Showing papers by "Johannes Kepler University of Linz published in 1998"
TL;DR: In this article, the elastic anisotropy in these artificial dot crystals acts in a manner similar to that of the directed chemical bonds of crystalline solids, which may be advantageous for optoelectronic device applications.
Abstract: The self-organization of pyramidal PbSe islands that spontaneously form during strained-layer epitaxial growth of PbSe/Pb1−xEuxTe (x = 0.05 to 0.1) superlattices results in the formation of three-dimensional quantum-dot crystals. In these crystals, the dots are arranged in a trigonal lattice with a face-centered cubic (fcc)–like A-B-C-A-B-C vertical stacking sequence. The lattice constant of the dot crystal can be tuned continuously by changing the superlattice period. As shown by theoretical calculations, the elastic anisotropy in these artificial dot crystals acts in a manner similar to that of the directed chemical bonds of crystalline solids. The narrow size distribution and excellent control of the dot arrangement may be advantageous for optoelectronic device applications.
397 citations
TL;DR: A new product cipher which encrypts large blocks of plaintext by repeated intertwined application of substitution and permutation operations is introduced, which offers many features that make them superior to contemporary bulk encryption systems when aiming at efficient image and video data encryption.
Abstract: To guarantee security and privacy in image and video archival applications, efficient bulk encryption techniques are necessary which are easily implementable in soft- and hardware and are able to cope with the vast amounts of data involved. Experience has shown that block-oriented symmetric product ciphers constitute an adequate design paradigm for resolving this task, since they can offer a very high level of security as well as very high encryption rates. In this contribution we introduce a new product cipher which encrypts large blocks of plaintext by repeated intertwined application of substitution and permutation operations. While almost all of the current product ciphers use fixed (predefined) permutation operations on small data blocks, our approach involves parametrizable (keyed) permutations on large data blocks (whole images) induced by specific chaotic systems (Kolmogorov flows). By combining these highly unstable dynamics with an adaption of a very fast shift register based pseudo-random number generator, we obtain a new class of computationally secure product ciphers which offer many features that make them superior to contemporary bulk encryption systems when aiming at efficient image and video data encryption. © 1998 SPIE and IS&T.
196 citations
TL;DR: In this paper, social representation theory is scrutinized for its capacity to ask new questions and to give new answers to social psychological problems, and its social constructionist implications and relationship to brute facts are investigated.
Abstract: Social representation theory is scrutinized for its capacity to ask new questions and to give new answers to social psychological problems. Its social constructionist implications and relationship to brute facts are investigated. It is shown that social representations result from collective symbolic coping with 'brute' facts. Consequently representations create the domesticated world of social objects which implies considering activity as part of a representation. Culture change in modern societies is shown to produce 'cognitive polyphasia' by adding alternative representations to existing ones instead of replacing them.
168 citations
TL;DR: In this article, the role of Trp proteins in store regulation of endothelial membrane conductances was tested by expression of an N-terminal fragment of hTrp3 (N-TRP), which exerts a dominant negative effect on Trp channel function presumably due to suppression of channel assembly.
165 citations
TL;DR: In this paper, a pump-probe transmission experiment was performed on PbSe above the fundamental absorption edge near 4 \ensuremath{mu}m in the temperature range 30 to 300 K, using the Dutch ps free-electron laser.
Abstract: Pump-probe transmission experiments have been performed on PbSe above the fundamental absorption edge near 4 \ensuremath{\mu}m in the temperature range 30 to 300 K, using the Dutch ps free-electron laser. For temperatures below 200 K and carrier densities above the threshold for stimulated emission, stimulated recombination represents the most efficient recombination mechanism with relatively fast kinetics in the 50--100-ps regime, in good agreement with earlier reports of photoluminescent emission. Above this temperature Auger recombination dominates, and the Auger coefficient C is determined from the pump-probe decay curves. In the low-temperature regime the Auger coefficient is determined from the decay curves at times beyond 100 ps. The Auger coefficient is approximately constant (with a value of about $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}28} {\mathrm{cm}}^{6}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1})$ between 300 and 70 K, and then drops a value of about $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}28} {\mathrm{cm}}^{6}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 30 K, in good agreement with the theory for nonparabolic near-mirror bands and nondegenerate statistics. It is found that C for PbSe is between one and two orders of magnitude lower than for ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te}$ of comparable band gap.
