Showing papers by "University of Stuttgart published in 1988"

Pseudopotentials for main group elements (IIIa through VIIa)

Abstract: Semiempirical pseudopotentials including core-polarization potentials have been generated for main group elements IIIa through VIIa, with main quantum numbers n ⩽ 5. Test calculations with these pseudopotentials have been performed for atomic ionization energies.

The Random Walk Method in Pollutant Transport Simulation

Abstract: Standard finite difference and finite element solution methods of the pollutant transport equation require restrictive spatial discretization in order to avoid numerical dispersion. The random walk method offers a robust alternative if for reasons of calculational effort discretization requirements cannot be met. The method is discussed for the case of an ideal tracer starting out from the Ito-Fokker-Planck-equation. Features such as chemical reactions and adsorption can be incorporated. Besides being an alternative to other solution methods for the classical transport equation the random walk deserves attention due to its generalizability allowing the incorporation of non-Fickian dispersion. A shortcoming of the method results from the general roughness of simulated distributions in space and time due to statistical fluctuations and resolution problems. The method is applied to a field case of groundwater pollution by chlorohydrocarbons.

Structure of nanometer-sized polycrystalline iron investigated by positron lifetime spectroscopy.

Abstract: Nanometer-sized polycrystalline materials are polycrystals prepared by compacting very small crystallites (5--10 nm in diameter) under high pressures. The initial studies of Gleiter and co-workers indicate a wide distribution of interatomic distances within the disordered intercrystalline phase, which can be investigated accurately due to its high relative volume fraction in these materials. In the present paper the investigation of nanometer-sized Fe polycrystals by positron lifetime spectroscopy is reported. The influence of the compacting pressure and thermal annealing was studied. The positron lifetimes ${\ensuremath{\tau}}_{1}$=180\ifmmode\pm\else\textpm\fi{}15 ps, ${\ensuremath{\tau}}_{2}$=360\ifmmode\pm\else\textpm\fi{}30 ps, and long-lived components between 1 and 5 ns, have been observed with saturation trapping of positrons. These values are different from the positron lifetimes in well-annealed bulk iron, in amorphous iron alloys, or in the uncompacted fine nanometer-sized iron crystals (\ensuremath{\tau}=443 ps). Based on the present results the lifetime ${\ensuremath{\tau}}_{1}$ in nanometer-sized Fe polycrystals is attributed to positron trapping in vacancy-size free volumes in the crystallite interfaces. This is in agreement with the hypothesis of an interfacial structure with a wide distribution of interatomic distances. The lifetime ${\ensuremath{\tau}}_{2}$ is ascribed to positron annihilation in microvoids at the intersections of interfaces. Hence, positron lifetime spectroscopy on nanometer-sized polycrystalline materials appears to supply a specific tool for studying the interfacial structure of solids. The long-lived components indicate ortho-positronium (o-Ps) formation in larger voids.

Metabolism of 2,4-dichlorophenoxyacetic acid, 4-chloro-2-methylphenoxyacetic acid and 2-methylphenoxyacetic acid by Alcaligenes eutrophus JMP 134

Abstract: Of eleven substituted phenoxyacetic acids tested, only three (2,4-dichloro-, 4-chloro-2-methyl- and 2-methylphenoxyacetic acid) served as growth substrates for Alcaligenes eutrophus JMP 134. Whereas only one enzyme seems to be responsible for the initial cleavage of the ether bond, there was evidence for the presence of three different phenol hydroxylases in this strain. 3,5-Dichlorocatechol and 5-chloro-3-methylcatechol, metabolites of the degradation of 2,4-dichlorophenoxyacetic acid and 4-chloro-2-methylphenoxyacetic acid, respectively, were exclusively metabolized via the ortho-cleavage pathway. 2-Methylphenoxyacetic acid-grown cells showed simultaneous induction of meta- and ortho-cleavage enzymes. Two catechol 1,2-dioxygenases responsible for ortho-cleavage of the intermediate catechols were partially purified and characterized. One of these enzymes converted 3,5-dichlorocatechol considerably faster than catechol or 3-chlorocatechol. A new enzyme for the cycloisomerisation of muconates was found, which exhibited high activity against the ring-cleavage products of 3,5-dichlorocatechol and 4-chlorocatechol, but low activities against 2-chloromuconate and muconate.

Reduction of quasilinear elliptic equations in cylindrical domains with applications

Abstract: For certain types of semilinear partial differential equations in a cylindrical domain small bounded solutions are known to lie on a so-called centre manifold of finite dimension. In this paper we prove the result for quasilinear partial differential equations of elliptic and parabolic type. Possible applications include free surface waves and problems in non-linear elasticity. Here we investigate, pars pro toto, the stationary inviscid potential flow in a channel with a free surface and an obstacle at the bottom.

