Showing papers by "University of Paderborn published in 2009"

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Abstract: QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

Hardware-Efficient Coherent Digital Receiver Concept With Feedforward Carrier Recovery for $M$ -QAM Constellations

Abstract: This paper presents a novel digital feedforward carrier recovery algorithm for arbitrary M-ary quadrature amplitude modulation (M-QAM) constellations in an intradyne coherent optical receiver. The approach does not contain any feedback loop and is therefore highly tolerant against laser phase noise. This is crucial, especially for higher order QAM constellations, which inherently have a smaller phase noise tolerance due to the lower spacing between adjacent constellation points. In addition to the mathematical description of the proposed carrier recovery algorithm also a possible hardware-efficient implementation in a parallelized system is presented and the performance of the algorithm is evaluated by Monte Carlo simulations for square 4-QAM (QPSK), 16-QAM, 64-QAM, and 256-QAM. For the simulations ASE noise and laser phase noise are considered as well as analog-to-digital converter (ADC) and internal resolution effects. For a 1 dB penalty at BER = 10-3, linewidth times symbol duration products of 4.1 x 10-4 (4-QAM), 1.4 x 10-4 (16-QAM), 4.0 x 10-5 (64-QAM) and 8.0 x 10-6 (256-QAM) are tolerable.

Electronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure

Abstract: Superconductivity was recently observed in the binary iron-based compound, FeSe. It is now shown that under pressure, the transition temperature can rise above 36 K. In addition, no static magnetic ordering is observed for this system, contrary to FeAs superconductors. The discovery of new high-temperature superconductors1 based on FeAs has led to a new ‘gold rush’ in high-TC superconductivity. All of the new superconductors share the same common structural motif of FeAs layers and reach TC values up to 55 K (ref. 2). Recently, superconductivity has been reported in FeSe (ref. 3), which has the same iron pnictide layer structure, but without separating layers. Here, we report the magnetic and electronic phase diagram of β-Fe1.01Se as a function of temperature and pressure. The superconducting transition temperature increases from 8.5 to 36.7 K under an applied pressure of 8.9 GPa. It then decreases at higher pressures. A marked change in volume is observed at the same time as TC rises, owing to a collapse of the separation between the Fe2Se2 layers. No static magnetic ordering is observed for the whole p–T phase diagram. We also report that at higher pressures (starting around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal NiAs-type structure and exhibits non-magnetic behaviour.

A virtual network mapping algorithm based on subgraph isomorphism detection

Abstract: Assigning the resources of a virtual network to the components of a physical network, called Virtual Network Mapping, plays a central role in network virtualization. Existing approaches use classical heuristics like simulated annealing or attempt a two stage solution by solving the node mapping in a first stage and doing the link mapping in a second stage.The contribution of this paper is a Virtual Network Mapping (VNM) algorithm based on subgraph isomorphism detection: it maps nodes and links during the same stage. Our experimental evaluations show that this method results in better mappings and is faster than the two stage approach, especially for large virtual networks with high resource consumption which are hard to map.

Magnetic Field-Induced Phase Transformation in NiMnCoIn Magnetic Shape-Memory Alloys—A New Actuation Mechanism with Large Work Output

Abstract: Magnetic shape memory alloys (MSMAs) have recently been developed into a new class of functional materials that are capable of magnetic-field-induced actuation, mechanical sensing, magnetic refrigeration, and energy harvesting. In the present work, the magnetic field-induced martensitic phase transformation (FIPT) in Ni{sub 45}Mn{sub 36.5}Co{sub 5}In{sub 13.5} MSMA single crystals is characterized as a new actuation mechanism with potential to result in ultra-high actuation work outputs. The effects of the applied magnetic field on the transformation temperatures, magnetization, and superelastic response are investigated. The magnetic work output of NiMnCoIn alloys is determined to be more than 1 MJ m{sup -3} per Tesla, which is one order of magnitude higher than that of the most well-known MSMAs, i.e., NiMnGa alloys. In addition, the work output of NiMnCoIn alloys is orientation independent, potentially surpassing the need for single crystals, and not limited by a saturation magnetic field, as opposed to NiMnGa MSMAs. Experimental and theoretical transformation strains and magnetostress levels are determined as a function of crystal orientation. It is found that [111]-oriented crystals can demonstrate a magnetostress level of 140 MPa T{sup -1} with 1.2% axial strain under compression. These field-induced stress and strain levels are significantly higher than those from existingmore » piezoelectric and magnetostrictive actuators. A thermodynamical framework is introduced to comprehend the magnetic energy contributions during FIPT. The present work reveals that the magnetic FIPT mechanism is promising for magnetic actuation applications and provides new opportunities for applications requiring high actuation work-outputs with relatively large actuation frequencies. One potential issue is the requirement for relatively high critical magnetic fields and field intervals (1.5-3 T) for the onset of FIPT and for reversible FIPT, respectively.« less