147 citations
TL;DR: In this article, the invariance properties have been used to provide a unified framework for convergence analysis for iterative methods for nonlinear ill-posed problems and prove convergence with rates for the Landweber and the iteratively regularized Gauss-Newton methods.
Abstract: For iterative methods for well-posed problems, invariance properties have been used to provide a unified framework for convergence analysis. We carry over this approach to iterative methods for nonlinear ill-posed problems and prove convergence with rates for the Landweber and the iteratively regularized Gauss-Newton methods. The conditions needed are weaker as far as the nonlinearity is concerned than those needed in earlier papers and apply also to severely ill-posed problems. With no additional effort, we can also treat multilevel versions of our methods.
126 citations
TL;DR: In this paper, convection artifacts in gradient-enhanced nuclear magnetic resonance spectroscopy are compensated by using modified pulse and gradient sequences. But the principle of convection compensation is applicable to a wide variety of gradientenhanced NMR experiments, in particular those where the interval between a defocusing and a refocusing pulsed field gradient is relatively long.
94 citations
TL;DR: The potential of diffusion-ordered 2D NMR spectroscopy (DOSY) for the analysis of solutions of polymer mixtures and polymers with complex molecular mass distributions is investigated in this paper.
Abstract: The potential of diffusion-ordered 2D NMR spectroscopy (DOSY) for the analysis of solutions of polymer mixtures and polymers with complex molecular mass distributions is investigated. Diffusion coefficient labeling in NMR is generally achieved by stepwise ramping up of the amplitudes of pairs of pulsed field gradients (PFGs). After Fourier transformation in the acquisition dimension and an inverse Laplace transform (ILT) with respect to the square of the gradient strength, 2D spectra are obtained that show the chemical shift along one dimension and the translational diffusion coefficient along the other. Since polymers may have broad, nonsymmetric or complex (bi- and multimodal) molecular weight distributions (MWDs), the diffusion coefficient distribution should follow the main features of the MWD. However, the calculation of the diffusion coefficient distribution involves a numerically unstable data inversion (ILT), which limits the resolution in the diffusion dimension. The applications of DOSY NMR tech...
94 citations
TL;DR: In this paper, thin films of crystalline polytetrafluoroethylene (PTFE) were prepared by pulsed-laser deposition using 248 nm UV-excimerlaser radiation.
92 citations
TL;DR: In this article, a convergence analysis for a modified Landweber iteration for the solution of nonlinear ill-posed problems is presented, and a priori and a posteriori stopping criteria for terminating the iteration are compared.
Abstract: In this paper a convergence analysis for a modified Landweber iteration for the solution of nonlinear ill-posed problems is presented. A priori and a posteriori stopping criteria for terminating the iteration are compared. Some numerical results for the solution of a parameter estimation problem are presented.
76 citations
TL;DR: In this article, the main manifestations of polaron pairs in the generation of free charge carriers, delayed luminescence, intra-chain triplet exciton generation, recombination of free carriers, persistence of the photoconductivity, displacement current in the transient photoconductivities experiments, and spin-dependent reactions are discussed.
TL;DR: In this paper, the shape functions of smart slender beams with integrated piezoelectric actuators and sensors are derived and formulated in the form of integral equations of the first kind for shape functions.
Abstract: Flexural vibrations of smart slender beams with integrated piezoelectric actuators and sensors are considered. A spatial variation of the sensor/actuator activity is achieved by shaping the surface electrodes and/or varying the polarization profile of the piezoelectric layers, and this variation is characterized by shape functions. Seeking shape functions for a desired purpose is termed a shaping problem. Utilizing the classical lamination theory of slender composite beams, equations for shaped sensors and actuators are derived. The interaction of mechanical, electrical and thermal fields is taken into account in the form of effective stiffness parameters and effective thermal bending moments. Self-sensing actuators are included. From these sensor/actuator equations, shaping problems with a practical relevance are formulated and are cast in the form of integral equations of the first kind for the shape functions. As a practical interesting aspect of these inverse problems, shape functions which fail to me...
