Showing papers by "University of Paderborn published in 1998"

Self-consistent-charge density-functional tight-binding method for simulations of complex materials properties

Abstract: We outline details about an extension of the tight-binding (TB) approach to improve total energies, forces, and transferability. The method is based on a second-order expansion of the Kohn-Sham total energy in density-functional theory (DFT) with respect to charge density fluctuations. The zeroth order approach is equivalent to a common standard non-self-consistent (TB) scheme, while at second order a transparent, parameter-free, and readily calculable expression for generalized Hamiltonian matrix elements may be derived. These are modified by a self-consistent redistribution of Mulliken charges (SCC). Besides the usual ``band structure'' and short-range repulsive terms the final approximate Kohn-Sham energy additionally includes a Coulomb interaction between charge fluctuations. At large distances this accounts for long-range electrostatic forces between two point charges and approximately includes self-interaction contributions of a given atom if the charges are located at one and the same atom. We apply the new SCC scheme to problems where deficiencies within the non-SCC standard TB approach become obvious. We thus considerably improve transferability.

Story Diagrams: A New Graph Rewrite Language Based on the Unified Modeling Language and Java

Abstract: Graph grammars and graph rewrite systems improved a lot towards practical usability during the last years. Nevertheless, there are still major problems to overcome in order to attract a broad number of software designers and developers to the usage of graph grammars and graph rewrite systems. Two of the main problems are, (1) that current graph grammar notations are too proprietary and (2) that there exists no seamless integration of graph rewrite systems with common (OO) design and implementation languages like UML and C++ or Java.

Multivalued perturbations ofm-accretive differential inclusions

Abstract: Given anm-accretive operatorA in a Banach spaceX and an upper semicontinuous multivalued mapF: [0,a]×X→2 X , we consider the initial value problemu′∈−Au+F(t,u) on [0,a],u(0)=x 0. We concentrate on the case when the semigroup generated by—A is only equicontinuous and obtain existence of integral solutions if, in particular,X* is uniformly convex andF satisfies β(F(t,B))≤k(t)β(B) for all boundedB⊂X wherek∈L 1([0,a]) and β denotes the Hausdorff-measure of noncompactness. Moreover, we show that the set of all solutions is a compactR δ-set in this situation. In general, the extra condition onX* is essential as we show by an example in whichX is not uniformly smooth and the set of all solutions is not compact, but it can be omited ifA is single-valued and continuous or—A generates aC o-semigroup of bounded linear operators. In the simpler case when—A generates a compact semigroup, we give a short proof of existence of solutions, again ifX* is uniformly (or strictly) convex. In this situation we also provide a counter-example in ℝ4 in which no integral solution exists.

An efficient algorithm for the minimal unsatisfiability problem for a subclass of CNF

Abstract: We consider the minimal unsatisfiability problem for propositional formulas over n variables with n+k clauses for fixed k. We will show that in case of at most n clauses no formula is minimal unsatisfiable. For n+1 clauses the minimal unsatisfiability problem is solvable in quadratic time. Further, we present a characterization of minimal unsatisfiable formulas with n+1 clauses in terms of a certain form of matrices.

Explaining Performance Changes in Newly Privatized Firms

Abstract: Much debate has been generated about whether privatization tends to enhance firm financial performance. The research presented here seeks to identify the strategic choices that differentiated firms with superior post-privatization performance from those with inferior post-privatization performance. Using agency theory as a theoretical foundation, it is hypothesized that superior post-privatization firm performance will be associated with (1) the government not retaining a significant stock holding, (2) changes in leadership, (3) management stock options being initiated, (4) employee head count being reduced, and (5) the company being restructured financially. The sample draws from 41 privatized firms from six industry classifications and 15 countries. To accommodate comparisons of small subsamples, non-parametric statistical methods are used. Controlling for size, industry and country (economic/regulatory effects), the hypotheses are generally supported except for the one relating to headcount.

