Showing papers by "University of Paderborn published in 2001"

Tight-binding approach to time-dependent density-functional response theory

Abstract: In this paper we propose an extension of the self-consistent charge-density-functional tight-binding (SCC-DFTB) method [M. Elstner et al., Phys. Rev. B 58, 7260 (1998)], which allows the calculation of the optical properties of finite systems within time-dependent density-functional response theory (TD-DFRT). For a test set of small organic molecules low-lying singlet excitation energies are computed in good agreement with first-principles and experimental results. The overall computational cost of this parameter-free method is very low and thus it allows us to examine large systems: we report successful applications to ${\mathrm{C}}_{60}$ and the polyacene series.

Color-Tuned Electroluminescence from Columnar Liquid Crystalline Alkyl Arenecarboxylates.

Abstract: Simple ester derivatives of polycyclic arenes offer access to light-emitting diodes of nearly any visible color by making use of the good charge-transport properties of the columnar liquid crystals of these derivatives. The picture shows the orange-red electroluminescence of a light-emitting diode containing the perylene 3,4,9,10-tetracarboxylic acid ethyl ester (structure shown, R=Et). Through use of multiple layers of different esters light-emitting diodes with almost white luminescence can be obtained.

Tight-binding molecular-dynamics simulation of impurities in ultrananocrystalline diamond grain boundaries

Abstract: Ultrananocrystalline diamond (UNCD) films grown from hydrogen-poor plasmas have grain sizes of 3-10 nm, resulting in a large number of grain boundaries. We repon on density-functional-based tight-binding molecular-dynamics calculations of high-energy high-angle twist (100) grain boundaries in diamond as a model for the UNCD grain boundaries. About one-half of the carbons in the grain boundary are threefold coordinated and are responsible for states introduced into the band gap. Simulations were also performed for N, Si, and H impurities in (100) twist grain boundaries where substitution energies, optimized geometries, and electronic structures were calculated. Substitution energies were found to be substantially lower for the grain boundaries compared to the bulk diamond crystal. Nitrogen increases the number of threefold-coordinated carbons while hydrogen saturates dangling bonds. The electronic structure of UNCD is characterized by a large number of states in the band gap attributed to the bonding disorder and impurities in the grain boundaries.

Pareto-Front Exploration with Uncertain Objectives

Abstract: We consider the problem of exploration of the set of all global optima (Pareto-points) or an approximation thereof in the context of multi-objective function optimization. Up to now, set oriented techniques assume that the evaluation of the m-dimensional vector of objectives can be done exactly which is important to steer the search process towards global optima. Here, we extend such techniques to allow objectives to be uncertain, i.e., vary within intervals. This may be often the case if the exact computation of objectives is computationally too expensive such that only estimates on the objective values of a design point may be derived. For objective values that are constrained by intervals, we derive a theory of probabilistic dominance, an extension of the definition of Pareto-dominance. Also, we show how this theory may be used in order to guide the selection process to approximate the Pareto-set.

Phonon Density of States of Iron up to 153 Gigapascals

Abstract: We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature. In comparison to the compressional and shear velocities from the preliminary reference Earth model (PREM) seismic model, our results suggest that Earth's inner core has a mean atomic number equal to or higher than pure iron, which is consistent with an iron-nickel alloy.

The Algorithms Behind GAIO — Set Oriented Numerical Methods for Dynamical Systems

Abstract: In a given dynamical system there are essentially two different types of information which could be of practical interest: on the one hand there is the need to describe the behavior of single trajectories in detail This information is helpful for the analysis of transient behavior and also in the investigation of geometric properties of dynamical systems On the other hand, if the underlying invariant set is generated by complicated dynamics then the computation of single trajectories may give misleading results In this case there still exists important set related information covering both topological and statistical aspects of the underlying dynamical behavior Within the DFG-Schwerpunkt we have focussed on the development of set oriented methods for the numerical approximation of invariant sets (eg invariant manifolds, global attractors, chain recurrent sets) (natural) invariant measures almost invariant sets

Finite state testing and analysis of graphical user interfaces

Abstract: Based on finite-state automata (FSA) and equivalent regular expressions, the paper introduces a holistic view of fault modeling that can be carried out as a complementary step to system modeling, revealing much rationalization potential. Appropriate formal notions will be used to introduce efficient algorithms to systematically generate and select test cases. The completeness of the test can be determined exploiting the link coverage of the state transition diagram of the FSA that models both the desired and undesired behavior of the system under test; this enables a precise scalability of the test and analysis process, leading to a better cost-effectiveness The elements of the approach will be narrated by realistic examples which will be used also to validate the approach.

