Universität Stuttgart
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Item Open Access Resonant Raman scattering from superconducting single crystals of (BEDT-TTF)2I3(1993) Pokhodnia, Konstantin I.; Graja, Andrzej; Weger, Meir; Schweitzer, DieterA 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.Item Open Access Matching rules for quasicrystals : the composition-decomposition method(1993) Gähler, FranzA general method is presented which proves that an appropriately chosen set of matching rules for a quasiperiodic tiling enforces quasiperiodicity. This method, which is based on self-similarity, is formulated in general terms to make it applicable to many different situations. The method is then illustrated with two examples, one of which is a new set of matching rules for a dodecagonal tiling.Item Open Access Liquid-crystalline blue phase III and structures of broken icosahedral symmetry(1993) Longa, Lech; Fink, Werner; Trebin, Hans-RainerThe structure of the liquid-crystalline blue phase III (BPIII) is still unknown and remains one of the mysteries of liquid-crystal physics. We take all icosahedral space-group symmetries of the reciprocal space for BPIII and study their thermodynamic stability within the frame of an extended de Gennes–Ginzburg–Landau free-energy expansion. The stability of the icosahedral structures is compared with that of the cholesteric phase and of the cubic blue phases. Strikingly, even though the extended model contains three extra parameters, we could not detect a region of parameter space where icosahedral structures are absolutely stable just below the isotropic phase.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 Optical elements for atoms : a beamsplitter and a mirror(1993) Sigel, Martin; Pfau, Tilman; Adams, Charles S.; Kurtsiefer, Christian; Seifert, Werner; Heine, Claus; Mlynek, Jürgen; Kaiser, Robin; Aspect, AlainIn the first part of this article we attempt to provide a very brief introduction to atom optics. In the second and third part we report on recent experiments in our group on two specific atom-optical elements that may be useful in atom interferometers and atom cavities: the demonstration of a new beam splitter based on the diffraction of atomic matter waves from a "magneto-optical grating" and the investigation of a mirror based on the reflection of atoms from an evanescent light field.Item Open Access Conformal degeneracy and conformal diffusion of vesicles(1993) Jülicher, Frank; Seifert, Udo; Lipowsky, ReinhardThe shape of vesicles with genus g=2, i.e., with two holes or two handles, is studied in the framework of curvature models. These vesicles exhibit a new phase which also persists for higher genus g>2. In this phase, the ground state of the vesicle is conformally degenerate even when the volume, the area, and the total mean curvature of the vesicle are kept constant. It is predicted that such vesicles undergo a new type of diffusive motion, termed conformal diffusion, which should be observable in experiments as pronounced shape fluctuations.Item Open Access Atomic beam splitters based on light(1993) Adams, Charles S.; Pfau, Tilman; Mlynek, JürgenIn this paper we review techniques to coherently split an atomic beam using the dipole force. We discuss the interaction of atoms with normal standing wave light field in the context of atomic beam splitters. The case where the atom enters the standing wave at a small angle such that Bragg diffraction and velocity selective resonances are observed is also considered. An alternative approach to realize a coherent beam splitter based on the optical Stem-Gerlach effect is discussed. Finally we consider the interaction of atoms with a magneto-optical potential formed by a polarization gradient light field and a static magnetic field. We show that the Stem-Gerlach effect and the magneto-optical potential produce a more effective beam splitting into states with high tranverse momentum than diffraction from a normal standing wave.Item Open Access Impulse response measurement of individual ear canals and impedances at the eardrum in man(1993) Joswig, ManfredDie Messung der Impulsantwort ist eine einfache Methode zur gleichzeitigen Bestimmung der individuellen Querschnittsfunktion des menschlichen Gehörgangs und der Abschlußimpedanz an der Stelle des Trommelfells in einem Frequenzbereich von 1-20 kHz. Zuerst wird die theoretische und numerische Lösung für das inverse Problem del Berechnung einer beliebigen Querschnittsfunktion aus der Impulsantwort angegeben. Zur praktischen Überprüfung dieser Methode wurden einige Versuchsmessungen an speziaiangefertigten Messingrohren ausgeführt, welche hervorragende Ergebnisse liefern. Auch stimmen die Impedanzwerte gut mit der Horn-Impedanz überein, die analytisch berechnet wurde. Des weiteren wird über Einzelergebnisse der Messung am Menschen für acht Personen berichtet.Item Open Access Fibre-optic probe hydrophone for ultrasonic and shock-wave measurements in water(1993) Staudenraus, Joachim; Eisenmenger, WolfgangAimed at lithotripter acoustic output measurements, a new fibre-optic probe hydrophone overcomes most of the problems involved with the use of piezoelectric hydrophone technology in non-linear ultrasonic and shock-wave fields. The fibre-optic principle allows for extremely wide bandwidth (larger than 1 GHz) and superior electromagnetic shielding. Contrary to hitherto existing hydrophones a high cavitation threshold at the water-silica interface provides undistorted detection of strong rarefractional pulse pressures. Considering pure compression there is good agreement between maximum pulse pressure derived from fibre-optic hydrophone theory and the corresponding amplitudes obtained from acoustically calibrated PVDF membrane and needle hydrophones.Item Open Access Double pulse-electronic speckle interferometry(1993) Pedrini, Giancarlo; Pfister, Berthold P.; Tiziani, Hans J.In this paper we describe a double-pulsed electronic speckle interferometry system. Two separated speckle patterns are recorded within few microseconds using a charge-coupled device camera, the two images are then stored in a frame grabber and the subtraction method is applied. A quantitative analysis of the fringes using the spatial-carrier phase-shift method is presented. The results show that this method is well suited to study transient vibrations.