@article{13535, author = {{Sanna, S. and Schmidt, Wolf Gero}}, issn = {{0885-3010}}, journal = {{IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control}}, number = {{9}}, pages = {{1925--1928}}, title = {{{Ferroelectric phase transition in LiNbO3: Insights from molecular dynamics}}}, doi = {{10.1109/tuffc.2012.2408}}, volume = {{59}}, year = {{2012}}, } @article{13538, author = {{Riefer, A. and Sanna, S. and Schmidt, Wolf Gero}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, number = {{12}}, title = {{{Polarization-dependent methanol adsorption on lithium niobate Z-cut surfaces}}}, doi = {{10.1103/physrevb.86.125410}}, volume = {{86}}, year = {{2012}}, } @article{62927, abstract = {{To model intermolecular excitation transfer between organic chromophores in the framework of Förster theory, the interaction matrix element is needed for all relative orientations and separations of chromophores. Simulations of extended multi-chromophoric systems thus require a fast but reliable approximation scheme to calculate these dipole interactions. By means of a comparative study of the dipole approximation with quantum chemistry, we demonstrate that the usual line-dipole theory, while suitable for short molecules, breaks down for longer molecules with inter-molecular separations similar to or smaller than the length of the interacting chromophores; a limit that is typically found in conjugated polymer thin films. As a remedy, we propose an improved way of distributing the sub-dipole moments within a line which provides results in very good agreement with the quantum chemistry, and is still simple enough to be used in large scale simulations.}}, author = {{Denis, Jean-Christophe and Schumacher, Stefan and Galbraith, Ian}}, issn = {{0021-9606}}, journal = {{The Journal of Chemical Physics}}, number = {{22}}, publisher = {{AIP Publishing}}, title = {{{Quantitative description of interactions between linear organic chromophores}}}, doi = {{10.1063/1.4768244}}, volume = {{137}}, year = {{2012}}, } @article{15869, author = {{Wiebeler, Christian and Tautz, Raphael and Feldmann, Jochen and von Hauff, Elizabeth and Da Como, Enrico and Schumacher, Stefan}}, issn = {{1520-6106}}, journal = {{The Journal of Physical Chemistry B}}, pages = {{4454--4460}}, title = {{{Spectral Signatures of Polarons in Conjugated Co-polymers}}}, doi = {{10.1021/jp3084869}}, year = {{2012}}, } @article{13820, author = {{Schmidt, Wolf Gero and Wippermann, S. and Sanna, S. and Babilon, M. and Vollmers, N. J. and Gerstmann, Uwe}}, issn = {{0370-1972}}, journal = {{physica status solidi (b)}}, number = {{2}}, pages = {{343--359}}, title = {{{In-Si(111)(4 × 1)/(8 × 2) nanowires: Electron transport, entropy, and metal-insulator transition}}}, doi = {{10.1002/pssb.201100457}}, volume = {{249}}, year = {{2012}}, } @article{15872, author = {{Montgomery, Neil A. and Hedley, Gordon J. and Ruseckas, Arvydas and Denis, Jean-Christophe and Schumacher, Stefan and Kanibolotsky, Alexander L. and Skabara, Peter J. and Galbraith, Ian and Turnbull, Graham A. and Samuel, Ifor D. W.}}, issn = {{1463-9076}}, journal = {{Physical Chemistry Chemical Physics}}, title = {{{Dynamics of fluorescence depolarisation in star-shaped oligofluorene-truxene molecules}}}, doi = {{10.1039/c2cp24141b}}, year = {{2012}}, } @article{18542, abstract = {{We present recent advances in numerical implementations of hybrid functionals and the GW approximation within the full-potential linearized augmented-plane-wave (FLAPW) method. The former is an approximation for the exchange–correlation contribution to the total energy functional in density-functional theory, and the latter is an approximation for the electronic self-energy in the framework of many-body perturbation theory. All implementations employ the mixed product basis, which has evolved into a versatile basis for the products of wave functions, describing the incoming and outgoing states of an electron that is scattered by interacting with another electron. It can thus be used for representing the nonlocal potential in hybrid functionals as well as the screened interaction and related quantities in GW calculations. In particular, the six-dimensional space integrals of the Hamiltonian exchange matrix elements (and exchange self-energy) decompose into sums over vector–matrix–vector products, which can be evaluated easily. The correlation part of the GW self-energy, which contains a time or frequency dependence, is