Integrated optical Ti:Er:LiNbO3 distributed Bragg reflector laser with a fixed photorefractive grating.

Abstract: For the first time to the authors' knowledge, an integrated optical distributed Bragg reflector laser with a fixed photorefractive grating in LiNbO(3) is demonstrated. Sample preparation, grating fabrication, and laser characteristics are reported. The device is pumped by a fiber pigtailed laser diode (lambda(p) approximately 1480 nm) through the Bragg grating in a double-pass configuration, yielding an emission in the backward direction at lambda=1531.7 nm . The laser threshold is 40 mW; as much as 5 mW of output power has been obtained at 110 mW of launched pump power in cw operation.

Second-harmonic imaging of ferroelectric domains in linbo3 with micron resolution in lateral and axial directions

Abstract: We demonstrate that scanning second-harmonic microscopy with a mode-locked laser can be used as a nondestructive technique to image ferroelectric domain structures with micron resolution in both lateral and axial directions. This method is expected to have significant impact particularly on the further development of nonlinear optical bulk and waveguide devices with periodically poled ferroelectric crystals. PACS: 07.60.Pb; 42.30; 77.80.Dj Ferroelectrics such as LiNbO3, KNbO3, or BaTiO3 play an important role in nonlinear optics and electro-optics because of their large second-order nonlinearities. A ferroelectric crystal may be composed of domains with different polar orientations.LiNbO3, for example, whose crystal symmetry is 3m, can show two different domain orientations. They differ in the signs of all non-vanishing components of the nonlinear optical tensor χ(2). Hence, for nonlinear optical experiments and applications, it is crucial to control the domain structure of the crystals. Devices for holographic data storage and optical parallel processing [1, 2] or for nonlinear frequency conversion with birefringent phase-matching usually require crystals with a single domain, whereas in recent years a lot of interest has been attracted by crystals fabricated with a periodic domain structure, mainly for quasi-phase-matched frequency conversion [3, 4]. The technique of quasi-phasematching (QPM) opens a number of very attractive possibilities in nonlinear optics: virtually any nonlinear interaction of waves within the transparency region of the crystal can be noncritically phase-matched at room temperature, and in addition QPM devices can be significantly less sensitive to the photorefractive effect [5], which is particularly important for the generation of visible light. The crucial prerequisite ∗ To whom correspondence should be addressed ∗∗ Present address:Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH-Hönggerberg, HPT E 17, CH-8093 Zürich, Switzerland, Phone: +41-1 /633-6825, Fax: +41-1 /633-1059, E-mail: paschott@iqe.phys.ethz.ch for using QPM is the ability to fabricate crystals with periodic domain structures of good quality, typically with periods in the range from3μm to 30μm. Various techniques have been used for this purpose, most successfully the technique of electric-field poling [6–8]. In any case, the further development of such methods requires techniques for the characterization of the obtained domain structures. First we briefly review the currently available techniques and later describe our new technique which is non-destructive, reproducible, quick and versatile, and provides images of domain structures with micron resolution in both lateral and axial directions. 1 Alternative techniques for the observation of ferroelectric domains Polarization microscopy is well suitable for the observation of the anisotropic grains in polycrystalline matter. The linear optical technique, however, does not allow us to distinguish between the antiparallel domains in ferroelectrics. Domain boundaries can cause visible structures due to stress birefringence or due to the electro-optic effect originating from charges on the boundaries. The unambiguous identification of domain structures, however, seems not to be possible with this method. Selective etching can be used to transform the domain structure on the surface of a crystal into a topographic structure [9, 10] that can be observed with a usual microscope. This method, however, is destructive, provides information about the domain structure on a polished surface but not from the interior of the specimen, and it works only on certain crystal faces (for example, on the Y and Z face, but not on the X face of aLiNbO3 crystal). Another method applied to crystal surfaces is the deposition of electrically charged powder particles [11]. This technique is non-destructive but suffers from a poor accuracy of the obtained images. Other nondestructive techniques such as atomic force microscopy [12– 14] and scanning secondary-electron microscopy [15, 16] have recently been applied to ferroelectric crystal surfaces. However, they also do not provide information on the interior domain structure. X-ray topography studies can reveal strain,