An overview on vehicle scheduling models

Abstract: The vehicle scheduling problem, arising in public transport bus companies, addresses the task of assigning buses to cover a given set of timetabled trips with consideration of practical requirements such as multiple depots and vehicle types as well as further extensions. An optimal schedule is characterized by minimal fleet size and/or minimal operational costs. Various publications were released as a result of extensive research in the last decades on this topic. Several modeling approaches as well as specialized solution strategies were presented for the problem and its extensions. This paper discusses the modeling approaches for different kinds of vehicle scheduling problems and gives an up-to-date and comprehensive overview on the basis of a general problem definition. Although we concentrate on the presentation of modeling approaches, also the basic ideas of solution approaches are given.

From model transformation to incremental bidirectional model synchronization

Abstract: The model-driven software development paradigm requires that appropriate model transformations are applicable in different stages of the development process The transformations have to consistently propagate changes between the different involved models and thus ensure a proper model synchronization However, most approaches today do not fully support the requirements for model synchronization and focus only on classical one-way batch-oriented transformations In this paper, we present our approach for an incremental model transformation which supports model synchronization Our approach employs the visual, formal, and bidirectional transformation technique of triple graph grammars Using this declarative specification formalism, we focus on the efficient execution of the transformation rules and how to achieve an incremental model transformation for synchronization purposes We present an evaluation of our approach and demonstrate that due to the speedup for the incremental processing in the average case even larger models can be tackled

Did the Hartz Reforms Speed-Up the Matching Process? A Macro-Evaluation Using Empirical Matching Functions

Abstract: Starting in January 2003, Germany implemented the first two so-calledHartz reforms, followed by the third and fourth packages of Hartzreforms in January 2004 and January 2005, respectively. The aim of thesereforms was to accelerate labor market flows and reduce unemploymentduration. Without attempting to evaluate the specific components ofthese Hartz reforms, this paper provides a first attempt to evaluate theoverall effectiveness of the first two reform waves, Hartz I/II and III,in speeding up the matching process between unemployed and vacant jobs.The analysis is conceptually rooted in the flow-based view underlyingthe reforms, estimating the structural features of the matching process.The results indicate that the reforms indeed had an impact in making thelabor market more dynamic and accelerating the matching process.

ReconOS: Multithreaded programming for reconfigurable computers

Abstract: Rising logic densities together with the inclusion of dedicated processor cores push reconfigurable devices from being applied for glue logic and prototyping towards implementing complete reconfigurable systems-on-chip. The mix of fast CPU cores and fine-grained reconfigurable logic allows to map both sequential, control-dominated code and highly parallel data-centric computations onto one platform. However, traditional design techniques that view specialized hardware circuits as passive coprocessors are ill-suited for programming these reconfigurable computers. In particular, the programming models for software—running on an embedded operating system—and digital hardware—synthesized to an FPGA—lack commonalities, which hinders design space exploration and severely impairs the potential for code reuse.In this article, we present ReconOS, an execution environment based on existing embedded operating systems that extends the multithreaded programming model established in the software domain to reconfigurable hardware. Using threads and common synchronization and communication services as an abstraction layer, ReconOS allows for the creation of portable and flexible multithreaded applications targeting CPU/FPGA systems. This article discusses the ReconOS programming model and its execution environment, presents implementations based on modern platform FPGAs and the operating systems eCos and Linux, evaluates time and area overheads of the proposed mechanisms and, finally, demonstrates the feasibility of the multithreading design approach on several case studies.