TL;DR: Algorithm qHyper for finding all solutions y ( x) of a linear homogeneous q -difference equation, such that y ( qx )= r ( x ) y (x ) where r (x) is a rational function of x .
TL;DR: An equation error method is developed which transforms the inverse problem of reconstructing the diffusion coefficient in a quasi-linear parabolic differential equation in divergence form into a system of linear operator equations for the diffusion coefficients which can be solved by the conjugate gradient method in a very efficient and stable manner.
Abstract: We consider the inverse problem of reconstructing the diffusion coefficient in a quasi-linear parabolic differential equation in divergence form from measurements of the solution at a finite number of points in the interior of the domain. An equation error method is developed which transforms the inverse problem into a system of linear operator equations for the diffusion coefficient and which can be solved by the conjugate gradient method in a very efficient and stable manner. A detailed error analysis relates the required number of measurements with their accuracy. Numerical results illustrate the performance of the method.
TL;DR: In this article, a class of Newton type methods for the approximate solution of nonlinear ill-posed operator equations, that use so-called filter functions for regularizing the linearized equation in each Newton step, is presented.
Abstract: This paper treats a class of Newton type methods for the approximate solution of nonlinear ill-posed operator equations, that use so-called filter functions for regularizing the linearized equation in each Newton step. For noisy data we derive an aposteriori stopping rule that yields convergence of the iterates to asolution, as the noise level goes to zero, under certain smoothness conditions on the nonlinear operator. Appropriate closeness and smoothness assumptions on the starting value and the solution are shown to lead to optimal convergence rates. Moreover we present an application of the Newton type methods under consideration to a parameter identification problem, together with some numerical results.
TL;DR: This paper shows how to construct and analyze preconditioners on the basis of domain decomposition techniques and the numerical results obtained confirm the theoretical analysis quite well.
Abstract: The finite element discretization of the Signorini Problem leads to a large scale constrained minimization problem. To improve the convergence rate of the projection method preconditioning must be developed. To be effective, the relative condition number of the system matrix with respect to the preconditioning matrix has to be small and the applications of the preconditioner as well as the projection onto the set of feasible elements have to be fast computable. In this paper, we show how to construct and analyze such preconditioners on the basis of domain decomposition techniques. The numerical results obtained for the Signorini problem as well as for contact problems in plane elasticity confirm the theoretical analysis quite well.
TL;DR: Using thermal energy helium atom scattering it was found that deposition of cobalt on the surface at 350 K leads to layer-by-layer growth of a pseudomorphic fcc(110) Co film as discussed by the authors.
Abstract: Using thermal energy helium atom scattering it is found that deposition of cobalt on the $\mathrm{Cu}(110)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ surface at 350 K leads to layer-by-layer growth of a pseudomorphic fcc(110) Co film. Auger electron spectroscopy reveals that the surface is O terminated even after deposition of more than 20 monolayer Co. A $(3\ifmmode\times\else\texttimes\fi{}1)$ superstructure is observed in the topmost Co layer, suggesting that the Co(110) phase is stabilized by an oxygen induced surface reconstruction.
TL;DR: This work presents a method by which the selection of well-defined coherence transfer pathways can be simulated efficiently and visualized easily and implemented in a MATLAB program, called CCCP (Complete Calculation of Coherence Pathways).
TL;DR: In this article, a semi-analytical approach to a quantitative analysis of thermal ns laser ablation is presented, where the nonlinear heat equation is reduced to three ordinary differential equations for the surface temperature, spatial width of the enthalpy distribution, and the ablated depth.
01 Jan 1998
TL;DR: This paper presents two types of coarsening algorithms based on the graph of the stiffness matrix, and observes, in some special cases of nested mesh refinement, that some geometrical version of the multi-grid method turns out to be a special case of the AMG algorithms.
