed. The spectra in the metallic phase are dominated by a broad electronic peak between 200 and 500 cm-1 which can be understood as due to transitions across the small gap whose origin is discussed. In the spectra a rich phonon peak structure is observed, mostly of vibronic origin. It is suggested that non totally symmetric modes of BEDT-TTF molecule can couple with electrons.Item Open Access 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.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.