Getting Started with ADOL-C

Abstract: The C++ package ADOL-C described in this paper facilitates the evaluation of first and higher derivatives of vector functions that are defined by computer programs written in C or C++. The numerical values of derivative vectors are obtained free of truncation errors at mostly a small multiple of the run time and a fix small multiple random access memory required by the given function evaluation program. Derivative matrices are obtained by columns, by rows or in sparse format. This tutorial describes the source code modification required for the application of ADOL-C, the most frequently used drivers to evaluate derivatives and some recent developments.

Lactide Polymerisation with Air‐Stable and Highly Active Zinc Complexes with Guanidine–Pyridine Hybrid Ligands

Abstract: The synthesis of zinc complexes of guanidine-pyridine hybrid ligands [Zn(DMEGpy)Cl 2 ] (Cl), [Zn-(TMGpy)Cl 2 ] (C2), [Zn(DMEGqu)Cl 2 ] (C3), [Zn(TMGqu)Cl 2 ] (C4), [Zn-(DMEGpy)(CH 3 COO) 2 ] (C5), [Zn-(TMGpy)(CH 3 COO) 2 ] (C6), [Zn-(DMEGqu)(CH 3 COO) 2 ] (C7), [Zn-(TMGqu)(CH 3 COO) 2 ] (C8), [Zn-(DMEGqu) 2 (CF 3 SO 3 )] [CF 3 SO 3 ] (C9) and [Zn(TMGqu)2(CF 3 SO 3 )] [CF 3 SO 3 ] (C10) is reported. These zinc complexes were completely characterised and screened regarding their activity in the ring-opening polymerisation of D,L-lactide. They proved to be active initiators in lactide bulk polymerisation, and polylactides with molecular weights (M w ) up to 176000 g mol -1 could be obtained. They combine high activity with robustness towards moisture and air. The influence of reaction temperature and of the anionic component of the zinc salt on the activity of the catalyst, as well as the occurrence of undesired side reactions, was investigated. By correlating these findings with the structural study on the zinc complexes we could deduce a structure-reactivity relationship for the zinc catalysts. This study was accompanied by DFT calculations. The bis-chelate triflate complexes C9 and C10, supported by quinoline-guanidine ligands L3 and L4, exhibit by far the highest reactivity. Systematic comparison of these complexes with their mono-chelate counterparts and their bis-guanidine analogues allows the attributes that promote polymerisation by neutral guanidine ligand systems to be elucidated: accessibility to the zinc centre and Lewis acidity.

Responsive hydrogel layers—from synthesis to applications

Abstract: Responsive polymer networks are interesting materials for a variety of different applications due to the fact that they can perform a large volume transition. However, the swelling transition is a diffusion limited process. Thus, the decrease of the feature size (e.g., in thin layers) is an appropriate way to create structures with reasonable response time. The possibility to pattern responsive polymer networks makes them useful for application in microsystem technology as well as in biomedicine. The transition behavior of these films showed similar trends to those of the corresponding linear polymers whereas confinement effects have been found for thin hydrogel layers. The ability to optimize the integration of these polymers is critical for the fabrication and development of platforms that harness the unique abilities of responsive polymer networks. Here, recent developments on chemically cross-linked hydrogel layers with respect to synthesis, characterization, and application are highlighted.

Lithium niobate photonic wires

Abstract: LN photonic wires of cross-section dimensions down to 1 x 0.73 microm2 were fabricated by Ar milling of a single-crystalline LiNbO3 (LN) film bonded to a SiO2/LiNbO3 substrate. Mode intensity distributions, propagation losses, and group indices of refraction were measured at 1.55 microm wavelength and compared with simulation results. Moreover, effective mode indices and end face reflectivities were numerically evaluated. The waveguide of 1 microm top width is the smallest LN photonic wire reported to date; it has a mode size of approximately 0.4 microm2 (0.5 microm2) only and propagation losses of 9.9 dB/cm (12.9 dB/cm) for qTM (qTE) polarization.

