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Subject: search.filters.subject.530Start date: 1990End date: 1992Institute: search.filters.UBSInstitut.Sonstige Einrichtung

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Now showing 1 - 10 of 81
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    Adhesion of vesicles and membranes
    (1991) Lipowsky, Reinhard; Seifert, Udo
    In 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.
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    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.
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    Fluid flow and heat transfer in an axially rotating pipe subjected to external convection
    (1992) Weigand, Bernhard; Beer, Hans
    The effects of external insulation and tube rotation on the heat transfer to a fluid flowing inside a tube are examined by analysis. The turbulent flow is assumed to be hydrodynamically fully developed. Heat transfer was found to be strongly suppressed by tube rotation. It is shown that the significance of external insulation on the Nusselt number increases with growing rotation rate of the pipe.
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    TRSS: a new version of program TRS for a different geometry
    (1992) Schmitz, Joachim; Trebin, Hans-Rainer; Rössler, Ulrich
    Quantum resonances in the bands of semiconductors under uniaxial stress provide very detailed information on the band parameters. However, the analysis of experimental data is difficult. Computer programs based on an adequate theoretical model make this task easier. Program TRSS calculates energy eigenvalues, wave functions and oscillator strengths for direct inter- and intraband dipole transitions. The magnetic field is applied parallel to the [001] crystal axis while the uniaxial stress is directed perpendicular [100] to it.
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    Theory of liquid crystalline phases in biaxial systems
    (1992) Longa, Lech; Trebin, Hans-Rainer
    General properties of SO(3) - symmetric free- energy expansion for biaxial systems are studied. In particular, all invariants in powers of a traceless and symmetric quadrupole tensor order parameter and a vector order parameter are identified and their relation to possible local structures are found. A new class of polar, chiral biaxial phases are predicted.
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    Spontaneous polarization in chiral biaxial liquid crystals
    (1990) Longa, Lech; Trebin, Hans-Rainer
    A phenomenological theory of polar structures in chiral biaxial liquid crystals is constructed exploiting the properties of a symmetric and traceless tensor order parameter field Q αβ(r) and of a polar field Pα(r). Full advantage is taken of the symmetry of the order parameters by systematic use of the method of integrity bases, which allows us to establish an expansion of the most general SO(3)-invariant free-energy density to arbitrary powers in the components Qαβ and Pα. A coordinate-independent parametrization of the invariants is introduced that yields a classification of local polar structures and some predictions about possible topologies of phase diagrams without the necessity of performing numerical calculations. As one prominent result, the theory predicts a polar, chiral biaxial state that exists due to a piezoelectric coupling of a chiral biaxial tensor field and the polarization field and which disappears if tensor is uniaxial. We then provide a general theory of flexopolarization in biaxial systems. A general biaxial system is described by 12 fundamental flexopolarization modes. Special cases, obtained by imposing symmetry restrictions to the tensor field Q, reduce the number of modes. Finally, the theory is applied to chiral phases. Simple polar chiral structures including cholesteric and smectic-C* liquid crystals are analyzed. In particular, it is shown that if the smectic-C* phase is stabilized due to the piezoelectric coupling between Q, P, and a density wave, then it must be described as a biaxial uniform spiral with at least two nonvanishing commensurate harmonics. The minimization of the quadratic part of the Landau–de Gennes energy supplemented by (flexo)polarization terms may give rise to incommensurate two- or three-dimensional polar structures that can be stabilized by cubic terms.
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    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, Dieter
    We 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.
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    Simulation mit Supercomputern : ein neues Werkzeug der Physik
    (1990) Ruder, Hanns; Ertl, Thomas; Geyer, Florian; Herold, Heinz; Kraus, Ute; Nollert, Hans-Peter; Rebetzky, Andreas; Schweizer, Wolfgang; Zahn, Corvin
    Unser Wissen über die Struktur des Kosmos und die darin enthaltenen Objekte stammt aus der sorgfältigen Analyse der auf der Erde einfallenden elektromagnetischen Strahlung,verbunden mit einer theoretischen Modellierung im Rahmen der von uns erforschten Naturgesetze. Die sprunghafte Zunahme unserer Kenntnisse in den letzten zwei Jahrzehnten verdanken wir vor allem den vielen Forschungssatelliten, die die Beobachtung kosmischer Objekte ohne die störenden Einflüsse unserer Atmosphäre erlauben. Speziell im weichen Röntgenbereich ist dadurch eine Beobachtung überhaupt erst möglich , und es ist gerade dieser Spektralbereich, der uns wesentlich neue Erkenntnisse über hochinteressante Objekte geliefert hat, da intensive Röntgenstrahlung nur unter sehr extremen physikalischen Bedingungen entsteht. Die in diesen Röntgenemissionsgebieten vorherrschenden Temperaturen, Magnet- und Gravitationsfelder sind so extrem, daß sie in irdischen Labors nicht realisiert werden können. Folglich bleibt nur die Möglichkeit, die Eigenschaften der Materie und die unter diesen Bedingungen ablaufenden physikalischen Prozesse theoretisch zu berechnen, um so - im Vergleich mit den Beobachtungen - zu zuverlässigen Aussagen über die Struktur dieser kosmischen Objekte zu gelangen.
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    (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, Dieter
    The 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.