Showing papers by "University of Stuttgart published in 1987"

Finite element methods with B-splines

Abstract: Preface 1. Introduction 2. Basic finite element concepts 3. B-splines 4. Finite element bases 5. Approximation with weighted splines 6. Boundary value problems 7. Multigrid methods 8. Implementation Appendix Notation and symbols Bibliography Index.

Investigation of Thermal Equilibrium Vacancies in Metals by Positron Annihilation

Abstract: The study of thermal vacancy formation in pure metals by positron annihilation and the determination of vacancy formation enthalpies is reviewed with particular emphasis to the potential of positron lifetime spectroscopy. The investigation of high-temperature vacancy migration by positron annihilation in pulse-heating experiments is outlined and first results are reported. Es wird ein Uberblick gegeben uber die Untersuchung von Leerstellen im thermischen Gleichgewicht in reinen Metallen mit Hilfe der Positronenzerstrahlung und uber die dabei gewonnenen Resultate der Leerstellenbildungsenthalpie. Hierbei wird insbesondere auf die spezifischen Moglichkeiten der Positronlebensdauerspektroskopie hingewiesen. Es wird zudem uber die Ergebnisse von Messungen der Positronenzerstrahlung in schnellen Aufheizexperimenten berichtet, mit denen die Leerstellenwanderung bei hohen Temperaturen untersucht werden kann.

Assemblage of ortho cleavage route for simultaneous degradation of chloro- and methylaromatics

Abstract: Genetic engineering is a powerful means of accelerating the evolution of new biological activities and has considerable potential for constructing microorganisms that can degrade environmental pollutants. Critical enzymes from five different catabolic pathways of three distinct soil bacteria have been combined in patchwork fashion into a functional ortho cleavage route for the degradation of methylphenols and methylbenzoates. The new bacterium thereby evolved was able to degrade and grow on mixtures of chloro- and methylaromatics that were toxic even for the bacteria that could degrade the individual components of the mixtures. Except for one enzymatic step, the pathway was fully regulated and its component enzymes were only synthesized in response to the presence of pathway substrates.

Numerical investigation of the three-dimensional development in boundary-layer transition

Abstract: A numerical method for solving the complete Navier-Stokes equations for incompressible flows is introduced that is applicable for investigating three-dimensional transition phenomena in a spatially growing boundary layer. Results are discussed for a test case with small three-dimensional disturbances for which detailed comparison to linear stability theory is possible. The validity of our numerical model for investigating nonlinear transition phenomena is demonstrated by realistic spatial simulations of the experiments by Kachanov and Levchenko1 for a subharmonic resonance breakdown and of the experiments of Klebanoff et al.2 for a fundamental resonance breakdown.

Heat and mass transfer in metal hydride reaction beds: Experimental and theoretical results

Abstract: A numerical model for transient heat and mass transfer within metal hydride reaction beds is presented. Theoretical predictions based on this model are compared with experimentally determined reaction rates and pressure-temperature distributions within the reaction beds. In addition, a mathematical treatment of the behaviour of coupled reaction beds is shown and as an example calculated hydrogen pressures are compared with experimental results.

Phase-locked modes, phase transitions and component oscillators in biological motion

Abstract: We review the results of joint experimental and theoretical work on coordinated biological motion demonstrating the close alliance between our observations and other nonequilibrium phase transitions in nature (e.g., the presence of critical fluctuations, critical slowing down). Order parameters are empirically determined and their (low-dimensional) dynamics used in order to explain specific pattern formation in movement, including stability and loss of stability leading to behavioral change, phase-locked modes and entrainment. The system's components and their dynamics are identified and it is shown how these may be coupled to produce observed cooperative states. This phenomenological synergetics approach is minimalist and operational in strategy, and may be used to understand other systems (e.g., speech), other levels (e.g., neural) and the linkage among levels. It also promotes the search for additional forms of order in multi-component, multi-stable systems.

Dimensionality dependence of the band-gap renormalization in two- and three-dimensional electron-hole plasmas in GaAs

Abstract: We have investigated the band-gap renormalization due to many-body effects in electron-hole plasmas in 2D GaAS-GaAlAs multiple quantum-well structures. A comparison of these data with corresponding 3D data and calculations for both dimensionalities shows that the band-gap shift increases absolutely but decreases in effective Rydberg units with decreasing dimensionality. The dimensionality dependence of the band-gap shift is traced to different screening efficiencies in 3D and 2D systems.

Quantum decay rates for dissipative systems at finite temperatures

Abstract: The decay of a metastable state of a system coupled to a heat-bath environment is studied. A functional-integral method is presented allowing for the calculation of decay rates at finite temperatures and in the presence of dissipation. The theory is utilized to determine the rate for a wide range of parameters. The temperature extends from the region where the decay is thermally activated down to very low temperatures where the system decays by tunneling from its ground state in the metastable well. The range of damping parameters covers the region from weakly damped to heavily overdamped motions. It is found that the transition between thermally activated decay and tunneling occurs near a crossover temperature ${T}_{0}$ which decreases with increasing damping strength. Well above ${T}_{0}$ the rate follows the classical Arrhenius law where the preexponential factor is affected by the frequency-dependent damping. As ${T}_{0}$ is approached, quantum corrections to the classical rate formula become increasingly important. In the vicinity of ${T}_{0}$ the rate follows a scaling law describing the crossover between thermally activated and quantum-mechanical decay. In the region below ${T}_{0}$ the decay rate can be determined analytically only in limiting cases. For a system with Ohmic dissipation and a cubic potential, accurate numerical calculations are presented exhausting the range of parameters not covered by analytical results.