Mechanical and wear properties of ultrafine-grained pure Ti produced by multi-pass equal-channel angular extrusion

Abstract: In this study, pure grade 2 Ti was processed via equal-channel angular extrusion (ECAE) for 8 and 12 passes following route-E at 300 °C. After processing, the microstructural evolution, tensile properties and wear behavior were investigated. ECAE-processed Ti exhibited a significant improvement in strength values with a slight decrease in ductility. However, the wear test results surprisingly showed that the strengthening of titanium by ECAE processing does not lead to the improvement of wear resistance at least for the pressures and sliding distances used in this study. This finding was mainly attributed to the tribochemical reaction leading to oxidative wear with the abrasive effect in Ti. Three distinct regions were formed on the subsurface of CG and UFG Ti after sliding wear, which are the tribolayer including titanium oxide with smeared wear material at the top, a deformed region having material structure oriented along the sliding direction in the middle, and the original unaffected bulk material at the bottom.

The role of monotonic pre-deformation on the fatigue performance of a high-manganese austenitic TWIP steel

Abstract: The fatigue performance of a high-manganese austenitic steel featuring the twinning-induced plasticity (TWIP) effect was investigated in both thermomechanically treated rolled and pre-deformed conditions A thorough set of mechanical experiments and microstructural analyses demonstrated that microstructural evolutions during high-strain monotonic and low-strain cyclic deformations differ significantly Specifically, a high twin density is obtained as a result of monotonic deformation, and a low twin density, yet a noticeable increase in twin thickness, is observed in the fatigued samples Furthermore, the fatigue performance of the pre-deformed TWIP steel samples is superior to that of the as-received steel, which is attributed to the enhanced interaction of glide dislocations with twins of increased density owing to the monotonic pre-deformation

Twenty Years of Service Guarantee Research: A Synthesis

Abstract: During the past two decades, service guarantees have received increased attention as a means for service firms to attract and retain customers and gain a competitive edge in the marketplace. Although many academic studies, referring to diverse service guarantee aspects, have appeared during this time, a synthesis of research is needed to clarify what researchers have learned about service guarantees and what remains unknown. To evaluate the state of published research on service guarantees, 109 articles published from 1985 to 2008 are collected and analyzed. The resultant review reveals a significant change in the type of research being performed, including a shift toward greater interest in the impact of service guarantees on consumer behavior and service firms. However, a significant shortfall marks empirical work directed toward the internal and operational effects of service guarantees. The effects of service guarantees on service performance, service recovery, and return on service guarantee investments are topics in need of further research.

Seasonal variability of the subpolar gyres in the Southern Ocean: a numerical investigation based on transfer operators

Abstract: . The detection of regions in the ocean that are coherent over an extended period of time is a fundamental problem in many oceanic applications. For instance such regions are important for studying the transport of marine species and for the distribution of nutrients. In this study we demonstrate the efficacy of transfer operators in detecting and analysing such structures. We focus first on the detection of the Weddell and Ross Gyre for the four seasons spanning December 2003–November 2004 within the 3-D oceanic domain south of 30° S, and show distinct seasonal differences in both the three-dimensional structure and the persistence of the gyres. Further, we demonstrate a new technique based on the discretised transfer operators to calculate the mean residence time of water within parts of the gyres and determine pathways of water leaving and entering the gyres.

Indistinguishable photons from independent semiconductor nanostructures.

Abstract: We demonstrate quantum interference between photons generated by the radiative decay processes of excitons that are bound to isolated fluorine donor impurities in ZnSe/ZnMgSe quantum-well nanostructures. The ability to generate single photons from these devices is confirmed by autocorrelation experiments, and the indistinguishability of photons emitted from two independent nanostructures is confirmed via a Hong-Ou-Mandel dip. These results indicate that donor impurities in appropriately engineered semiconductor structures can portray atomlike homogeneity and coherence properties, potentially enabling scalable technologies for future large-scale optical quantum computers and quantum communication networks.

PermaDAQ: A scientific instrument for precision sensing and data recovery in environmental extremes

Abstract: The PermaSense project has set the ambitious goal of gathering real-time environmental data for high-mountain permafrost in unattended operation over multiple years. This paper discusses the specialized sensing and data recovery architecture tailored to meet the precision, reliability and durability requirements of scientists utilizing the data for model validation. We present a custom sensor interface board including specialized sensors and redundancy features for end-to-end data validation. Aspects of high-quality data acquisition, design for reliability by strict separation of operating phases and analysis of energy efficiency are discussed. The system integration using the Dozer protocol scheme achieves a best-in-class average power consumption of 148µA considerably exceeding the lifetime requirement.

