15 Fakultätsübergreifend / Sonstige Einrichtung

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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.
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.
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.
Open Access
Cs deposition on c-axis-oriented thin films of α-(BEDT-TTF)2I3: evidence for electron doping and intercalation
(1994) Söderholm, Svante; Loppinet, Benoit; Schweitzer, Dieter
The deposition of Cs on c-axis-oriented thin films of α-(BEDT-TTF)2I3 causes a shift of the chemical potential in agreement with addition of electrons to the system. At high Cs doses a chemical reaction between Cs+ and I3- occurs, with the formation of CsI and probably mainly I2. In the valence-band regime new structures appear, due to I- ,i.e, CsI, and molecular orbitals of I2 derived from I 5p. The hybridization between these orbitals and the BEDT-TTF orbitals is negligible. The remarkable changes in the I 4d spectrum caused by Cs deposition can be understood within the framework of the proposed chemical reaction.
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.
Open Access
Electrical transport in the organic superconductor β-(BEDT-TTF)2AuI2 [with BEDT-TTF≡bis(ethylenedithio)tetrathiafulvalene] : influence of x-ray-induced defects on the normal phase and superconducting ground state
(1992) Dolanski Babic, Sanja; Biskup, Neven; Tomic, Silvia; Schweitzer, Dieter
We have measured the in-plane and out-of-plane resistivities of the organic superconductor β-(BEDT-TTF)2AuI2 with disorder introduced by x-ray irradiation. [Here BEDT-TTF is bis(ethylenedithio)- tetrathiafulvalene]. For defect concentrations less than 0.15% per mole, Matthiessen’s rule is obeyed at all temperatures and a T2 term in the resistivity is found in the temperature region between 5 and 20 K. The long-range superconducting order is destroyed by 0.1% per mole and the transition is completely smeared out above 0.2% per mole of defects. The overall temperature behavior of the resistivity is discussed and the possible importance of the electron-electron interaction and two-dimensional localization for the stabilization of the superconducting ground state is emphasized.
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.
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.
Open Access
FT-IR investigations of (BEDT-TTF)2I3 radical salts
(1993) Moldenhauer, Jörg; Pokhodnia, Konstantin I.; Schweitzer, Dieter; Keller, Heimo J.
Fourier transform infrared absorption spectra of structurally different phasis of the radical cation salt (BEDT-TTF)2I3 in the range of the molecular vibration of the donor are presented. The low-temperature spectra provide exact fingerprints of the different phases as long they have different unit cells. Additional differences concerning the position of ethylene group reflect themselves in the CH2-stretching vibrations. As a result, it could be shown that the ethylene groups of the αt-phase and β-phase must have a different order. Concerning the tempering of the different phases, it is shown that a least αt- and χt-(BEDT-TTF)2I3 are the same.
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.
Open Access
De Haas-van Alphen effect in the organic superconductor κ-(BEDT-TTF)2I3
(1993) Heinecke, Martin; Winzer, Klaus; Schweitzer, Dieter
In the two-dimensional organic superconductor kappa-(BEDT-TTF)2I3 de Haas-van Alphen oscillations were observed at magnetic fields above 5 T and temperatures between 0.4 and 2 K. We found two dHvA frequencies at 3.846 kT and 0.570 kT, which correspond to the cross-sectional areas of the Fermi surface expected from a tight-binding calculation. From the temperature dependence of the oscillation amplitudes the effective mass belonging to the larger orbit was found to be 3.80mo. Precise measurements of the angular dependence of the dHvA frequency show no deviation from that expected for a cylindrical Fermi surface. The angular dependence of the amplitude including spin splitting zeroes can essentially be described by a two-dimensional Fermi surface. Certain systematical deviations, however, hint for a slight corrugation.
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.
