15 Fakultätsübergreifend / Sonstige Einrichtung
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Item Open Access 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.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 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 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.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 Preparation and physical properties of highly conducting metal (M = Ni, Co, Cu) coordination polymers(1993) Dahm, Stefan; Strunz, Werner; Keller, Heimo J.; Schweitzer, DieterCoordination polymers of nickel, cobalt and copper with the linking polydentate ligands tetrathiafulvalenetetrathiolate, benzene-1,2,4,5-tetrathiolate and phthalocyaninate have been prepared. The environmentally very stable materials have dc conductivities ranging between 10-2 S·cm-1 for the polyphthalocyaninato derivatives and 10² S·cm-1 for the tetrathiafulvalenetetrathiolates. The temperature dependence of the dc conductivity of the latter compounds is remarkably low.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 FT-IR absorption spectroscopy of BEDT-TTF radical salts: charge transfer and donor-anion interaction(1993) Moldenhauer, Jörg; Horn, Christof; Pokhodnia, Konstantin I.; Schweitzer, Dieter; Heinen, Ilsabe; Keller, Heimo J.IR absorption spectra from various K-phases of BEDT-TTF radical cation salts as well as from α-(BEDT-TTF)2I3 are presented. From tbese spectra one special mode ν27 representing the asymmetric ring C=C stretching vibration is assigned and the dependence of its frequency from the average charge on the donor molecule is given. The investigation of the metal-insulator phase transition of α-(BEDT-TTF)2I3 at 135 K with respect to this vibrational model ν27 gives strong indication of a charge localization in at least one of the two crystallographic different stacks. Furthermore, we correlate the frequencies of the CH2 stretching modes of a variety of BEDT-TTF radical salts with the superconducting phase transition temperatures. These frequencies are evidently not charge dependent, but significantly reflect the strength of the interaction of the donor molecule with the respective anion. It could be shown that phases with a higher Tc of their superconducting transition exhibit a smaller red shift of these frequencies, i.e., a less attractive donor-anion interaction.Item Open Access Detection of CH2 stretch vibrations in α-, αt- and β-(BEDT-TTF)2I3 by FT-IR absorption spectroscopy(1991) Gärtner, Stephan; Schweitzer, Dieter; Keller, Heimo J.We have investigated the CH2 stretch vibrations of α-, αt- and β-(BEDT-TTF)2I3 as well as those of the protonated and deuterated pure donor BEDT-TTF by FT-IR absorption spectroscopy. The crystals were pulverized and diluted in pressed pellets of KBr. Results in the spectral range around 2900 cm-1 taken at room temperature are presented. The spectra clearly reveal absorption lines varying in position and intensity between the different species in a characteristic manner.Item Open Access (BEDO-TTF)2ReO4·(H2O): a new organic superconductor(1991) Kahlich, Siegfried; Schweitzer, Dieter; Heinen, Ilsabe; Song, En Lan; Nuber, Bernhard; Keller, Heimo J.; Winzer, Klaus; Helberg, Hans W.The structure and the temperature dependence of the resistivity, thermopower and ac-susceptibility of the new organic metal (BEDO-TTF)2ReO4(H2O) was investigated. The resistivity and thermopower data indicate phase transitions at 213K, around 90K and 35K. Below 2.5K an onset to superconductivity is observed in the resistivity data. Superconductivity was suppressed in the resistivity at 1.3K by applying a magnetic field of about 0.2T. Ac-susceptibility data indicate that superconductivity is a bulk effect in (BEDO-TTF)2ReO4(H2O) but the onset for superconductivity observed in the ac-susceptibility is only at 0.9K and the transition seems to be complete only at temperatures below 50 mK. This broad transition might be due to some disorder in the structure created by the low temperature phase transitions.