Abstract: This paper is devoted to the construction of Algebraic Multi-Grid (AMG) methods, which are especially suited for the solution of large sparse systems of algebraic equations arising from the finite element discretization of second-order elliptic boundary value problems on unstructured, fine meshes in two or three dimensions. The only information needed is recovered from the stiffness matrix. We present two types of coarsening algorithms based on the graph of the stiffness matrix. In some special cases of nested mesh refinement, we observe, that some geometrical version of the multi-grid method turns out to be a special case of our AMG algorithms. Finally, we apply our algorithms on two and three dimensional heat conduction problems in domains with complicated geometry (e.g. micro-scales), as well as to plane strain elasticity problems with jumping coefficients.
TL;DR: Fast solution techniques for the linear systems of equations obtained by the BEM (BE-equations) utilizing the non-overlapping domain decomposition (DD) are discussed and an elementary spectral equivalence analysis of the B EM Schur complement is given that provides the tool for constructing and analysing appropriate preconditioners.
Abstract: The boundary element method (BEM) is of advantage in many applications including far-field computations in magnetostatics and solid mechanics as well as accurate computations of singularities. Since the numerical approximation is essentially reduced to the boundary of the domain under consideration, the mesh generation and handling is simpler than, for example, in a finite element discretization of the domain. In this paper, we discuss fast solution techniques for the linear systems of equations obtained by the BEM (BE-equations) utilizing the non-overlapping domain decomposition (DD). We study parallel algorithms for solving large scale Galerkin BE–equations approximating linear potential problems in plane, bounded domains with piecewise homogeneous material properties. We give an elementary spectral equivalence analysis of the BEM Schur complement that provides the tool for constructing and analysing appropriate preconditioners. Finally, we present numerical results obtained on a massively parallel machine using up to 128 processors, and we sketch further applications to elasticity problems and to the coupling of the finite element method (FEM) with the boundary element method. As shown theoretically and confirmed by the numerical experiments, the methods are of
$O(h^{-2})$
algebraic complexity and of high parallel efficiency, where
$h$
denotes the usual discretization parameter.
TL;DR: In this article, the authors show that the concept of the degree of nonlinearity combined with source conditions can be used to characterize the local ill-posedness and to derive a posteriori estimates for nonlinear illposed problems.
Abstract: The characterization of the local ill-posedness and the local degree of nonlinearity are of particular importance for the stable solution of nonlinear ill-posed problems. We present assertions concerning the interpendence between the ill-posedness of the nonlinear problem and its linearization. Moreover, we show that the concept of the degree of nonlinearity combined with source conditions can be used to characterize the local ill-posedness and to derive a posteriori estimates for nonlinear ill-posed problems. A posteriori estimates are widely used in finite element and multigrid methods for the solution of nonlinear partial differential equations, but these techniques are in general not applicable to inverse and ill-posed problems. Additionally we show for the well known Landweber method and the iteratively regularized Gauss-Newton method that they satisfy a posteriori estimates under source conditions; this can be used to prove convergence rate results. A numerical test is presented that confirms the theoretical assertions.
12 Oct 1998
TL;DR: In this article, the roll eccentricity may be caused by inexact roll grinding or nonuniform thermal expansion of the rolls, e.g. due to inexact rolling grinding or thermal expansion.
Abstract: This paper is concerned with the active compensation of the roll-eccentricity-induced periodic disturbances in the strip exit thickness of hot and cold rolling mills. The roll eccentricity may be caused by different reasons, e.g., inexact roll grinding or nonuniform thermal expansion of the rolls. The increasing demands on the thickness tolerances require new methods for the active compensation of the contribution of the roll eccentricity to the final thickness deviation. The presented method is based on the factorization approach over the set of stable transfer functions in combination with an adaptive least-mean-squares algorithm derived from the projection theorem. Here, the authors take advantage of the fact that the eccentricity-caused disturbance is periodic with a frequency proportional to the measured angular velocity of the rolls. Furthermore, it turns out that the presented concept fits the conventional control circuit of automatic gauge control in an optimal way. Simulation results for a cold rolling mill and measurement results for a hot strip mill demonstrate the feasibility and the excellent performance of the design.