calculated on the imaginary frequency axis with a subsequent analytic continuation to the real axis or, alternatively, by a direct frequency convolution of the Green function G and the dynamically screened Coulomb interaction W along a contour integration path that avoids the poles of the Green function. Hybrid-functional and GW calculations are notoriously computationally expensive. We present a number of tricks that reduce the computational cost considerably, including the use of spatial and time-reversal symmetries, modifications of the mixed product basis with the aim to optimize it for the correlation self-energy and another modification that makes the Coulomb matrix sparse, analytic expansions of the interaction potentials around the point of divergence at k=0, and a nested density and density-matrix convergence scheme for hybrid-functional calculations. We show CPU timings for prototype semiconductors and illustrative results for GdN and ZnO. }}, author = {{Friedrich, Christoph and Betzinger, Markus and Schlipf, Martin and Blügel, Stefan and Schindlmayr, Arno}}, issn = {{1361-648X}}, journal = {{Journal of Physics: Condensed Matter}}, number = {{29}}, publisher = {{IOP Publishing}}, title = {{{Hybrid functionals and GW approximation in the FLAPW method}}}, doi = {{10.1088/0953-8984/24/29/293201}}, volume = {{24}}, year = {{2012}}, } @article{4104, abstract = {{We report on the molecular beam epitaxy growth of cubic GaN on 3C–SiC (001) nanostructures. Transmission electron microscopy (TEM) studies show phase-pure cubic GaN crystals with a low defect density on top of the post shaped 3C–SiC nanostructures and GaN grown on their sidewalls, which is dominated by {111} planar defects. The nanostructures, aligned parallel and perpendicular to the [110] directions of the substrate, are located in anti-phase domains of the 3C–SiC/Si (001) substrate. These anti-phase domains strongly influence the optimum growth of GaN layers in these regions. TEM measurements demonstrate a different stacking fault density in the cubic GaN epilayer in these areas.}}, author = {{Kemper, R.M. and Hiller, L. and Stauden, T. and Pezoldt, J. and Duschik, K. and Niendorf, T. and Maier, H.J. and Meertens, D. and Tillmann, K. and As, D.J. and Lindner, Jörg}}, issn = {{0022-0248}}, journal = {{Journal of Crystal Growth}}, location = {{Nara (Japan)}}, pages = {{291--294}}, publisher = {{Elsevier BV}}, title = {{{Growth of cubic GaN on 3C–SiC/Si (001) nanostructures}}}, doi = {{10.1016/j.jcrysgro.2012.10.011}}, volume = {{378}}, year = {{2012}}, } @article{3974, abstract = {{We study the quantum properties and statistics of photons emitted by a quantum-dot biexciton inside a cavity. In the biexciton-exciton cascade, fine-structure splitting between exciton levels degrades polarization-entanglement for the emitted pair of photons. However, here we show that the polarization-entanglement can be preserved in such a system through simultaneous emission of two degenerate photons into cavity modes tuned to half the biexciton energy. Based on detailed theoretical calculations for realistic quantum-dot and cavity parameters, we quantify the degree of achievable entanglement.}}, author = {{Schumacher, Stefan and Förstner, Jens and Zrenner, Artur and Florian, Matthias and Gies, Christopher and Gartner, Paul and Jahnke, Frank}}, issn = {{1094-4087}}, journal = {{Optics Express}}, keywords = {{tet_topic_qd}}, number = {{5}}, pages = {{5335--5342}}, publisher = {{OSA}}, title = {{{Cavity-assisted emission of polarization-entangled photons from biexcitons in quantum dots with fine-structure splitting}}}, doi = {{10.1364/oe.20.005335}}, volume = {{20}}, year = {{2012}}, } @article{40405, author = {{Sharapova, Polina and Tikhonova, O V}}, issn = {{1063-7818}}, journal = {{Quantum Electronics}}, keywords = {{Electrical and Electronic Engineering, Atomic and Molecular Physics, and Optics, Statistical and Nonlinear Physics, Electronic, Optical and Magnetic Materials}}, number = {{3}}, pages = {{199--207}}, publisher = {{IOP Publishing}}, title = {{{Dynamics of ionisation and entanglement in the 'atom + quantum electromagnetic field' system}}}, doi = {{10.1070/qe2012v042n03abeh014805}}, volume = {{42}}, year = {{2012}}, } @inproceedings{3965, abstract = {{We design the geometrical shape of plasmonic nanostructures to achieve field patterns with desired properties. For this, we combine Maxwell simulations and automatic optimization techniques. By allowing variations of the geometrical shape, which can be based on either boxes or arbitrary polygons, we maximize the desired objective.