Enzyme catalyzed cyanohydrin synthesis in organic solvents

Abstract: An identical ESR spectrum is observed for the diene 3 subjected to the same treatment; in line with the lower oxidation potential, the appearance voltage in the electrolysis is substantially lower than for 1 . The results of the ESR studies are thus fully consistent with the postulated cycloreversion 5-7 , so that the structure of the radical cation should be properly described by formula 7. Assignment of the large coupling constant (1.544 mT) to the eight equivalent p protons, i.e., to those that are separated by one C-C bond from the n system of 7, is straightforward, considering the D2/, symmetry. The coupling constants, aH”, of s protons arise from hyperconjugation and obey the relationship‘’] uH”= B.p”-cos’B, where p“ is the spin population at the adjacent n center and 6’ stands for the dihedral angle between the 2p, axis at this center and the C-HI’ bond. Since the total rr-spin population in 7 is evenly distributed among the four n centers, p” amounts to 0.25. Taking + 6 mT as the proportionality factor B appropriate for radical cations and setting B= 10” on the basis of structural data for 3,“’l one obtains uH11= + 1.5 mT. According to the evidence from TRIPLE, the positive sign thus predicted for 1.544 mT also holds for 0.051 mT, whence 0.1 17 and 0.065 mT must be negative. Assignments of the three last-mentioned coupling constants to the three sets of four equivalent y protons in specific positions of 7 are hardly feasible without further information. The colored radical cation (A,,,,, =610 nm) generated under similar conditions from [2.2. I . Ilpagodane 2 is also rather persistent; however, it decays faster than 7 . By analogy to 7, its structure should be described by formula 8. The ESR spectrum (g = 2.0040 -t 0.000 1) reflects the lower C,, symmetry of 8 by exhibiting two large coupling constants, 1.76 * 0.0 i and 0.96 k 0.0 I mT, each due to a set of four equivalent p protons. The hyperfine splittings of the sixteen y protons belonging to six different sets are not resolved in the ESR spectrum; they d o not exceed 0. I mT, as indicated by the ENDOR spectrum.’“’] The persistence of the radical cations 718 is closely connected with their polycyclic skeletons. Under a variety of oxidation conditions, i t was not possible, by ESR spectroscopy, to detect radical cations generated from the “molecular moieties” of 1/3 (8/9 in Ref. [2]). The opening of the four-membered ring, which occurs upon formation of the dications 11/12 from the pagodanes 112, presumably also occurs in 516 (ECE) but not in 9/10 (EEC). Owing to the small exothermicity of the opening 1 3 (AAH;‘= -2.4 kcal/mol),”’l and presumably also of 5-7 , and owing to the rigidity of the [I.l.l.l]pagodane 1 , there is a good chance that 5 might be identified directly at low temperature1’21 and that 5 and 7 as well as 9 and 11 might be differentiated in the gas-phase oxidation (“charge stripping”),[ 131

An extension of the Landau-Ginzburg-de Gennes theory for liquid crystals

Abstract: Using angular momentum representation a method is proposed that allows the systematic construction of a generalized Landau-de Gennes elastic free energy of liquid crystals, in powers of a symmetric and traceless tensor order parameter, polarization field, of external fields and all respective derivatives. By this method all linearly independent elastic invariants and surface terms are constructed for nematics and cholesterics up to fourth order terms. In particular it is shown that up to fourth order in the tensor order parameter there are nineteen bulk elastic constants and four surface terms in the free energy of a general, biaxial nematic. In addition, the stability of this expansion is studied in detail. Some special cases of the elastic free energy of liquid crystals, already discussed in the literature, are reexamined and discrepancies with our results are emphasized. Finally, a thermo-dynamically correct way of establishing contact between the generalized de Gennes elastic free energy and ...

Reliability analysis of parameter estimation in linear models with application to mensuration problems in computer vision

Abstract: The analysis of a mensuration problem aims at an evaluation of the suitability of the design of the measuring process for a specific task and at an assessment of the actually obtained measurements and of their influence onto the result. The concept of quality control, as it has been developed by the Netherlands geodesist W. Baarda is outlined. This theory provides objective quality measures, which take the geometry of the design and the used estimation and testing procedure into account: The evaluation of the design is based on measures for the precision, the controllability, and the robustness, which themselves can be used for planning purposes. The evaluation of the data is based on a statistical test, the estimated size of possible blunders and on the influence of the observed values onto the result. Three examples, namely template matching and absolute and relative orientation of cameras, demonstrate that the measures make intuitive evaluation precise and that they seem to besuitable for automatic quality control of mensuration problems encountered in computer vision.

