Showing papers by "University of Paderborn published in 2005"

the stable derived category of a noetherian scheme

Abstract: for a noetherian scheme, we introduce its unbounded stable derived category. this leads to a recollement which reflects the passage from the bounded derived category of coherent sheaves to the quotient modulo the subcategory of perfect complexes. some applications are included, for instance an analogue of maximal cohen–macaulay approximations, a construction of tate cohomology, and an extension of the classical grothendieck duality. in addition, the relevance of the stable derived category in modular representation theory is indicated.

Theoretical crack path prediction

Abstract: In many practical cases, the crack growth leads to abrupt failure of components and structures. For reasons of a reliable quantification of the endangerment due to sudden fracture of a component, therefore, it is of enormous importance to know the threshold values, the crack paths and the growth rates for the fatigue crack growth as well as the limiting values for the beginning of unstable crack growth (fracture toughness). This contribution deals with the complex problem of a-however initiated-crack, that is subjected to a mixed-mode loading. It will present the hypotheses and concepts, which describe the superposition of Mode I and Mode II (plane mixed mode) as well as the superposition of all three modes (Mode I, II and III) for spatial loading conditions. Those concepts admit a quantitative appraisal of such crack situations and a characterization of possible crack paths.

Attracted by long-range electron correlation: adenine on graphite.

Abstract: The adsorption of adenine on graphite is analyzed from first-principles calculations as a model case for the interaction between organic molecules and chemically inert surfaces. Within density-functional theory we find no chemical bonding due to ionic or covalent interactions, only a very weak attraction at distances beyond the equilibrium position due to the lowering of the kinetic energy of the valence electrons. Electron exchange and correlation effects are much more important for the stabilization of the adsystem. They are modeled by the local density or generalized gradient approximation supplemented by the London dispersion formula for the van der Waals interaction.

Calculating absorption shifts for retinal proteins: Computational challenges

Abstract: Rhodopsins can modulate the optical properties of their chromophores over a wide range of wavelengths. The mechanism for this spectral tuning is based on the response of the retinal chromophore to external stress and the interaction with the charged, polar, and polarizable amino acids of the protein environment and is connected to its large change in dipole moment upon excitation, its large electronic polarizability, and its structural flexibility. In this work, we investigate the accuracy of computational approaches for modeling changes in absorption energies with respect to changes in geometry and applied external electric fields. We illustrate the high sensitivity of absorption energies on the ground-state structure of retinal, which varies significantly with the computational method used for geometry optimization. The response to external fields, in particular to point charges which model the protein environment in combined quantum mechanical/molecular mechanical (QM/MM) applications, is a crucial feature, which is not properly represented by previously used methods, such as time-dependent density functional theory (TDDFT), complete active space self-consistent field (CASSCF), and Hartree-Fock (HF) or semiempirical configuration interaction singles (CIS). This is discussed in detail for bacteriorhodopsin (bR), a protein which blue-shifts retinal gas-phase excitation energy by about 0.5 eV. As a result of this study, we propose a procedure which combines structure optimization or molecular dynamics simulation using DFT methods with a semiempirical or ab initio multireference configuration interaction treatment of the excitation energies. Using a conventional QM/MM point charge representation of the protein environment, we obtain an absorption energy for bR of 2.34 eV. This result is already close to the experimental value of 2.18 eV, even without considering the effects of protein polarization, differential dispersion, and conformational sampling.

A survey of methods for computing (un)stable manifolds of vector fields

Abstract: The computation of global invariant manifolds has seen renewed interest in recent years. We survey different approaches for computing a global stable or unstable manifold of a vector field, where we concentrate on the case of a two-dimensional manifold. All methods are illustrated with the same example — the two-dimensional stable manifold of the origin in the Lorenz system.

