Universität Stuttgart
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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 Excited electronic states of flavin-containing coenzyme models(1988) Gückel, Friedemann; Schweitzer, Dieter; Becker, Katja; Schirmer, Rolf Heiner; Zipplies, Matthias F.; Staab, Heinz A.In order to gain further insight into the physical basis of flavin-catalyzed reactions, the interactions of a flavin with a second flavin and with other aromatic ring systems were studied. For this purpose compounds through 1 were synthesized These compounds contain the interacting units in defined geometric orientation. A monomeric flavin and glutathione reductase a flavoenzyme of known active-site chemistry, were included as reference molecules. The present report deals with excited triplet states of the flavin compounds as studied by optical spectroscopy and optically detected magnetic resonances (ODMR) at 1.3K.Item Open Access Electrons and holes in InSb under crossed magnetic and stress fields. 1, Theory(1988) Trebin, Hans-Rainer; Wolfstädter, Bernd; Pascher, Harald; Häfele, Hans-GeorgIn a series of four papers magnetooptical transitions are presented for InSb crystals, which are subjected to uniaxial stress perpendicular to the magnetic field. Here, in the first paper, we establish an 8×8 k⋅p Hamiltonian matrix for stress Tǁ[100] and field Bǁ[001] and diagonalize it exactly. The dependence of valence and conduction states on stress and longitudinal momentum is discussed and compared with the geometry of parallel fields TǁBǁ[001]. Characteristic features are extracted for inter- and intraband transitions. Under crossed fields, the levels are separated much stronger with stress, yielding more insight than in the parallel configuration.Item Open Access Quasicrystal structures from the crystallographic viewpoint(1988) Gähler, Franz; Fröhlich, J. (Prof. Dr.)Quasikristalle sind neuartige Phasen, die in schnell abgekühlten Metall-Legierungen vorkommen. Ihre wichtigste Eigenschaft ist, dass Ihre Fourier-Transformierte aus scharfen Bragg-Peaks besteht, deren Positionen und Intensitäten eine Punktsymmetrie haben, die mit einer dreidimensionalen periodischen Struktur nicht verträglich ist. Die Positionen der Bragg-Peaks eines Quasikristalls können jedoch alle als ganzzahlige Linearkombinationen von endlich vielen fundamentalen Wellenvektoren geschrieben werden; dies legt nahe, diese Strukturen als Schnitt durch eine höherdimensionale, periodische Struktur aufzufassen.Item Open Access Quasiperiodic tilings : a generalized grid projection method(1988) Korepin, Vladimir E.; Gähler, Franz; Rhyner, JakobWe generalize the grid-projection method for the construction of quasiperiodic tilings. A rather general fundamental domain of the associated higher-dimensional lattice is used for the construction of the acceptance region. The arbitrariness of the fundamental domain allows for a choice which obeys all the symmetries of the lattice, which is important for the construction of tilings with a given non-trivial point-group symmetry in Fourier space. As an illustration, the construction of a two-dimensional quasiperiodic tiling with 12-fold orientational symmetry is described.Item Open Access Anwendung der synthetischen Holografie in der Meßtechnik(1988) Tiziani, Hans J.Ein wichtiges Einsatzgebiet ist die Prüfung asphärischer Flächen mit computergenerierten Hologrammen. Darüber wird in diesem Beitrag besonders berichtet. Asphärische Flächen werden zunehmend in optischen Geräten und Systemen zur Realisierung extremer Eigenschaften bezüglich Öffnungsverhältnis und Abbildungsfehlerkorrektur verwendet. Die Herstellung derartiger Flächen erfordert zwangsläufig ein Meßgerät, das eine Qualitätskontrolle der erreichten Flächenform mit Genauigkeiten von 0,1 μm und besser ermöglicht. Die geforderten Genauigkeiten liefern interferometrische Meßmethoden, die außerdem eine visuelle und quantitative Beurteilung der gesamten Fläche erlauben.Item Open Access Testing of aspheric surfaces with computer generated holograms(1988) Tiziani, Hans J.; Packroß, Bernd; Schmidt, GerhardAspherical surfaces are becoming more important and can even be mass producted. There is a need for flexible test methods of high accuracy. The paper describes the combination of a computer generated holograms with a partially compensating lens as a powerful tool. An example for testing a steep aspheric surface will be given.Item Open Access Electrons and holes in InSb under crossed magnetic and stress fields. 4, Stimulated Raman scattering in the valence bands(1988) Wolfstädter, Bernd; Trebin, Hans-Rainer; Pascher, Harald; Häfele, Hans-GeorgIn a series of four papers magnetooptical transitions are presented for InSb crystals, which are subjected to uniaxial stress perpendicular to the magnetic field. Here, in the first paper, we establish an 8×8 k⋅p Hamiltonian matrix for stress Tǁ[100] and field Bǁ[001] and diagonalize it exactly. The dependence of valence and conduction states on stress and longitudinal momentum is discussed and compared with the geometry of parallel fields TǁBǁ[001]. Characteristic features are extracted for inter- and intraband transitions. Under crossed fields, the levels are separated much stronger with stress, yielding more insight than in the parallel configuration.Item Open Access Temperature and pressure dependence of the resistivity of β-(BEDT-TTF)2X (X=I3, I2Au) and αt-(BEDT-TTF)2I3(1988) Weger, Meir; Bender, Klaus; Klutz, Thomas; Schweitzer, Dieter; Gross, Frieder; Heidmann, Claus Peter; Probst, Christian; Andres, KlausThe temperature dependence of the resistivity of the organic metals and superconductors {β-(BEDT-TTF)2I3, β-{3-(BEDT-TTF)2I2Au and αt(BEDT-TTF)2I3 was measured at ambient and at several isotropic pressures (up to 2.5 kbar). By the application of a relatively low pressure, it is possible to measure the term in the resistivity that is linear in temperature. This term is masked by the larger T2 term at ambient pressure. It is shown that in the temperature range below 120 K, the external modes are mainly responsible for the resistivity, while above this temperature the flipping of the CH2 groups becomes important. The linear electron-phonon coupling corresponds to a value of λ between 1 and 1.5.Item Open Access Polarization distributions in isotropic, stretched or annealed PVDF films(1988) Bihler, Eckardt; Holdik, Karl; Eisenmenger, WolfgangThe spatial distribution of the polarization in polyvinylidene fluoride (PVDF) films was measured at room temperature with the PPS (piezoelectric pressure step) method. In order to investigate the time development under external fields, a thin insulation polyethylene terephthalate (PET) film covering an evaporated aluminium electrode was inserted between the sample and the measuring electrode. The observed development of inhomogeneous and internal polarization zones is attributed to charge injection and charge trapping at the polarization zone boundaries. The polarization zone develops at a position where the critical field strength for dipole orientation in the crystallites is exceeded by the approach of injected homocharges (or without injection by the depletion of internal homocharges and the excess of heterocharges). The critical field for α-crystallites corresponds to an electric field phase transition at 1.2 MV/cm. The development of central polarization zones in PVDF containing β-crystallites indicates injection of charges with both signs and almost equal mobility. The results indicate that the β-crystallites determine the charge injection rate or the mobility or both.