}}, author = {{Hildebrandt, Andre and Reichelt, Matthias and Meier, Torsten and Förstner, Jens}}, keywords = {{tet_topic_optical antenna, tet_topic_plasmonics}}, location = {{Bad Honnef}}, number = {{59}}, publisher = {{AIP AIP Conference Proceedings 1475}}, title = {{{Optimization of the intensity enhancement in plasmonic nanoantennas}}}, doi = {{10.1063/1.4750095}}, year = {{2012}}, } @article{4136, abstract = {{Results of atomistic simulations aimed at understanding precipitation of the highly attractive wide band gap semiconductor material silicon carbide in silicon are presented. The study involves a systematic investigation of intrinsic and carbon-related defects as well as defect combinations and defect migration by both, quantummechanical first-principles as well as empirical potential methods. Comparing formation and activation energies, ground-state structures of defects and defect combinations as well as energetically favorable agglomeration of defects are predicted. Moreover, accurate ab initio calculations unveil limitations of the analytical method based on a Tersoff-like bond order potential. A work-around is proposed in order to subsequently apply the highly efficient technique on large structures not accessible by first-principles methods. The outcome of both types of simulation provides a basic microscopic understanding of defect formation and structural evolution particularly at non-equilibrium conditions strongly deviated from the ground state as commonly found in SiC growth processes. A possible precipitation mechanism, which conforms well to experimental findings and clarifies contradictory views present in the literature is outlined.}}, author = {{Zirkelbach, F. and Stritzker, B. and Nordlund, K. and Schmidt, Wolf Gero and Rauls, E. and Lindner, Jörg K. N.}}, issn = {{1862-6351}}, journal = {{physica status solidi (c)}}, number = {{10-11}}, pages = {{1968--1973}}, publisher = {{Wiley}}, title = {{{First-principles and empirical potential simulation study of intrinsic and carbon-related defects in silicon}}}, doi = {{10.1002/pssc.201200198}}, volume = {{9}}, year = {{2012}}, } @article{3972, abstract = {{Using a finite-difference time-domain method, we theoretically investigate the optical spectra of crossing perpendicular photonic crystal waveguides with quantum dots embedded in the central rod. The waveguides are designed so that the light mainly propagates along one direction and the cross talk is greatly reduced in the transverse direction. It is shown that when a quantum dot (QD) is resonant with the cavity, strong coupling can be observed via both the transmission and crosstalk spectrum. If the cavity is far off-resonant from the QD, both the cavity mode and the QD signal can be detected in the transverse direction since the laser field is greatly suppressed in this direction. This structure could have strong implications for resonant excitation and in-plane detection of QD optical spectroscopy.}}, author = {{Song, Xiaohong and Declair, Stefan and Meier, Torsten and Zrenner, Artur and Förstner, Jens}}, issn = {{1094-4087}}, journal = {{Optics Express}}, keywords = {{tet_topic_phc, tet_topic_qd}}, number = {{13}}, pages = {{14130--14136}}, publisher = {{The Optical Society}}, title = {{{Photonic crystal waveguides intersection for resonant quantum dot optical spectroscopy detection}}}, doi = {{10.1364/oe.20.014130}}, volume = {{20}}, year = {{2012}}, } @inproceedings{3967, abstract = {{We simulate the linear and nonlinear optical response from split-ring resonator (SRR) arrays to study collective effects between the constituent SRRs that determine spectral properties of the second harmonic generation (SHG). We apply the Discontinuous Galerkin Time Domain (DGTD) method and the hydrodynamic Maxwell-Vlasov model to calculate the SHG emission. Our model is able to qualitatively reproduce and explain the non-monotonic dependence of the spectral SHG transmission measured experimentally for SRR arrays with different lattice constants}}, author = {{Grynko, Yevgen and Meier, Torsten and Linden, Stefan and Niesler, Fabian B. P. and Wegener, Martin and Förstner, Jens}}, keywords = {{tet_topic_meta, tet_topic_shg}}, location = {{Bad Honnef}}, number = {{1}}, pages = {{128--130}}, publisher = {{AIP Conference Proceedings}}, title = {{{Near-field coupling and second-harmonic generation in split-ring resonator arrays}}}, doi = {{10.1063/1.4750118}}, volume = {{1475}}, year = {{2012}}, } @article{3970, abstract = {{Optical experiments on second-harmonic generation from split-ring-resonator square arrays show a nonmonotonic dependence of the conversion efficiency on the lattice constant. This finding is interpreted in terms of a competition between dilution effects and linewidth or near-field changes due to interactions among the individual elements in the array.