Thermal processing of polycrystalline NiTi shape memory alloys

Abstract: The objective of this study is to examine the effect of heat treatment on polycrystalline Ti–50.9 at.% Ni in hot-rolled and cold-drawn states. In particular, we examine microstructure, transformation temperatures as well as mechanical behavior in terms of both uniaxial monotonic testing and instrumented Vickers micro-indentation. The results constitute a fundamental understanding of the effect of heat treatment on thermal/stress-induced martensite and resistance to plastic flow in NiTi, all of which are critical for optimizing the mechanical properties. The high temperature of the hot-rolling process caused recrystallization, recovery, and hindered precipitate formation, essentially solutionizing the NiTi. The subsequent cold-drawing-induced a high density of dislocations and martensite. Heat treatments were carried out on hot-rolled, as well as, hot-rolled then cold-drawn materials at various temperatures for 1.5 h. Transmission Electron Microscopy observations revealed that Ti 3 Ni 4 precipitates progressively increased in size and changed their interface with the matrix from being coherent to incoherent with increasing heat treatment temperature. Accompanying the changes in precipitate size and interface coherency, transformation temperatures were observed to systematically shift, leading to the occurrence of the R-phase and multiple-stage transformations. Room temperature stress–strain tests illustrated a variety of mechanical responses for the various heat treatments, from pseudoelasticity to shape memory. The changes in stress–strain behavior are interpreted in terms of shifts in the primary martensite transformation temperatures, rather then the occurrence of the R-phase transformation. The results confirm that Ti 3 Ni 4 precipitates can be used to elicit a desired isothermal stress–strain behavior in polycrystalline NiTi. Instrumented micro-indention tests revealed that Martens (Universal) Hardness values are more dependent on the resistance to dislocation motion than measured uniaxial pseudoelastic or shape memory response. Based on comparison of hardness and the stress required to induce martensite, it is shown that the resistance to dislocation motion and the ease of the stress-induced martensite transformation cannot be simultaneously maximized, although an optimal combination should exist. Measuring indentation depth before and after heating more distinctly confirmed shape memory or pseudoelastic behavior.

Discrete Talbot effect in waveguide arrays

Abstract: We report the first observation of discrete Talbot revivals in one-dimensional waveguide arrays. Unlike continuous systems where the Talbot self-imaging effect always occurs irrespective of the pattern period, in discrete configurations this process is only possible for a specific set of periodicities. Recurrence of different input periodic patterns is observed in good agreement with theory.

Phase noise-tolerant synchronous QPSK/BPSK baseband-type intradyne receiver concept with feedforward carrier recovery

Abstract: Quadrature phase-shift keying (QPSK) is attractive to increase transmission lengths and capacity, especially when it is combined with polarization division multiplex. Baseband processing at the symbol rate allows to keep the required electronic bandwidth low. So far, external cavity lasers seemed to be indispensable for such transmission systems due to linewidth requirements. We propose a feedforward carrier recovery scheme based on regenerative intradyne frequency dividers, i.e., the well-known regenerative frequency divider is extended to process baseband in-phase and quadrature (I and Q) signals. An IF linewidth tolerance of up to 0.001 times the QPSK symbol rate is predicted, 2 decades more than for an optical phase locked loop with a realistic loop delay. This means that commercially available DFB lasers shall suffice for synchronous optical QPSK/BPSK transmission.

Covering Pareto Sets by Multilevel Subdivision Techniques

Abstract: In this work, we present a new set-oriented numerical method for the numerical solution of multiobjective optimization problems. These methods are global in nature and allow to approximate the entire set of (global) Pareto points. After proving convergence of an associated abstract subdivision procedure, we use this result as a basis for the development of three different algorithms. We consider also appropriate combinations of them in order to improve the total performance. Finally, we illustrate the efficiency of these techniques via academic examples plus a real technical application, namely, the optimization of an active suspension system for cars.

Coresets in dynamic geometric data streams

Abstract: A dynamic geometric data stream consists of a sequence of m insert/delete operations of points from the discrete space 1,…,Δd [26]. We develop streaming (1 + e)-approximation algorithms for k-median, k-means, MaxCut, maximum weighted matching (MaxWM), maximum travelling salesperson (MaxTSP), maximum spanning tree (MaxST), and average distance over dynamic geometric data streams. Our algorithms maintain a small weighted set of points(a coreset) that approximates with probability 2/3 the current point set with respect to the considered problem during the m insert/delete operations of the data stream. They use poly (e-1, log m, log Δ) space and update time per insert/delete operation for constant k and dimension dHaving a coreset one only needs a fast approximation algorithm for the weighted problem to compute a solution quickly. In fact, even an exponential algorithm is sometimes feasible as its running time may still be polynomial in n. For example one can compute in poly(log n, exp(O((1+log (1⁄e)⁄e)d-1))) time a solution to k-median and k-means [21] where n is the size of the current point set and k and d are constants. Finding an implicit solution to MaxCut can be done in poly(log n, exp((1⁄e)O(1))) time. For MaxST and average distance we require poly(log n, e-1) time and for MaxWM we require O(n3) time to do this.

A critical evaluation of different QM/MM frontier treatments with SCC-DFTB as the QM method.