Temperature stabilized surface reconstructions at polar ZnO(0001).

Abstract: The atomic structure of the polar ZnO(0001) surfaces in a dry and humid oxygen environment is studied combining diffraction experiments and density-functional theory. Our results indicate that for similar stoichiometries a large number of very different, but energetically almost degenerate reconstructions exist. Thus vibrational entropy, which could be safely neglected for most semiconductor surfaces becomes dominant, giving rise to a hitherto not reported strong dependence of surface phase diagrams on temperature. Based on this insight we are able to consistently describe and explain the experimentally observed surface structures on polar ZnO(0001) surfaces.

Globalization and Factor Mobility: The Impact of the ``Bosman-Ruling'' on Player Migration in Professional Soccer

Abstract: Since the 1960s, increasing numbers of football players from Eastern Europe, South America, Africa, and Asia have been migrating to the top leagues in Western Europe (England, France, Germany, Italy, and Spain). This development has been massively fostered by the ``Bosman ruling'' of the European Court of Justice in December 1995. Perhaps surprisingly, the decreasing playing time that is now available to ``local players'' has not yet resulted in an increasing competitive balance of the national teams of countries importing players and those exporting players.

A distributed polylogarithmic time algorithm for self-stabilizing skip graphs

Abstract: Peer-to-peer systems rely on scalable overlay networks that enable efficient routing between its members. Hypercubic topologies facilitate such operations while each node only needs to connect to a small number of other nodes. In contrast to static communication networks, peer-to-peer networks allow nodes to adapt their neighbor set over time in order to react to join and leave events and failures. This paper shows how to maintain such networks in a robust manner. Concretely, we present a distributed and self-stabilizing algorithm that constructs a (variant of the) skip graph in polylogarithmic time from any initial state in which the overlay network is still weakly connected. This is an exponential improvement compared to previously known self-stabilizing algorithms for overlay networks. In addition, individual joins and leaves are handled locally and require little work.

Autonomous Sensor Nodes for Aircraft Structural Health Monitoring

Abstract: The current trend towards the use of novel materials and design concepts for aircraft structures may demand structural health monitoring (SHM) systems in the future. Wireless sensor networks (WSNs) could fulfill the required monitoring tasks concerning fatigue, damage or stress of structural parts. Autonomous sensor nodes (ASNs) are key elements for a WSN in order to allow a self-sufficient and maintenance free operation, without any complex wiring for power supply or communication purposes.

Pricing Randomized Allocations

Abstract: Randomized mechanisms, which map a set of bids to a probability distribution over outcomes rather than a single outcome, are an important but ill-understood area of computational mechanism design. We investigate the role of randomized outcomes (henceforth, "lotteries") in the context of a fundamental and archetypical multi-parameter mechanism design problem: selling heterogeneous items to unit-demand bidders. To what extent can a seller improve her revenue by pricing lotteries rather than items, and does this modification of the problem affect its computational tractability? Our results show that the answers to these questions hinge on whether consumers can purchase only one lottery (the buy-one model) or purchase any set of lotteries and receive an independent sample from each (the buy-many model). In the buy-one model, there is a polynomial-time algorithm to compute the revenue-maximizing envy-free prices (thus overcoming the inapproximability of the corresponding item pricing problem) and the revenue of the optimal lottery system can exceed the revenue of the optimal item pricing by an unbounded factor as long as the number of item types exceeds 4. In the buy-many model with n item types, the profit achieved by lottery pricing can exceed item pricing by a factor of O(log n) but not more, and optimal lottery pricing cannot be approximated within a factor of O(n^eps) for some eps>0, unless NP has subexponential-time randomized algorithms. Our lower bounds rely on a mixture of geometric and algebraic techniques, whereas the upper bounds use a novel rounding scheme to transform a mechanism with randomized outcomes into one with deterministic outcomes while losing only a bounded amount of revenue.