Equations of state for solids under strong compression

Abstract: The historical and theoretical background for the different mathematical forms commonly used to represent equations of state (EOS) for solids is reviewed and some criteria are discussed which allow to select some specific forms, which are not only convenient mathematical forms but bring out the physical meaning of the data. Some examples are discussed with emphasis on recent experimental and theoretical data including noble gas solids, simple metals and a few metals with special “anomalies” to illustrate the differences involved in the use of different EOS forms and to point out the specific physical reasons for differences in specific EOS forms.

Truly efficient parallel algorithms: 1-optimal multisearch for an extension of the BSP model

Abstract: In this paper we design and analyse parallel algorithms with the goal to get exact bounds on their speed-ups on real machines. For this purpose we define an extension of Valiant's BSP model, BSP*, that rewards blockwise communication, and uses Valiant's notion of c-optimality. Intuitively a c-optimal parallel algorithm for p processors achieves speed-up close to p/c. We consider the Multisearch problem: Assume a strip in 2D to be partitioned into m segments. Given n query points in the strip, the task is to locate, for each query, its segment. For m ≤ n we present a deterministic BSP* algorithm that is 1-optimal, if n = Ω(p log2p). For m > n, we present a randomized BSP* algorithm that is (1 + δ)-optimal for arbitrary δ>0, m ≤ 2p and n=Ω(p log2p). Both results hold for a wide range of BSP* parameters where the range becomes larger with growing input sizes m and n. We further report on implementation work in progress. Previous parallel algorithms for Multisearch were far away from being c-optimal in our model and do not consider blockwise communication.

Improved analytical modeling of conductive losses in gapped high-frequency inductors

Abstract: An improved method to predict conductive losses in gapped high frequency inductors is presented and used for parametrization of an equivalent small signal circuit model. The method is based on the superposition of power losses resulting from the well known one-dimensional field calculation and losses due to eddy currents caused by the fringing field of airgaps determined from new analytical 2D-field calculations. Losses due to reactive currents in the windings caused by the self capacitance are also considered An accuracy improvement of at least 50% percent compared to the known methods is proved by measurements.

Observation of a minimum in the temperature-dependent electrical resistance above the magnetic-ordering temperature in Gd2PdSi3

Abstract: Results on electrical resistivity, magnetoresistance, magnetic susceptibility, heat capacity and 155Gd Mossbauer measurements on a Gd-based intermetallic compound, Gd2PdSi3, are reported. A finding of interest is that the resistivity unexpectedly shows a well-defined minimum at about 45 K, well above the long-range magnetic-ordering temperature (21 K), a feature which gets suppressed by the application of a magnetic field. This observation in a Gd alloy presents an interesting scenario. On the basis of our results, we propose electron localization induced by s-f (or d-f) exchange interaction prior to long-range magnetic order as a mechanism for the electrical resistance minimum.

Embedding of Hypercubes into Grids

Abstract: We consider one-to-one embeddings of the n-dimensional hypercube into grids with 2n vertices and present lower and upper bounds and asymptotic estimates for minimal dilation, edge-congestion, and their mean values. We also introduce and study two new cost-measures for these embeddings, namely the sum over i=1, ..., n of dilations and the sum of edge-congestions caused by the hypercube edges of the ith dimension. It is shown that, in the simulation via the embedding approach, such measures are much more suitable for evaluating the slowdown of uniaxial hypercube algorithms then the traditional cost measures.

Functional flexibility: merely reacting or acting strategically?

Abstract: This article raises the question of to what extent functional flexibility is an operatively designed approach with which European companies confront current short‐term changes in their environment, and whether functional flexibility is integrated into long‐term human resource strategies. The proposition is tested that organisations with a strategic human resource management in the sense of a coordinated, objective‐oriented personnel management pattern, show a higher probability of using job rotation than organisations lacking a strategic‐oriented human resource management approach.