Extended Luenberger observer for non-linear multivariable systems

Abstract: For non-linear multiple-input multiple-output systems [xdot] = f(x, u), y = h(x, u), nonlinear observers are designed using a transformation into the non-linear observer canonical form and an extended linearization The differential equation of observer error in canonical coordinates is linearized about the reconstructed trajectory, and dimensioned by eigenvalue assignment With reference to the extended Kalman filter algorithm, this non-linear observer design is called the extended Luenberger observer This observer design is possible for all sufficiently smooth and locally observable systems In comparison with single-output systems, the non-linear observer design can be essentially simplified using the degrees of freedom available in the case of multiple outputs

Nonlinearly Resonant Surface Waves and Homoclinic Bifurcation

Abstract: Publisher Summary This chapter examines the nonlinearly resonant surface waves and homoclinic bifurcation. The behavior of steady nonlinear water waves on the surface of an inviscid heavy fluid layer has received much attention, both from the mathematical and from the physical side. The basic equations for the interaction of traveling nonlinear surface waves with in-phase external pressure waves are derived. An inviscid fluid layer of mean depth is considered under gravity. On its free upper surface, where capillary forces may also act, it supports nonlinear surface waves of permanent form, traveling from right to left with constant speed. The upper bound could be estimated explicitly by quantitatively showing the validity of subsequently described, almost identical transformations and estimates on noncritical eigenvalues of the operator. The linear dispersion relation for cnoidal waves corresponds to the imaginary eigenvalues. It is found that the reduced phase space is four-dimensional and the existence of a homoclinic orbit is equivalent to the intersection of two curves. The existence of solitary waves on a curve in the parameter space is also elaborated.

A unified scheme for processing GPS dual-band phase observations

Abstract: A unified computational scheme is presented for sequential least-squares processing ofGPS dual-band carrier-beat-phase observations in network-mode positioning with orbit relaxation, and in orbit determination applications. This scheme is applicable to any spatial and temporal distribution of stations and satellites during a particularGPS experiment. Full covariance matrices can be specified for carrier-beat-phases and for weighted constraints on the ionosphere in order to improve phase ambiguity resolution. Physically meaningful choices for these covariance matrices are developed.

Enzyme‐Catalyzed Synthesis of 1‐Deoxymannojirimycin, 1‐Deoxynojirimycin, and 1,4‐Dideoxy‐1,4‐imino‐D‐arabinitol

Abstract: The title compounds 1, 2, and 3 are highly effective glycosidase inhibitors. They were synthesized from achiral starting compounds by enzyme-catalyzed CC coupling of dihydroxyacetone phosphate (1, 2) or hydroxypyruvate (3) with racemic 3-azido-2-hydroxypropanal via fructose, sorbose and xylulose derivatives and their subsequent enantioselective catalytic hydrogenation.

High-Tc superconductivity induced by ferromagnetic clustering

Abstract: We propose a theoretical model which allows to explain high- T c superconductivity in perovskites. It starts from a Hubbard-type Hamiltonian and from a description of the antiferromagnetic state. We show that when doping with holes immobile ferromagnetic clusters are formed which at higher hole concentrations percolate to metallic lines. The superconductivity occurring along these one-dimensional lines is caused by deformation coupling to Jahn-Teller active optical vibrations. Only a small isotope effect is predicted. The model also explains the spin-glass behaviour for intermediate concentrations and the destruction of superconductivity at high concentrations.

Measuring spatial orientation of short fiber reinforced thermoplastics by image analysis

Abstract: A computer-based measuring strategy to determine spatial orientation of short glass fibers from the cross-section of single fibers is presented A way of correcting the inclination-dependent probability of hitting a fiber is shown as well as the correction of fiber-end intersections The latter is done by pattern recognition and application of a set of FORTRAN subroutines 9 references

Pattern recognition and associative memory as dynamical processes in a synergetic system

Abstract: We first present numerical results for the decomposition procedure for complex scenes described in Part I of these papers. Part II then mainly deals with a formalism that allows a formulation of our approach to pattern recognition and associative memory that is simultaneously invariant against translation, rotation, and scaling, Part II thus contains an explicit elaboration of ideas of Part I.

Variations in vocal patterns of Senegal and South African lesser bushbabies and their implications for taxonomic relationships.

Abstract: Vocalizations of Senegal and South African lesser bushbabies were compared with respect to their acoustic properties analysed from spectrograms and oscillograms. Homologous calls could be identified w

A combined density functional and configuration interaction method

Abstract: Correlation energies are divided into two parts. One contribution is given by a configuration interaction calculation in the space of the natural orbitals with occupation numbers larger than an arbitrary threshold u. The remaining part is obtained from a u-dependent functional of the electronic density. Representative examples (for which the existing spin-density functionals fail) are (1) the correlation energies in the He and Be series and (2) the contribution of the correlation energy to the dissociation energy of the first-row dimers. It is shown that even for large values of v the errors remain on the order of 0.01 hartree.

Pattern recognition and associative memory as dynamical processes in a synergetic system. I. Translational invariance, selective attention, and decomposition ofscenes

Abstract: We consider a model for associative memory and pattern recognition which was devised by Haken (1987b). This model treats the activity of the neurons as continuous variables and exploits an analogy with pattern formation in synergetic systems. The capability of such a system to act as associative memory is demonstrated by the reconstruction of faces which are partially offered to the system, and which are restored by the corresponding dynamical process. We demonstrate how this model can be cast into a form which is translation invariant and how partially hidden faces in scenes can be recognized by means of the control of attention parameters of specific patterns.