Puzzles and polytope isomorphisms.

Abstract: We show that an isomorphism between the graphs of two simple polytopes of arbitrary dimension can always be extended to an isomorphism between the polytopes themselves. It has been convenient to study the dual situation, involving what we like to call the puzzle of a simplicial polytope.

A stable superconducting state at 8 K and ambient pressure in $$\alpha _t - (BEDT - TTF^ \star )_2 I_3 ^1 $$

Abstract: We report bulk superconductivity at 8 K and ambient pressure in crystals of α t (BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α t -crystals after tempering.

Gangliosides and Modulation of Neuronal Functions

Abstract: Gangliosides (= amphiphilic sialic acid containing glycosphingolipids) are components of all plasma membranes and as such ubiquitous in vertebrate tissues. They are especially abundant and complex in neuronal membranes, where they are anchored in the outer leaflet of the membrane bilayer by their hydrophobic ceramide moiety, while exposing their negatively charged hydrophilic and spacious sialo-oligosaccharide residue into the extracellular space (Fig. 1). Together with neutral glycolipids, glycoproteins and glycosaminoglycans, gangliosides comprise the glycocalix of the cell surface.

Structure and tensile behavior of irradiation-and peroxide-crosslinked polyethylenes

Abstract: Three grades of polyethylene differing mainly in their molecular weight were irradiation and peroxide crosslinked. Their gel content, degree of swelling, melting temperature, degree of crystallinity, and tensile properties were determined. The irradiation and peroxide crosslinking of the same polymers makes it possible to properly compare the effects of the two crosslinking methods. Upon irradiation the competition between crosslinking and chain scission reactions determines the level of the critical dose required to form the first gel and the magnitude of maximum attainable gel content. Crosslinking causes trapping of entanglements which then contribute to the effective network density determined by solvent swelling. crystalline polyethylene enhances the degree of crystallinity and crystal perfection while subsequent crystallization from the melt is hindered by the presence of crosslinks. The homogeneity of the crosslinked network, or distribution of crosslinks, depends on the crosslinking metho...

General relation between band-gap renormalization and carrier density in two-dimensional electron-hole plasmas.

Abstract: We have analyzed the dependence of the band-gap renormalization due to many-body effects on the carrier density in electron-hole plasmas in quasi-two-dimensional systems. A comparison of the data obtained at low lattice temperatures in various $\mathrm{GaAs}/{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$, GaSb/AlSb, and ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}/\mathrm{InP}$ multiple-quantum-well structures shows that the band-gap shift, if scaled in effective excitonic units, is independent of the material properties. This new general relation governing the dependence of the band-gap renormalization on the plasma density in two-dimensional systems is confirmed by theoretical considerations.

A dynamic phase transition model for spatial agglomeration processes.

Abstract: A nonlinear model of population migration is presented in order to provide a dynamic explanation for the formation of metropolitan areas. "In Section 2 the model is introduced in terms of the rate equations for the mean values of the regional population numbers with specifically chosen individual transition rates. Section 3 gives a survey of concepts and results for the convenience of the reader not interested in the details of the mathematical derivations. Section 4 derives the stationary solutions of the rate equations, that is, the equilibria of the system. Section 5 treats the time dependent solutions of the model equations focussing on the exact analytic solutions along so-called symmetry paths. Section 6 analyzes the dynamic stability of the symmetry path solutions and decides which stationary states are unstable and which are stable equilibrium states."

Influence of friction and temperature on coherent quantum tunneling

Abstract: The dynamics of a quantum particle tunneling through the barrier of an almost symmetric double-well potential is studied. The coherent quantum oscillations characteristic for isolated tunneling systems are shown to be strongly affected by the interaction with a heat bath environment. Attention is focused on the situation where the heat bath causes an Ohmic damping. This case is of potential relevance to the problem of macroscopic quantum coherence in SQUID rings. The range of parameters in which quantum coherence survives is determined and the time evolution of the occupation probabilities in the two wells is calculated.

Enzymkatalysierte Cyanhydrin‐Synthese in organischen Lösungsmitteln

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Group Commit Timers and High Volume Transaction Systems

Abstract: When "Group Commit Timers and High-Volume Transaction Systems" wasfirst published in the proceedings of the Second InternationalWorkshop on High-Performance Transaction Systems (Asilomar,California, September 1987), the graphs in Figures 4 and 5 wereaccidentally switched. The authors apologize for the error. Inthis publication, the graphs are correct.

Systematics of laser operation in GaAs/AlGaAs multiquantum well heterostructures

Abstract: Important laser parameters of GaAs/AlGaAs MQWH's were measured by means of optical gain spectroscopy. Unsaturated optical net gain spectra are carefully analyzed using a model for band-to-band transitions including momentum conservation and an energy-and density-dependent lifetime broadening. The calculated dependence of the peak gain on the carrier density agrees well with experimental data. The description of energetic positions of the peak gain has to include a bandgap shrinkage with carrier density, present at laser threshold. Temperature-dependent measurements of the onset of optical net gain reveal a pure thermodynamic behavior of the absolute threshold values as a function of L z and of the characteristic temperature T 0 .