TL;DR: The metal-insulator transition has been unexpectedly discovered in Si-MOS field effect transistors and has been verified in many other semiconducting materials afterwards as discussed by the authors, which caused much attention as it is in apparent contradiction to the scaling behavior of non-interacting two-dimensional electrons.
Abstract: The metal-insulator transition has been unexpectedly discovered in Si-MOS field effect transistors and has been verified in many other semiconducting materials afterwards. The transition caused much attention as it is in apparent contradiction to the scaling behavior of non-interacting two-dimensional electrons. This work gives an overview about recent experimental work and theoretical models on the metal-insulator transition and the metallic state in two dimensions.
TL;DR: In this article, a formalism for the calculation of diffusely scattered x-ray intensity from spatially inhomogeneous strain fields in Ge rich islands and in the surrounding Si matrix of SiGe/Si multilayers is presented.
Abstract: We report on a formalism for the calculation of diffusely scattered x-ray intensity from spatially inhomogeneous strain fields in Ge rich islands and in the surrounding Si matrix of SiGe/Si multilayers. The data analysis is based on a theory considering the two-dimensional statistical distribution of the dot positions, which allows a common formalism for both coplanar and grazing incidence scattering geometries. The strain fields were simulated based on the approach of the elastic Green function, taking the influence of the elastic strain relaxation at the sample surface into account. From these simulations the degree of relaxation of the islands was obtained, which compared very well with experimental data derived from x-ray reciprocal space maps. @S0163-1829 ~98!02736-2#
TL;DR: In this article, a method for the analysis of free amino acids in beverage samples by capillary electrophoresis and direct UV detection at 185 nm is presented, where separation were performed in a strongly acidic carrier electrolyte containing an alkanesulfonic acid and varying amounts of acetonitrile.
TL;DR: In this paper, single-ion transfer properties such as Gibbs energies, entropies and enthalpies are employed to account for the interactions of ions with solvents.
TL;DR: In this paper, it was shown that Er is incorporated in another surrounding and proposed Si-Er-O nano-precipitates since the spectra of other candidates, like Er 2 O 3, are clearly different.
01 Jul 1998-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this article, a polyethylene (PE) was irradiated with 63 keV Ar+ and 155 keV Xe+ ions to fluences of 1.3 to 3.5 cm−2 with ion energies being chosen in order to achieve approximately the same penetration depth for both species.
Abstract: Polyethylene (PE) was irradiated with 63 keV Ar+ and 155 keV Xe+ ions to fluences of 1 × 1013 to 3 × 1015 cm−2 with ion energies being chosen in order to achieve approximately the same penetration depth for both species. The PE surface morphology was examined by means of atomic force microscopy (AFM), whereas the concentration of free radicals and conjugated double bonds, both created by the ion irradiation, were determined using electron paramagnetic resonance (EPR) and UV–VIS spectroscopy, respectively. As expected, the degradation of PE was higher after irradiation with heavier Xe+ ions but the changes in the PE surface morphology were more pronounced for Ar+ ions. This newly observed effect can be explained by stronger compaction of the PE surface layer in the case of the Xe+ irradiation, connected with a reduction of free volume available.
TL;DR: The numerical performance of this multi level algorithm is compared with Landweber's iteration and an advantage of this method is that the convergence analysis can be considered in a family of finite dimensional spaces.
Abstract: The convergence analysis of Landweber's iteration for the solution of nonlinear ill–posed problem has been developed recently by Hanke, Neubauer and Scherzer. In concrete applications, sufficient conditions for convergence of the Landweber iterates developed there (although quite natural) turned out to be complicated to verify analytically. However, in numerical realizations, when discretizations are considered, sufficient conditions for local convergence can usually easily be stated. This paper is motivated by these observations: Initially a discretization is fixed and a discrete Landweber iteration is implemented until an appropriate stopping criterion becomes active. The output is used as an initial guess for a finer discretization. An advantage of this method is that the convergence analysis can be considered in a family of finite dimensional spaces. The numerical performance of this multi level algorithm is compared with Landweber's iteration.