08 Fakultät Mathematik und Physik
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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 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 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 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 Investigations of the transport properties of the organic superconductors β-(BEDT-TTF)2I3 and βt-(BEDT-TTF)2I3(1990) Kahlich, Siegfried; Schweitzer, Dieter; Keller, Heimo J.The temperature dependence of the resistivity, thermopower and ac-susceptibility of the organic superconductors β-(BEDT-TTF )2I3 and βt-(BEDT-TTF)2I3 were investigated. A metal-metal phase transition in β-(BEDT-TTF)2I3 at around 125K, which was found earlier already in thermopower measurements, was observed in resistivity measurements. βt-(BEDT-TTF)2I3 crystals were prepared by annealing β-(BEDT-TTF)2I3 at 85 K for several weeks. The superconducting transition temperatures of the βt-(BEDT-TTF)2I3 crystals were found between 4.5K and 7K, that means at much higher temperatures than the observed Tc = 1.3K of β-(BEDT-TTF)2I3 crystals. Ac-susceptibility measurements indicate that the superconducting state is a bulk effect in βt-(BEDT-TTF)2I3.Item Open Access Resonant Raman scattering on single crystals of (BEDT-TTF)2Cu(NCS)2(1990) Zamboni, Roberto; Schweitzer, Dieter; Keller, Heimo J.Resonant Raman spectra of the organic superconductor (BEDT-TTF)2Cu(NCS)2 have been measured at 1.3 K as well as at 120 K. The Raman data do not show a superconducting gap opening in the investigated two dimensional superconducting system. The vibrational modes found in the Raman spectra are discussed and assigned. Possible mechanisms for resonance conditions in such two-dimensional organic superconducting systems are proposed.Item Open Access CH2 vibrations in radical cation salts based on the donor BEDT-TTF(1991) Gärtner, Stephan; Schweitzer, Dieter; Keller, Heimo J.Investigations of the CH2 stretch vibrations in radical cation salts based on the donor BEDT-TTF and the neutral donor itself are presented. The spectra taken by Fourier transform infrared absorption technique not only demonstrate the importance of the relationship between the possible positions of the CH2 groups in the BEDT-TIF molecule and the physical properties of the organic metal but also offer the opportunity to achieve structural information by the use of a simple method.Item Open Access Proton relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2(1991) Klutz, Thomas; Haeberlen, Ulrich; Schweitzer, Dieter; Keller, Heimo J.The origin of the nonexponentiallity of the proton spin-lattice relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2 is clarified. In fine powders the nonexponentiallity results from localized paramagnetic centers whose concentration is nonuniform over the powder sample. In coarse grains at low temperatures it is due to finite penetration of the rf-field into the conducting grains.