Abstract: The performance of different link atom based frontier treatments in QM/MM simulations was evaluated critically with SCC-DFTB as the QM method. In addition to the analysis of gas-phase molecules as in previous studies, an important element of the present work is that chemical reactions in realistic enzyme systems were also examined. The schemes tested include all options available in the program CHARMM for SCC-DFTB/MM simulation, which treat electrostatic interactions due to the MM atoms close to the QM/MM boundary in different ways. In addition, a new approach, the divided frontier charge (DIV), has been implemented in which the partial charge associated with the frontier MM atom ("link host") is evenly distributed to the other MM atoms in the same group. The performance of these schemes was evaluated based on properties including proton affinities, deprotonation energies, dipole moments, and energetics of proton transfer reactions. Similar to previous work, it was found that calculated proton affinities and deprotonation energies of alcohols, carbonic acids, amino acids, and model DNA bases are very sensitive to the link atom scheme; the commonly used single link atom approach often gives error on the order of 15 to 20 kcal/mol. Other schemes give better and, on average, mutually comparable results. For proton transfer reactions, encouragingly, both activation barriers and reaction energies are fairly insensitive (within a typical range of 2-4 kcal/mol) to the link atom scheme due to error cancellation, and this was observed for both gas-phase and enzyme systems. Therefore, the effect of using different link atom schemes in QM/MM simulations is rather small for chemical reactions that conserve the total charge. Although the current study used an approximate DFT method as the QM level, the observed trends are expected to be applicable to QM/MM methods with use of other QM approaches. This observation does not mean to encourage QM/MM simulations without careful benchmark in the study of specific systems, rather it emphasizes that other technical details, such as the treatment of long-range electrostatics, tend to play a more important role and need to be handled carefully.

Validation of the density-functional based tight-binding approximation method for the calculation of reaction energies and other data

Abstract: We investigated the performance of the approximative density functional method DFTB versus BLYP and G2 with respect to zero-point corrected reaction energies, vibrational frequencies, and geometry parameters for a set of 28 reactions and 22 representative molecules containing C, H, N, and O (DFTB—density-functional based tight-binding approximation). The DFTB reaction energies show a mean absolute deviation versus the G2 reference of 4.3kcal∕mol only. The corresponding value for the vibrational frequencies amounts to 75cm−1 versus BLYP/cc-pVTZ. With very few exceptions bond lengths and angles are in excellent agreement with the results of higher-level methods.

Defects in SiO 2 as the possible origin of near interface traps in the SiC ∕ SiO 2 system: A systematic theoretical study

Abstract: A systematic study of the level positions of intrinsic and carbon defects in ${\mathrm{SiO}}_{2}$ is presented, based on density functional calculations with a hybrid functional in an $\ensuremath{\alpha}$-quartz supercell. The results are analyzed from the point of view of the near interface traps (NIT), observed in both $\mathrm{SiC}∕{\mathrm{SiO}}_{2}$ and $\mathrm{Si}∕{\mathrm{SiO}}_{2}$ systems, and assumed to have their origins in the oxide. It is shown that the vacancies and the oxygen interstitial can be excluded as the origin of such NIT, while the silicon interstitial and carbon dimers give rise to gap levels in the energy range inferred from experiments. The properties of these defects are discussed in light of the knowledge about the $\mathrm{SiC}∕{\mathrm{SiO}}_{2}$ interface.

Towards a Formal Semantics of UML 2.0 Activities.

Abstract: The new version 2.0 of the Unified Modeling Language (UML) was targeted at improving expressiveness and semantic precision. These developments are particularly evident in activity diagrams which have not only acquired many new features, but a completely new metamodel and semantic foundation. The UML contains some hints that Petri-nets are the inspirational source for the new semantics. In this paper we will investigate how strong the alignment of UML’s activity diagrams to Petri-nets really is. We start by providing a mapping of the basic elements of activity diagrams to Petri-nets and discuss the problems arising when trying to extend this approach to some of the advanced features of activity diagrams, namely exceptions, traverse-to-completion, and streaming. This examination raises several syntactic and semantic questions concerning activities. We conclude that for basic activities, the analogy works pretty well, but for higher-level constructs, no such intuitive connection exists.

Theoretical study of the mechanism of dry oxidation of 4H-SiC

Abstract: Possible defect structures, arising from the interaction of ${\mathrm{O}}_{2}$ molecules with an ideal portion of the $\mathrm{SiC}∕\mathrm{Si}{\mathrm{O}}_{2}$ interface, have been investigated systematically using density functional theory. Based on the calculated total energies and assuming thermal quasiequilibrium during oxidation, the most likely routes leading to complete oxidation have been determined. The defect structures produced along these routes will remain at the interface in significant concentration when stopping the oxidation process. The results obtained for their properties are well supported by experimental findings about the $\mathrm{SiC}∕\mathrm{Si}{\mathrm{O}}_{2}$ interface. It is found that carbon-carbon bonds can explain most of the observed interface states but not the high density near the conduction band of $4H$-SiC.

