Universität Stuttgart
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Item Open Access Polarization distribution in PVDF obtained by poling under constant current condition(1990) Neumann, Guido; Bihler, Eckardt; Eberle, Gernot; Eisenmenger, WolfgangSince the resistivity of PVDE films increases when the films are poled, the voltage across the samples must be increased during the poling process in order to keep the current constant. Under these conditions, the formation of a polarization zone in the center of the film depth is observed. The thickness of the polarization-free zone close to the film surfaces is found to decrease with larger current densities. The results are consistent with a model assuming charge trapping in deep traps associated with the orientation of polar crystallites.Item Open Access 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, CorvinUnser 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.Item Open Access Phono-conduction spectroscopy of shallow states in semiconductors(1990) Lassmann, Kurt; Gienger, Martin; Groß, PeterPhonoconduction spectroscopy of shallow states in semiconductors by help of superconduction Al-junctions as phonon sources with high spectral resolution is a sensitive new technique with phonon frequencies well up in the Debye range. The large k-vector of the phonons involved opens new possibilities for the investigation of electron dynamics and states in semiconductors.Item Open Access Proton relaxation in the superconducting organic solid (BEDT-TTF)2Cu(NCS)2: evidence for relaxation by localized paramagnetic centers(1990) Klutz, Thomas; Haeberlen, Ulrich; Schweitzer, DieterThe nonexponential character of the build-up of the nuclear magnetization usually increases on lowering the temperature. Superconducting fluctuations have been offered in a speculative way as an explanation for the nonexponential spin relaxation. We have observed the same relaxation behaviour in our investigation of the proton spin relaxation in (BEDT-TTF)2Cu(NCS)2 which is an organic conductor at room temperature and becomes a superconductor at Tc=10.4 K at ambient pressure. As we noticed during preliminary measurements that the degree of deviation from exponential relaxation depends on the sample under study we decided to do experiments under controlled conditions of sample preparation.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 High frequency phonon transmission through amorphous films(1990) Mebert, Joachim; Maile, Bernd; Eisenmenger, WolfgangWe now report on phonon spectroscopy measurements in the frequency range from 300 GHz - 1 THz on thin amorphous films of several materials (i.e. e-beam evaporated SiO2., α-Si and α-Ge). Some films were prepared in a residual gas atmosphere as H20 and N2.Item Open Access Dual wavelength heterodyne interferometry for rough surface measurements(1990) Fischer, Edgar; Sodnik, Zoran; Ittner, Thomas; Tiziani, Hans J.For interferometric topography measurements of optically rough surfaces dual wavelength heterodyne Interferometry (DWHI) is a powerful tool. A DWHI system based on a two-wavelength HeNe laser and a matched grating technique is described. This set-up improves system stability and simple heterodyne frequency generation.Item Open Access Influence of charge injection on the formation of remanent polarization in P(VDF-TrFE) copolymers(1990) Bihler, Eckardt; Neumann, Guido; Eberle, Gernot; Eisenmenger, WolfgangThe influence of charge injection on the formation of remanent polarization of P(VDF-TrFE) copolymers was studied by inserting an insulating barrier between the metal electrodes and the P(VDF-TrFE) copolymer during the poling process. The resulting remanent polarization is significantly smaller than that of identically poled samples for the same field strength and time duration, but with directly contacting metal electrodes. This shows that the orientation of the dipoles alone is not a sufficient condition for the formation of remanent polarization in PVDF and its copolymers. The experiments indicate that the stabilization of the polarization is mediated by charge injection and charge trapping at polarized crystallite surfaces.Item Open Access Structural aspects of organic superconductors(1990) Schweitzer, DieterSome physical properties of the organic superconductors with the highest transition temperatures are presented. In addition the influence of small structural changes on the superconducting transition temperature is discussed. Finally bulk superconductivity in polycrystalline pressed pellets of organic metals is reported.Item Open Access Classification of space groups for quasicrystals with (2+1)-reducible point group(1990) Gähler, FranzQuasicrystalline structures can be viewed as a cut through a periodic structure in a higherdimensional space. In this paper, the crystallography of 5-dimensional periodic structures which can give rise to a quasi crystal in this way is developed. For the five 5d Bravais classes giving rise to quasicrystals periodic in one direction and with symmetry of order 5, 8, 10 or 12 in the plane perpendicular to that direction, a complete classification of point groups and space groups is given. We find a total of 150 distinct space groups, which can be attributed to 44 point groups. For each of these space groups, a set of generators of its point group and the associated non-primitive translations are tabulated, and for the non-symmorphic space groups the indices of the systematic extinctions are given. All the results are expressed in terms of a geometrically natural lattice basis.