15 Fakultätsübergreifend / Sonstige Einrichtung
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Item Open Access Molecular solids containing BEDT-TTF and structurally related transition metal complexes(1986) Keller, Heimo J.; Niebl, Roland; Nöthe, Dietrich; Schweitzer, Dieter; Weber, AngelikaThe special physical properties of BEDT-TTF salts, e.g. their two-dimensional behaviour, are caused by the typical geometry of this donor and the arrangement of the cations in the crystal lattice of its salts. In order to extend the range of physical properties produced by BEDT-TTF like compounds, we synthesized sulfur containing transition metal complexes with a molecular structure very similar to BEDT-TTF. The bis(5.6-dihydro-1.4-dithin-2.3-dithiolate)metallates of the nickel triad can be prepared in different oxidation states as anions and in neutral form. The anionic complexes can be reacted with BEDT-TTF cations.Item Open Access 1,3,5-Tris(dimethylamino)benzol mit Iod und 1,3,5-Tricyanobenzol : eine ungewöhnliche Dimerisierung und ein "normaler" Donor/Akzeptor-Komplex(1988) Keller, Heimo J.; Niebl, Roland; Renner, Gerd; Ruhr, Dorothea von der; Schweitzer, DieterWir sind auf der Suche nach "organischen" Ferromagneten. Als Zielsubstanz haben wir kristallisierte Radikal-Ionen mit bahnentarteten Grundzuständen ausgewählt. Molekulare Festkörper aus planaren organischen Radikalionen zeigen ein breites Spektrum teilweise recht interessanter physikalischer Eigenschaften, die durch kollektives Elektronenverhalten zu erklären sind. In den letzten Jahren wurde vor allem das elektrische Verhalten dieser Substanzen untersucht. Dies führte zur Entdeckung einer Vielzahl "organischer Metalle", von denen einige bei tiefer Temperatur auch supraleitend werden.Item Open Access Zukunft der Optik(1970) Tiziani, Hans J.Die Optik hat in den letzten Jahren neuen Aufschwung genommen. Mitverantwortlich für diese Entwicklung sind sowohl die Verwendung von leistungsfähigen Computern als auch die Nachfrage nach immer besserer Auflösung und Bildqualität. Besonders positiv haben sich die Entwicklungen des Laser (1960) und der Holographie ausgewirkt. Immer neue Applikationen des gebündelten, kohärenten Laserlichtes werden erschlossen, doch will ich mich in diesem Zusammenhang auf die mögliche Weiterentwlckung der erwähnten Gebiete beschränken.Item Open Access Zero bias anomalies in point-contact characteristics of αt-(BEDT-TTF)2I3(1994) Ernst, Gabriele; Nowack, Andreas; Weger, Meir; Schweitzer, DieterThe zero-bias anomaly in point-contact characteristics of the organic superconductor α-(BEDT-TTF)2I3 is investigated as a function of temperature and magnetic field. It is found that the zero-bias anomaly is insensitive to magnetic fields up to 5 T. In contrast, a structure at 5 meV, conventionally designated as the superconducting gap - but which is 4 times larger than the expected BCS gap - is strongly affected by magnetic fields above 1 T.Item Open Access Temperature and angular dependence of the upper critical field in β-(BEDT-TTF)2I3 and its deuterated analogue(1986) Heidmann, Claus Peter; Andres, Klaus; Schweitzer, DieterWe present measurements of the temperature and angular dependence of the upper critical fields of β-ET2I3 and d-β-ET2I3. The data are analyzed in terms of an anisotropic effective mass model. The results of cooling rate experiments are reported and the effect of deuteration on superconductivity is discussed.Item Open Access A study of the use of laser speckle to measure small tilts of optically rough surfaces accurately(1972) Tiziani, Hans J.A new method is proposed where small tilts can be measured with high accuracy even in the presence of a lateral shift. For this purpose the speckling of the object, an optically rough surface, is recorded in the Fourier plane before and after a tilt. By illuminating the developed plate with a laser, fringeswith spacings inversely proportional to the tilt angle can be observed.Item Open Access Lattice dynamics of 3-dimensional tilings modelling icosahedral quasicrystals(1993) Los, Joop; Janssen, Ted; Gähler, FranzA study of the lattice dynamics of three-dimensional tilings modelling icosahedral quasicrystals is presented. The phonon density of states is calculated, and the character of the eigenstates is determined. Three different types of commensurate approximants are considered, namely symmetrized, perfect and randomized approximants. It appears that the density of states is smoothed by randomization. The participation ratio, which measures the rate of localization of an eigenmode, is given as a function of frequency. Only the states at the very upper end of the frequency spectrum appear to be localized, whereas all other states are extended. The density of states at low frequencies is analyzed in more detail, by applying a Brillouin zone integration over the lowest branches. It is found that these lowest branches scale for successive approximants.Item Open Access Plasma reflectance of the one-dimensional organic metals of the type (arene)2XF6(arene=perylene, pyrene or fluoranthene ; X=P, As, Sb)(1983) Geserich, Hans Peter; Koch, Bertram; Ruppel, Wolfgang; Wilckens, Rudolf; Schweitzer, Dieter; Enkelmann, Volker; Wegner, Gerhard; Wieners, Gerhard; Keller, Heimo J.We have investigated the spectral position and the form of the plasma edge of different radical cation salts of the type (arene)zXF, independence of the type of the organic donor molecule, and of the charge transfer between donor and acceptor molecules.Item Open Access Adhesion of vesicles and membranes(1991) Lipowsky, Reinhard; Seifert, UdoIn the presence of an attractive surface, a vesicle can undergo shape transformations between two different free states, between a free and a bound state, and between two different bound states. Adhesion can also lead to topological changes such as vesicle rupture and vesicle fusion. The interaction between the vesicle membrane and the surface is renormalized by thermally excited shape fluctuations. This renormalization leads to unbinding phenomena both for fluid and for polymerized (or solid-like) membranes.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.