Randomized protocols for low-congestion circuit routing in multistage interconnection networks

Abstract: In this paper we study randomized algorithms for circuit switching on multistage networks related to the butterfly. We devise algorithms that route messages by constructing circuits (or paths) for the messages with small congestion, dilation, and setup time. Our algorithms are based on the idea of having each message choose a route from two possibilities, a technique that has previously proven successful in simpler load balancing settings. As an application of our techniques, we propose a novel design for a data server.

A Freely Available Morphological Analyzer, Disambiguator and Context Sensitive Lemmatizer for German

Abstract: In this paper we present Morphy, an integrated tool for German morphology, part-of-speech tagging and context-sensitive lemmatization. Its large lexicon of more than 320, 000 word forms plus its ability to process German compound nouns guarantee a wide morphological coverage. Syntactic ambiguities can be resolved with a standard statistical part-of-speech tagger. By using the output of the tagger, the lemmatizer can determine the correct root even for ambiguous word forms. The complete package is freely available and can be downloaded from the World Wide Web.

Zeros of the Zak transform on locally compact abelian groups

Abstract: Let G be a locally compact abelian group. The notion of Zak transform on L2 (Rd) extends to L2 (G). Suppose that G is compactly generated and its connected component of the identity is non-compact. Generalizing a classical result for L2 (R), we then prove that if f E L2(G) is such that its Zak transform Zf is continuous on G x G, then Zf has a zero.

Effect of increasing running velocity on electroencephalogram in a field test

Abstract: This study was designed to measure the electroencephalogram (EEG) after exercise with increasing intensity. In a field test with increments in running velocity a 2-min EEG was recorded, together with blood lactate concentration and heart rate, after each stage. An individual protocol was used, with up to six stages of running to ensure comparability of exercise intensity among the subjects, in each of 19 athletes (17 men, 2 women) experienced in leisure-time running. The exercise consisted initially of three running stages of aerobic exercise intensity without blood lactate accumulation followed by stages with an increase of lactate concentration. The protocol of the field test led to a progressive increase in cortical activity directly after the stages without blood lactate accumulation mainly in the δ frequency band, followed by θ and α-1 frequency band, and less pronounced in the α-2 and in the β frequency bands. After the stages with an onset and further increase of blood lactate accumulation significant decreases in the β-2, β-1 and α-1 frequency bands occurred predominantly in temporal (T3, T4, T5, and T6) and occipital (O1, and O2) electrode positions, indicating a stage-by-stage decrease of activity. This decrease may be explained by feed-back from working muscle, via afferents to the cortex from intero- and proprio-receptors and affective processes. This could suggest that through a higher running intensity indicated by an onset of blood lactate accumulation metabolic and mechanical changes led to alterations within the afferent systems influencing the level of cortical activity.

Automatic polarisation mode dispersion compensation in 40 Gbit/s optical transmission system

Abstract: Polarisation mode dispersion impedes the development of high capacity optical trunk lines but can be compensated for by an adaptive compensator in the receiver. By using three polarisation mode dispersion penalty extraction signals, differential group delays exceeding one bit duration are compensated for in a 40 Gbit/s optical transmission system for the first time.

The transport mechanism in micro-crystalline silicon

Abstract: We investigate the transport mechanism of μc-Si:H with a typical grain size in the 200 nm range. We study in particular a series of doped μc-Si:H samples with a wide range of crystallinities, x. The conductivity (and also the Hall mobility) vary by more than two orders of magnitude with crystallinity. In contrast, there is little variation of these quantities with temperature from room temperature to 10 K. We discuss these experimental data by comparison with theoretical network calculations of the dc conductivity and the Hall mobility for a random assembly of highly conducting crystalline and essentially non-conducting amorphous grains of concentrations x and (1-x), respectively. We show, that for x larger than the critical percolation concentration, P c , the sample conductivity can be less than the crystalline conductivity by more than two orders of magnitude. We further show that for x > P c , the Hall experiment, evaluated with the standard formulae valid for a homogeneous sample, yields the mean mobility of the mobile carriers to a good approximation. This agreement provides a theoretical basis for the use of Hall data in these inhomogeneous systems. The remaining temperature dependence of the conductivities for highly doped μc-Si:H is ascribed to the affects of barriers between the crystalline grains.