A methodology for specifying and analyzing consistency of object-oriented behavioral models

Abstract: Object-oriented modeling favors the modeling of object behavior from different viewpoints and the successive refinement of behavioral models in the development process. This gives rise to consistency problems of behavioral models. The absence of a formal semantics for UML models and the numerous possibilities of employing behavioral models within the development process lead to the rise of a number of different consistency notions. In this paper, we discuss the issue of consistency of behavioral models in the UML and present a general methodology how consistency problems can be dealt with. According to the methodology, those aspects of the models relevant to the consistency are mapped to a semantic domain in which precise consistency tests can be formulated. The choice of the semantic domain and the definition of consistency conditions can be used to construct different consistency notions. We show the applicability of our methodology by giving an example of a concrete consistency problem of concurrent object-oriented models.

Physics and current understanding of X-ray storage phosphors

Abstract: BaFBr doped with Eu 2+ is the most successful X-ray storage phosphor used for digital radiography so far. The mechanisms of information storage and subsequent read-out are, however, not yet fully understood. The present understanding of the structures and reactions of the radiation-induced point defects involved in these processes are critically discussed. The shortcomings of X-ray storage phosphor screens based on BaFBr:Eu 2+ are the inferior spatial resolution compared to conventional X-ray films. Also reviewed are therefore alternative storage phosphors such as alkali halides doped with Eu 2+ . Ga + , In + , or Tl + or elpasolites doped with Ce 3+ or Eu + which are also promising with high figures of merit and potentially higher spatial resolution. A completely new approach is the use of suitably doped glasses and glass ceramics. First results on fluorozirconate glass ceramics with good storage and read-out properties are reviewed.

Sliding friction in the presence of ultrasonic oscillations: superposition of longitudinal oscillations

Abstract: It was often observed that friction forces can be reduced significantly if ultrasonic oscillations are superposed to the macroscopic sliding velocity. This phenomenon can be used to improve machining processes by addition of ultrasonic vibration to tools or workpieces, and forms the basis for many processes of ultrasonic machining. On the other hand, ultrasonic vibrations can be used to generate motion. The thrusting force of ultrasonic motors is provided to the rotor through friction. In the present paper, a simple theoretical model for friction in the presence of ultrasonic oscillations is derived theoretically and validated experimentally. The model is capable of predicting the reduction of the macroscopic friction force as a function of the ultrasonic vibration frequency and amplitude and the macroscopic sliding velocity.

Tubular structures of silicon

Abstract: In this paper we demonstrate, using density-functional tight-binding theory, that certain classes of silicon-based tubular nanostructures are stable and energetically viable. Specifically, we consider silicide and SiH nanotubes. The structures adopted by these nanotubes are very similar to those of previously reported phosphorus nanotubes. As in that case, the Si-based nanotubes have a semiconducting gap, which in contrast to carbon nanotubes is largely independent of the tube diameter and chirality. We further report on the mechanical properties of the Si-based nanotubes and suggest possible routes towards their synthesis.

Quantum mechanics simulation of protein dynamics on long timescale.

Abstract: Protein structure and dynamics are the keys to a wide range of problems in biology. In principle, both can be fully understood by using quantum mechanics as the ultimate tool to unveil the molecular interactions involved. Indeed, quantum mechanics of atoms and molecules have come to play a central role in chemistry and physics. In practice, however, direct application of quantum mechanics to protein systems has been prohibited by the large molecular size of proteins. As a consequence, there is no general quantum mechanical treatment that not only exceeds the accuracy of state-of-the-art empirical models for proteins but also maintains the efficiency needed for extensive sampling in the conformational space, a requirement mandated by the complexity of protein systems. Here we show that, given recent developments in methods, a general quantum mechanical-based treatment can be constructed. We report a molecular dynamics simulation of a protein, crambin, in solution for 350 ps in which we combine a semiempirical quantum-mechanical description of the entire protein with a description of the surrounding solvent, and solvent-protein interactions based on a molecular mechanics force field. Comparison with a recent very high-resolution crystal structure of crambin (Jelsch et al., Proc Natl Acad Sci USA 2000;102:2246-2251) shows that geometrical detail is better reproduced in this simulation than when several alternate molecular mechanics force fields are used to describe the entire system of protein and solvent, even though the structure is no less flexible. Individual atomic charges deviate in both directions from "canonical" values, and some charge transfer is found between the N and C-termini. The capability of simulating protein dynamics on and beyond the few hundred ps timescale with a demonstrably accurate quantum mechanical model will bring new opportunities to extend our understanding of a range of basic processes in biology such as molecular recognition and enzyme catalysis.

The randomized z-buffer algorithm: interactive rendering of highly complex scenes

Abstract: We present a new output-sensitive rendering algorithm, the randomized z-buffer algorithm. It renders an image of an arbitrary three-dimensional scene consisting of triangular primitives by reconstruction from a dynamically chosen set of random surface sample points. This approach is independent of mesh connectivity and topology. The resulting rendering time grows only logarithmically with the numbers of triangles in the scene. We were able to render walkthroughs of scenes of up to 1014 triangles at interactive frame rates. Automatic identification of low detail scene components ensures that the rendering speed of the randomized z-buffer cannot drop below that of conventional z-buffer rendering. Experimental and analytical evidence is given that the image quality is comparable to that of common approaches like z-buffer rendering. The precomputed data structures employed by the randomized z-buffer allow for interactive dynamic updates of the scene. Their memory requirements grow only linearly with the number of triangles and allow for a scene graph based instantiation scheme to further reduce memory consumption.