}}, author = {{Linden, S. and Niesler, F. B. P. and Förstner, Jens and Grynko, Yevgen and Meier, Torsten and Wegener, M.}}, issn = {{0031-9007}}, journal = {{Physical Review Letters}}, keywords = {{tet_topic_shg, tet_topic_meta}}, number = {{1}}, publisher = {{American Physical Society (APS)}}, title = {{{Collective Effects in Second-Harmonic Generation from Split-Ring-Resonator Arrays}}}, doi = {{10.1103/physrevlett.109.015502}}, volume = {{109}}, year = {{2012}}, } @article{43200, abstract = {{A novel adaptive approach to compute the eigenenergies and eigenfunctions of the two-particle (electron-hole) Schrödinger equation including Coulomb attraction is presented. As an example, we analyze the energetically lowest exciton state of a thin one-dimensional semiconductor quantum wire in the presence of disorder which arises from the non-smooth interface between the wire and surrounding material. The eigenvalues of the corresponding Schrödinger equation, i.e., the one-dimensional exciton Wannier equation with disorder, correspond to the energies of excitons in the quantum wire. The wavefunctions, in turn, provide information on the optical properties of the wire. We reformulate the problem of two interacting particles that both can move in one dimension as a stationary eigenvalue problem with two spacial dimensions in an appropriate weak form whose bilinear form is arranged to be symmetric, continuous, and coercive. The disorder of the wire is modelled by adding a potential in the Hamiltonian which is generated by normally distributed random numbers. The numerical solution of this problem is based on adaptive wavelets. Our scheme allows for a convergence proof of the resulting scheme together with complexity estimates. Numerical examples demonstrate the behavior of the smallest eigenvalue, the ground state energies of the exciton, together with the eigenstates depending on the strength and spatial correlation of disorder.}}, author = {{Meier, Torsten and Mollet, Christian and Kunoth, Angela}}, journal = {{Communications in Computational Physics}}, number = {{1}}, pages = {{21--47}}, publisher = {{Cambridge University Press}}, title = {{{Excitonic Eigenstates of Disordered Semiconductor Quantum Wires: Adaptive Wavelet Computation of Eigenvalues for the Electron-Hole Schrödinger Equation}}}, doi = {{10.4208/cicp.081011.260712a}}, volume = {{14}}, year = {{2012}}, } @article{7495, author = {{Urbanski, Martin and Piegdon, Karoline A. and Meier, Cedrik and Kitzerow, Heinz-Siegfried}}, issn = {{0267-8292}}, journal = {{Liquid Crystals}}, number = {{4}}, pages = {{475--482}}, publisher = {{Informa UK Limited}}, title = {{{Investigations on the director field around microdisc resonators}}}, doi = {{10.1080/02678292.2011.552742}}, volume = {{38}}, year = {{2011}}, } @phdthesis{26976, author = {{Mracek, Boris}}, publisher = {{Verlagsschriftenreihe des Heinz Nixdorf Instituts, Paderborn; Band 297}}, title = {{{Untersuchung des dynamischen Verhaltens gekoppelter piezoelektrischer Ultraschallmotoren mit Stoßkontakt}}}, volume = {{Band 297}}, year = {{2011}}, } @phdthesis{23195, author = {{Bruns, Torsten}}, publisher = {{Universität Paderborn}}, title = {{{Trajektorienplanung mittels Diskretisierung und kombinatorischer Optimierung am Beispiel des autonomen Kreuzungsmanagements für Kraftfahrzeuge.}}}, volume = {{299}}, year = {{2011}}, } @article{39733, author = {{Redler, Andreas and Hoischen, Andreas and Kitzerow, Heinz-Siegfried}}, issn = {{1542-1406}}, journal = {{Molecular Crystals and Liquid Crystals}}, keywords = {{Condensed Matter Physics, General Materials Science, General Chemistry}}, number = {{1}}, pages = {{97/[1787]--107/[1797]}}, publisher = {{Informa UK Limited}}, title = {{{Liquid Crystal/Polymer Composites: Kinetic Study of the Grating Formation in Holographic Polymer-Dispersed Liquid Crystals}}}, doi = {{10.1080/15421406.2011.572798}}, volume = {{547}}, year = {{2011}}, }