Hydrogenated and deuterated iron clusters : infrared spectra and density functional calculations

Abstract: Iron clusters react sequentially with hydrogen molecules to form multiply hydrogenated products. The increases in cluster ionization potential upon reaction verify that hydrogen chemisorbs dissociatively to form iron cluster–hydride complexes, FenHm. At low source temperatures, the cluster–hydride complexes take up additional hydrogen molecules which are shown to be physisorbed onto the underlying FenHm complexes to form FenHm(H2)p species. The infrared spectra of FenHm and FenDm (n=9–20) were obtained by the photodissociation action spectroscopic method in which depletion of the FenHm(H2)p and FenDm(D2)p species was the signature of absorption. The spectra, recorded in the 885–1090 cm−1 region, consist of several overlapping bands, each approximately 20 cm−1 in width. The dissimilarity of each FenHm(H2)p spectrum with the corresponding FenDm(D2)p spectrum indicates that the carrier involves hydrogen and is not merely due to absorption by the underlying iron cluster. Density functional calculations were performed on model complexes, Fe13H14 and Fe13D14, the iron portion of which was assumed to have Th symmetry. The infrared-active vibrational frequencies involving hydrogen bending and deuterium stretching are predicted to lie within the experimental frequency range of the experiment, well removed from the skeletal modes of the underlying iron cluster. The complexity of the observed spectra as compared to simulations based on the assumed (high-symmetry) model imply that the experimentally produced complexes possess low symmetry.

Analyzing an Infinite Parallel Job Allocation Process

Abstract: In recent years the task of allocating jobs to servers has been studied with the "balls and bins" abstraction. Results in this area exploit the large decrease in maximum load that can be achieved by allowing each job (ball) a very small amount of choice in choosing its destination server (bin). The scenarios considered can be divided into two categories: sequential, where each job can be placed at a server before the next job arrives, and parallel, where the jobs arrive in large batches that must be dealt with simultaneously. Another, orthogonal, classification of load balancing scenarios is into fixed time and infinite. Fixed time processes are only analyzed for an interval of time that is known in advance, and for all such results thus far either the number of rounds or the total expected number of arrivals at each server is a constant. In the infinite case, there is an arrival process and a deletion process that are both defined over an infinite time line. In this paper, we present an algorithm for allocating jobs arriving in parallel over an infinite time line. While there have been several results for the infinite sequential case, no analogous results exist for the infinite parallel case. We consider the process where m jobs arrive in each round to n servers, and each server is allowed to remove one job per round. We introduce a simple algorithm, where it is sufficient for each job to choose between 2 random servers, that obtains the following result: if m ≤ n/6e, then for any given round, the probability that any job has to wait more than O(log log n) rounds before being processed is at most 1/nα for any constant α. Furthermore, we analyze the distribution on waiting times: with the same probability, the number of jobs of any given round that have to wait t + c rounds to be processed is at most O(n/2(2t)) for a small constant c. These results are comparable with existing results for the infinite sequential case.

Efficient Q-switched Ti:Er:LiNbO3 waveguide laser

Abstract: Diode-pumped Q-switched Ti:Er:LiNbO/sub 3/ waveguide lasers have been developed with a monolithically integrated folded Mach-Zehnder type modulator of high extinction ratio as the Q-switch. The lasers are operated at 1562 nm wavelength, emitting pulses of 4.3 ns halfwidth with up to 1.44 kW peak power at 1 kHz repetition frequency. Good agreement between experimental and theoretical results has been obtained.