Synthesis and Optical Properties of Terminally Substituted Conjugated Polyenes

Abstract: Long-chain, conjugated polyenes can function as molecular wires. This is shown by the photophysical properties of polyenes containing naphthyl, anthryl, and tetraphenylporphyrinyl (TPP) terminal groups. When the anthryl group in 1, which is obtainable by a sequence of Wittig reactions, is selectively excited, strong TPP emission is observed.

On Asymptotic Error Estimates for Combined Bem and Fem

Abstract: As can be seen from several recent engineering applications, the coupling of finite elements with boundary elements plays an increasingly important role in computational methods. This paper presents a survey on the corresponding current mathematical analysis in the framework of asymptotic convergence and error estimates. The presented analysis covers classical coupling with boundary element Galerkin methods as for the Laplacian and corresponding variants with faster mesh refinement of the boundary elements on the coupling boundary or — alternatively — of the finite elements which already apply to a large class of elliptic boundary value problems. Further we present recent symmetric Galerkin formulations for general strongly elliptic boundary value problems in accordance with the Hellinger-Reissner principle. For two-dimensional problems we present some new results for boundary element collocation on the coupling boundary.

Adaptive boundary element methods for strongly elliptic integral equations

Abstract: Integral operators are nonlocal operators. The operators defined in boundary integral equations to elliptic boundary value problems, however, are pseudo-differential operators on the boundary and, therefore, provide additional pseudolocal properties. These allow the successful application of adaptive procedures to some boundary element methods. In this paper we analyze these methods for general strongly elliptic integral equations and obtain a-posteriori error estimates for boundary element solutions. We also apply these methods to nodal collocation with odd degree splines. Some numerical examples show that these adaptive procedures are reliable and effective.

Saint-Venant's problem and semi-inverse solutions in nonlinear elasticity

Abstract: Saint-Venant's problem consists in finding elastic deformations of an infinite prismatic body taking given values for the cross-sectional resultants of force and moment. Using the center manifold approach we show that all deformations having sufficiently small bounded strains lie on a finite-dimensional manifold. In particular, the flow on this manifold is described by a set of equations having exactly the form of the classical rod equations. Moreover, the set of semi-inverse solutions can be analyzed locally.

Influence of the processing conditions on morphology and deformation behavior of poly(butylene terephthalate) (PBT)

Abstract: This paper discusses the effect of varying cooling rates during crystallization on the spherulitic structure of poly(butalene terephthalate). Light microscopy, transmission electron microscopy, and differential scanning calorimetry were used to examine specimens. The mechanical properties of the polymers studied were examined by a variety of methods.

Relaxation dispersion and zero-field spectroscopy of thermotropic and lyotropic liquid crystals by fast field-cycling N.M.R.

Abstract: Systematic field-cycling measurements of the T 1 relaxation dispersion in numerous nematic liquid crystals (azoxybenzenes, Schiff's bases, biphenyls, phenyl-cyclohexanes, cyclo-cyclo-hexanes) confirm our previous observations obtained for PAA and MBBA that order fluctuations of the nematic director are a significant relaxation contribution only at low Lamor frequencies v, i.e. far below the usual megahertz range. Their significance is demonstrated most convincingly by the characteristic square-root dispersion law, T 1 ∼ v 1/2, which occurs in the kilohertz range and which completely disappears above the nematic–isotropic phase transition. The strength of the collective relaxation mechanism varies by more than two orders of magnitude in the sequence (selection) PAA-d 8 PAA, PAA-d 6 PAB, OCB7, MBBA, CB7, PCH7, MBBA-d 6 MBBA-d 13 and CCH7. This finding can be understood almost quantitatively by the widely differing separations and orientations of proton pairs on the molecules, together with the diff...

Nonradiative recombination via deep impurity levels in semiconductors: The excitonic Auger mechanism.

Abstract: We present a theoretical investigation of excitonic Auger recombination via deep impurity levels in semiconductors. A calculation of the transition matrix elements is carried out for several levels of approximation. The influence of carrier density and temperature is studied by taking into account the screening of the Coulomb interaction at high carrier densities and the thermal ionization of the excitons. The results of our calculations are then compared to our experimental results, which originally led to the development of our model of excitonic Auger recombination via deep impurities.

Bacterial catabolism of sulfanilic acid via catechol-4-sulfonic acid

Abstract: A sulfanilic acid (4-aminobenzenesulfonic acid) degrading culture consisting of two strains (strain S1 and S2), was studied. Only strain S1 was able to attack sulfanilic acid. When strain S1 was cultavated in a mineral medium with sulfanilic acid an intensive violet colour was observed. The accumulating metabolite was isolated from the culture supernatant. By comparison with an authentic compound the metabolite was identified as catechol-4-sulfonic acid by thin layer and high performance liquid chromatography and by UV- and H-NMR spectroscopy. The occurrence of catechol-4-sulfonic acid indicates that there is no release of the sulfonic group before ring cleavage.