Open Access
Organic metals from chiral BEDT-TTF donors
(1991) Chen, Benming; Deilacher, Frank; Hoch, Matthias; Keller, Heimo J.; Wu, Peiji; Armbruster, Kurt; Geiger, Rolf; Kahlich, Siegfried; Schweitzer, Dieter
We have recently described our project to prepare methylated BEDT-TTF derivatives in order to obtain different radical ion salts of these donors (1). Meanwhile we have obtained 7,7'-Dimethyl-BEDT-TTF ("DIET"), 7,8'-Dimethyl-BEDT-TTF ("DIET") and 7,7',8,8'-Tetramethyl-BEDT-TTF ("TMET") as isomeric mixtures and some of them in an "optically pure" form. The isomeric mixtures have been prepared as reported earlier (1) corresponding to the "classical" synthesis of BEDT-TTF (2). The optically pure compounds have been obtained starting from the optically pure 1,2-propanediol and 2,3-butanediol respectively, depicted on page 4.
Open Access
Organische Supraleiter : chemische Physik
(1994) Dressel, Martin; Schweitzer, Dieter; Keller, Heimo J.
Vor fünfzehn Jahren wurde erstmals Supraleitung in einem organischen Festkörper - dem quasi-eindimensionalen Leiter (TMTSF)2PF6 - nachgewiesen. Zehn Jahre ist es her, daß in β-(BEDT-TIF)2I3 Supraleitung bei Normaldruck gefunden wurde, und diese Klasse der zweidimensionalen organischen Metalle mit dem Donor BEDT-TTF erreicht heute eine Sprungtemperatur von 13 K. Was haben wir in diesen Jahren gelernt? Wo steht die Forschung auf dem Gebiet der organischen Metalle heute?
Open Access
Photochromic thiophenefulgides : photokinetics of two isopropyl derivatives
(1991) Ulrich, Karl; Port, Helmut; Wolf, Hans Christoph; Wonner, Johann; Effenberger, Franz; Ilge, Hans-Dieter
For two photochromic isopropyl-thiophenefulgides a complete photokinetic analysis is given. For one compound (5b), the partial quantum yield for the colouring process (ΦEC = 54%) is the highest value found for thiophenefulgides. EZ-isomerisations do not play an important role. The colouring quantum yield is temperature independent down to 20 K, whereas bleaching requires little thermal activation. In case of an adamantylidene substituted isopropyl-thiophenefulgide (5c), the bleaching efficiency (ΦCE = 31%) is very high. Upon heating of the cyclic products, thermal bleaching occurs with an activation energy of 1 eV.
Open Access
Point contact spectroscopy measurements on αt-(BEDT-TTF)2I3
(1991) Weger, Meir; Nowack, Andreas; Schweitzer, Dieter
We 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.
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, Dieter
Coordination 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.
Open Access
Resonant Raman scattering from superconducting single crystals of (BEDT-TTF)2I3
(1993) Pokhodnia, Konstantin I.; Graja, Andrzej; Weger, Meir; Schweitzer, Dieter
A 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.
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.
Open Access
Transport properties of single crystals and polycristalline pressed samples of (BEDT-TTF)2X salts and related coordination polymers
(1991) Schweitzer, Dieter; Kahlich, Siegfried; Gärtner, Stephan; Gogu, Emil; Grimm, Hans; Heinen, Ilsabe; Klutz, Thomas; Zamboni, Roberto; Keller, Heimo J.; Renner, Gerd
Ten years ago in 1979, superconductivity was observed for the first time in an organic metal. today, about 30 different organic metals are known, which become superconducting under pressure or ambient pressure. The organic superconductors with the highest transition temperatures are all radical salts of the donor bis(ethylenedithioio)-tettathiafulvalene (BEDT-TTF), namely at ambient pressure (BEDT-TTF)2CU(NCS)2 (Tc = 10.4 K) and αt,-(BEDT-TTF)2I3 (Tc = 8 K) and under isotropic pressure β H- (BEDT-TTF)2I3(0.5 kbar, Tc = 7.5 K) The latter β H-phase can even become superconducting at 8 K and ambient pressure, after a special pressure-temperature cycling procedure i.e. pressurization up to 1 kbar at room temperature, and release of the helium gas pressure at temperarures below 125 K. Nevertheless, this superconducting state at 8 K in β H-(BEDT -TTF)2I3 is only metastable , since warming up the crystal above 125 K and cooling down again under ambient pressure, results only in superconductivity at 1.3 K, the so-called β L - or β-phase.