08 Fakultät Mathematik und Physik
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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 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, DieterWe 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.Item Open Access (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, DieterThe 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.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 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 Field- and frequency dependent transport in the two-dimensional organic conductor α-(BEDT-TTF)2I3(1994) Dressel, Martin; Grüner, George; Pouget, Jean-Paul; Breining, Angela; Schweitzer, DieterWe studied the electrodynamic response of α-(BEDT-TTF)2I3 in a wide range of frequency, covering microwave and millimeter wave frequencies as well as the optical spectral range, and found a frequency dependent conductivity up to 1000 cm-1 in the low temperature phase. This is accompanied be a non-linear transport with a smooth onset at about 10 V /cm. Our X-ray studies show no indication of superstructure reflections and clearly rule out the formation of a charge density wave ground state. The lack of a temperature dependence in the millimeter wave conductivity between 20 K and 100 K makes hopping transport unlikely.Item Open Access A stable superconducting state at 8K and ambient pressure in αt-(BEDT-TTF)2I3(1987) Schweitzer, Dieter; Bele, Petra; Brunner, Hermann; Gogu, Emil; Haeberlen, Ulrich; Hennig, Ingolf; Klutz, Thomas; Swietlik, Roman; Keller, Heimo J.We report bulk superconductivity at 8 K and ambient pressure in crystals of α t (BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α t -crystals after tempering.Item Open Access Microwave hall mobility and conductivity in crystals of various (BEDT-TTF) radical salts(1991) Dressel, Martin; Helberg, Hans Wilhelm; Schweitzer, DieterWe 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.