Universität Stuttgart
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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 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 BEDT-TTF radical salts: organic metals and superconductors(1987) Schweitzer, DieterA review of the structural, electronic and superconducting properties of some BEDT-TTF-radical salts is given.Item Open Access Kinematics and dynamics for computer animation(1994) Ruder, Hanns; Ertl, Thomas; Gruber, Karin; Günther, Michael; Hospach, Frank; Ruder, Margret; Subke, Jörg; Widmayer, KarinThis tutorial will focus on the physical principles of kinematics and dynamics. After explaining the basic equations for point masses and rigid bodies a new approach for the dynamic simulation of multi-linked models with wobbling mass is presented, which has led to new insight in the field of biomechanics, but which has not been used in computer animation so far.Item Open Access Molecular metals and superconductors: BEDT-TTF radical salts(1987) Schweitzer, Dieter; Keller, Heimo J.The first discovery of superconductivity in an organic metal - in the radical salts of TMTSF (tetramethyltetraselenafulvalene) - under pressure and ambient pressure has demonstrated that in addition to the usual intrastack contacts between the donor molecules in quasi one dimensional metals intermolecular contacts between molecules in neighbouring stacks are important. These interstack contacts result in a less pronounced one dimensional electronic behaviour leading to a stabilization of the metallic character down to low temperatures.Item Open Access Shallow traps correlated with deep impurities in silicon as obtained by phonon induced conductance(1986) Burger, Wilfried; Lassmann, Kurt; Holm, Claus; Wagner, PeterAt low temperatures shallow neutral donors and acceptors in silicon can bind an extra carrier to form the so-called D- and A+ centers. With the method of phonon-induced electrical conductivity (PIC) we find the same threshold energies for the detachment of these carriers associated with the shallow impurities P and B, as have been obtained previously by FIR measurements. This shows that the detachment is by a one-phonon process. We find that there is no central cell correction for the binding to the deeper acceptors Al and Ga, whereas for In+ the binding energy is as large as 5,8 meV. We interprete this dependence on acceptor species as another example of the shallow-deep instability of the binding energy with the variation of the central cell potential.Item Open Access Influence of defects on the splitting of the acceptor ground state in silicon(1984) Ambrosy, Anton; Lassmann, Kurt; Goer, Anne M. de; Salce, Bernard; Zeile, Heinrich-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 Diffraction of atoms from optical potentials(1994) Pfau, Tilman; Schnetz, Andreas; Adams, Charles S.; Kurtsiefer, Christian; Sigel, Martin; Mlynek, JürgenIn the first part of this article we attempt to provide a very brief introduction to atom optics. In the second part we report on a recent experiment in our group on an atom-optical element 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".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.