University of Paderborn

About: University of Paderborn is a education organization based out in Paderborn, Nordrhein-Westfalen, Germany. It is known for research contribution in the topics: Computer science & Context (language use). The organization has 6684 authors who have published 16929 publications receiving 323154 citations.

A Self-Consistent Charge Density-Functional Based Tight-Binding Scheme for Large Biomolecules

Abstract: (a) Department of Physics, Harvard University, Cambridge MA 02138, USA(b) Theoretische Physik, Universita¨t Paderborn, D-33098 Paderborn, Germany(c) Molekulare Biophysik, Deutsches Krebsforschungszentrum, D-69120 Heidelberg,Germany(Received August 10, 1999)A common feature of traditional tight-binding (TB) methods is the non-self-consistent solution ofthe eigenvalue problem of a Hamiltonian operator, represented in a minimal basis set. These TBschemes have been applied mostly to solid state systems, containing atoms with similar electrone-gativities. Recently self-consistent TB schemes have been developed which now allow the treat-ment of systems where a redistribution of charges, and the related detailed charge balance be-tween the atoms, become important as e.g. in biological systems. We discuss the application ofsuch a method, a self-consistent charge density-functional based TB scheme (SCC-DFTB), to bio-logical model compounds. We present recent extensions of the method: (i) The combination of thetight binding scheme with an empirical force field, that makes large scale simulations with severalthousand atoms possible. (ii) An extension which allows a quantitative description of weak-bond-ing interactions in biological systems. The latter include an improved description of hydrogenbonding achieved by extending the basis set and improved molecular stacking interactionsachieved by incorporating the dispersion contributions empirically. In applying the method, we pre-sent benchmarks for conformational energies, geometries and frequencies of small peptides andcompare with ab initio and semiempirical quantum chemistry data. These developments provide afast and reliable method, which can handle large scale quantum molecular dynamic simulations inbiological systems.

Stability and electronic structure of GaN nanotubes from density-functional calculations

Abstract: Density-functional calculations are used to predict the stability and electronic structures of GaN nanotubes. Strain energies of GaN nanotubes are comparable to those of carbon nanotubes, suggesting the possibility for the formation of GaN nanotubes. The zigzag nanotube is a semiconductor with direct band gap, whereas the armchair nanotube has an indirect band gap. The band gaps decrease with decreasing diameter, contrary to the case of carbon nanotubes. We further discuss possible ways of synthesizing GaN nanotubes in conjunction with carbon nanotubes.

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.

Physics of solids under strong compression

Abstract: Progress in high pressure physics is reviewed with special emphasis on recent developments in experimental techniques, pressure calibration, equations of state for simple substances and structural systematics of the elements. Short sections are also devoted to hydrogen under strong compression and general questions concerning new electronic ground states. This review was received in February 1995

Metasurface holography: from fundamentals to applications

Abstract: Holography has emerged as a vital approach to fully engineer the wavefronts of light since its invention dating back to the last century. However, the typically large pixel size, small field of view and limited space-bandwidth impose limitations in the on-demand high-performance applications, especially for three-dimensional displays and large-capacity data storage. Meanwhile, metasurfaces have shown great potential in controlling the propagation of light through the well-tailored scattering behavior of the constituent ultrathin planar elements with a high spatial resolution, making them suitable for holographic beam-shaping elements. Here, we review recent developments in the field of metasurface holography, from the classification of metasurfaces to the design strategies for both free-space and surface waves. By employing the concepts of holographic multiplexing, multiple information channels, such as wavelength, polarization state, spatial position and nonlinear frequency conversion, can be employed using metasurfaces. Meanwhile, the switchable metasurface holography by the integration of functional materials stimulates a gradual transition from passive to active elements. Importantly, the holography principle has become a universal and simple approach to solving inverse engineering problems for electromagnetic waves, thus allowing various related techniques to be achieved.