The influence of a taurine containing drink on cardiac parameters before and after exercise measured by echocardiography.

Abstract: To determine the effect of the taurine containing drink "Red Bull" on cardiac parameters thirteen endurance trained subjects performed an exhaustive bout of endurance exercise at three different times Prior to the exercise the original "Red Bull" drink, a similar drink without taurine, containing caffeine, and a "placebo" drink without caffeine and without taurine were ingested by the subjects in a double-blind cross-over design Echocardiographic examinations were performed before the drinks, 40 minutes after the drinks prior to the exercise and in the regeneration period after exercise Stroke volume was significantly influenced only in the "Red Bull group" (80,4 ± 21,4 ml before drink vs 97,5 ± 26,2 ml in the regeneration period), mainly due to a reduced endsystolic diameter and volume Furthermore in this group the peak late diastolic inflow (VA) in the regeneration period was significantly higher compared with the pre-exercise levels This observation was also made in the caffeine group but without any consequences on ventricular function The results of the present study show an influence of the original caffeine and taurine containing drink (Red Bull) on parameters of the cardiac contractility

Application of an approximate density-functional method to sulfur containing compounds

Abstract: We present the results of an investigation of sulfur containing compounds using a tight-binding (TB) approach, which is a second-order approximation to density-functional theory (DFT). The performance of the so-called self-consistent charge density-functional based TB (SCC-DFTB) method is analyzed via comparison of the results for several molecular properties with other semi-empirical methods as well as with full DFT calculations. We also examine different conformers of the amino acid cysteine in order to estimate the reliability of our method for large-scale biomolecular systems. We find that SCC-DFTB provides, in general, good agreement with full DFT calculations and a similar deviation from experiment.

The simulation semantics of systemC

Abstract: We present a rigorous but transparent semantics definition of SystemC that covers method, thread, and clocked thread behavior as well as their interaction with the simulation kernel process. The semantics includes watching statements, signal assignment, and wait statements as they are introduced in SystemC V1.O. We present our definition in form of distributed Abstract State Machines (ASMs) rules reflecting the view given in the SystemC User's Manual and the reference implementation. We mainly see our formal semantics as a concise, unambiguous, high-level specification for SystemC-based implementations and for standardization. Additionally, it can be used as a sound basis to investigate SystemC interoperability with Verilog and VHDL.

Optimal FPGA module placement with temporal precedence constraints

Abstract: We consider the optimal placement of hardware modules in space and time for FPGA architectures with reconfiguration capabilities, where modules are modeled as three-dimensional boxes in space and time. Using a graph-theoretic characterization of feasible packings, we are able to solve the following problems. (a) Find the minimal execution time of the given problem on an FPGA of fixed size, (b) Find the FPGA of minimal size to accomplish the tasks within a fired time limit. Furthermore, our approach is perfectly suited for the treatment of precedence constraints for the sequence of tasks, which are present in virtually all practical instances. Additional mathematical structures are developed that lead to a powerful framework for completing optimal solutions. The usefulness is illustrated by computational results.

Efficient cascaded difference frequency conversion in periodically poled Ti:LiNbO3 waveguides using pulsed and cw pumping

Abstract: Using an FM-mode-locked Ti:Er:LiNbO3 waveguide laser as the fundamental source, wavelength conversion by cascaded χ(2):χ(2)-difference frequency generation with a conversion efficiency of up to +3(-4.6) dB was demonstrated at a pulse repetition rate of about 2 (10) GHz. In addition, multi-channel conversion was demonstrated with a fully packaged wavelength converter using a continuous fundamental source.

Rule-Based Specification of Behavioral Consistency Based on the UML Meta-model

Abstract: Object-oriented modeling favors the modeling of object behavior from different viewpoints and at different levels of abstraction. This gives rise to consistency problems between overlapping or semantically related submodels. The absence of a formal semantics for the UML and the numerous ways of employing the language within the development process lead to a number of different consistency notions. Therefore, general meta-level techniques are required for specifying, analyzing, and communicating consistency constraints. In this paper, we discuss the issue of consistency of behavioral models in the UML and present techniques for specifying and analyzing consistency. Using meta-model rules we transform elements of UML models into a semantic domain. Then, consistency constraints can by specified and validated using the language and the tools of the semantic domain. This general methodology is exemplified by the problem of protocol statechart inheritance.

Structure of the silicon vacancy in 6H-SiC after annealing identified as the carbon vacancy–carbon antisite pair

Abstract: We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S=1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite–carbon vacancy pair (CSi-VC) in the doubly positive charge state can explain all experimental findings. We propose that the (CSi-VC) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy.

A regularity class for the Navier-Stokes equations in Lorentz spaces

Abstract: We extend Serrin's regularity class for weak solutions of the Navier-Stokes equations to a larger class replacing the Lebesgue spaces by Lorentz spaces.