REPLICA: a bitstream manipulation filter for module relocation in partial reconfigurable systems

Abstract: The feature of partial reconfiguration provided by currently available field programmable gate arrays (FPGAs) makes it possible to change hardware modules while others keep working. The combination of this feature and the high gate capacity enables the integration of dynamic systems that can be adapted to changing demands during runtime. Placing the dynamically changing modules along a horizontal communication infrastructure does not only provide communication facilities it also enables the relocation of pre-synthesized modules by bitstream manipulations. The exact placement of an incoming module is determined according to the current resource allocation, which results in an online placement problem. In order to prevent any extra configuration overhead for the relocation process, we developed the REPLICA (relocation per online configuration alteration) filter, which is capable of performing the necessary bitstream manipulations during the regular download process. The filter architecture, a configuration manager and an evaluation example are presented in this paper.

Transport in Dynamical Astronomy and Multibody Problems

Abstract: We combine the techniques of almost invariant sets (using tree structured box elimination and graph partitioning algorithms) with invariant manifold and lobe dynamics techniques. The result is a new computational technique for computing key dynamical features, including almost invariant sets, resonance regions as well as transport rates and bottlenecks between regions in dynamical systems. This methodology can be applied to a variety of multibody problems, including those in molecular modeling, chemical reaction rates and dynamical astronomy. In this paper we focus on problems in dynamical astronomy to illustrate the power of the combination of these different numerical tools and their applicability. In particular, we compute transport rates between two resonance regions for the three-body system consisting of the Sun, Jupiter and a third body (such as an asteroid). These resonance regions are appropriate for certain comets and asteroids.

ClassSheets: automatic generation of spreadsheet applications from object-oriented specifications

Abstract: Spreadsheets are widely used in all kinds of business applications. Numerous studies have shown that they contain many errors that sometimes have dramatic impacts. One reason for this situation is the low-level, cell-oriented development process of spreadsheets.We improve this process by introducing and formalizing a higher-level object-oriented model termed ClassSheet. While still following the tabular look-and feel of spreadsheets, ClassSheets allow the developer to express explicitly business object structures within a spreadsheet, which is achieved by integrating concepts from the UML (Unified Modeling Language). A stepwise automatic transformation process generates a spreadsheet application that is consistent with the ClassSheet model. Thus, by deploying the formal underpinning of ClassSheets, a large variety of errors can be prevented that occur in many existing spreadsheet applications today.The presented ClassSheet approach links spreadsheet applications to the object-oriented modeling world and advocates an automatic model-driven development process for spreadsheet applications of high quality.

Sampling in dynamic data streams and applications

Abstract: A dynamic geometric data stream is a sequence of m Add/Remove operations of points from a discrete geometric space (1,...,Δ)d [21]. Add(p) inserts a point p from (1,...,Δ)d into the current point set, Remove(p) deletes p from P. We develop low-storage data structures to (i) maintain e-approximations of range spaces of P with constant VC-dimension and (ii) maintain an e-approximation of the weight of the Euclidean minimum spanning tree of P. Our data structures use O(log3e • log3(1/e) • log(1/e)/e2) and O(log (1/δ) • (log Δ/e)O(d)) bits of memory, respectively (we assume that the dimension d is a constant), and they are correct with probability 1-δ. These results are based on a new data structure that maintains a set of elements chosen (almost) uniformly at random from P.

Partial key exposure attacks on RSA up to full size exponents

Abstract: We present several attacks on RSA that factor the modulus in polynomial time under the condition that a fraction of the most significant bits or least significant bits of the private exponent is available to the attacker. Our new attacks on RSA are the first attacks of this type that work up to full size public or private exponent.

Automatic conformance testing of web services

Abstract: Web Services are the basic building blocks of next generation Internet applications, based on dynamic service discovery and composition. Dedicated discovery services will store both syntactic and behavioral descriptions of available services and guarantee their compatibility with the requirements expressed by clients. In practice, however, interactions may still fail because the Web Service's implementation may be faulty. In fact, the client has no guarantee on the quality of the implementation associated to any service description. In this paper, we propose the idea of high-quality service discovery incorporating automatic testing for validating Web Services before allowing their registration. First, the discovery service automatically generates conformance test cases from the provided service description, then runs the test cases on the target Web Service, and only if the test is successfully passed, the service is registered. In this way, clients bind with Web Services providing a compatible signature, a suitable behavior, and a high-quality implementation.

Quantum optical properties of a single In x Ga 1 − x As − Ga As quantum dot two-level system

Abstract: We report on a two-level system, defined by the ground-state exciton of a single $\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dot. Saturation spectroscopy combined with ultrahigh spectral resolution gives us a complete description of the system in the steady-state limit. Rabi oscillations and quantum interference experiments, on the other hand, provide a detailed insight into the coherent high excitation regime. All fundamental properties of the two-level system show an excellent quantitative agreement in both domains, even though obtained under entirely different experimental conditions. We thus are able to demonstrate control over an almost ideal two-level system, suitable for possible applications in quantum information processing.