360 Works

Flexible registration method for light-stripe sensors considering sensor misalignments

W. Gorschenew, M. Kaestner & Eduard Reithmeier
In many application areas such as object reconstruction or quality assurance, it is required to completely or partly measure the shape of an object or at least the cross section of the required object region. For complex geometries, therefore, multiple views are needed to bypass undercuts respectively occlusions. Hence, a multi-sensor measuring system for complex geometries has to consist of multiple light-stripe sensors that are surrounding the measuring object in order to complete the measurements...

Engineering interactions and anyon statistics by multicolor lattice-depth modulations

Lorenzo Cardarelli, Sebastian Greschner & Luis Santos
We show that a multicolor modulation of the depth of an optical lattice allows for a flexible independent control of correlated hopping, occupation-dependent gauge fields, effective on-site interactions without Feshbach resonances, and nearest-neighbor interactions. As a result, the lattice-depth modulation opens the possibility of engineering with minimal experimental complexity a broad class of lattice models in current experiments with ultracold atoms, including Hubbard models with correlated hopping, peculiar extended models, and two-component anyon-Hubbard models. ©...

Polar molecules in frustrated triangular ladders

Tapan Mishra, Sebastian Greschner & Luis Santos
Polar molecules in geometrically frustrated lattices may result in a very rich landscape of quantum phases, due to the nontrivial interplay between frustration, and two- and possibly three-body intersite interactions. In this paper we illustrate this intriguing physics for the case of hard-core polar molecules in frustrated triangular ladders. Whereas commensurate lattice fillings result in gapped phases with bond order and/or density-wave order, at incommensurate fillings we find chiral, two-component, and pair superfluids. We show...

Density-dependent synthetic gauge fields using periodically modulated interactions

Sebastian Greschner, G. Sun, D. Poletti & Luis Santos
We show that density-dependent synthetic gauge fields may be engineered by combining periodically modulated interactions and Raman-assisted hopping in spin-dependent optical lattices. These fields lead to a density-dependent shift of the momentum distribution, may induce superfluid-to-Mott insulator transitions, and strongly modify correlations in the superfluid regime. We show that the interplay between the created gauge field and the broken sublattice symmetry results, as well, in an intriguing behavior at vanishing interactions, characterized by the appearance...

Exploring unconventional hubbard models with doubly modulated lattice gases

Sebastian Greschner, Luis Santos & D. Poletti
Recent experiments show that periodic modulations of cold atoms in optical lattices may be used to engineer and explore interesting models. We show that double modulation combining lattice shaking and modulated interactions allows for the engineering of a much broader class of lattice with correlated hopping, which we study for the particular case of one-dimensional systems. We show, in particular, that by using this double modulation it is possible to study Hubbard models with asymmetric...

Closing the gap between spatial and spin dynamics of electrons at the metal-to-insulator transition

Jan G. Lonnemann, Eddy P. Rugeramigabo, Michael Oestreich & Jens Hübner
We combine extensive precision measurements of the optically detected spin dynamics and magneto-transport measurements in a contiguous set of n-doped bulk GaAs structures, in order to unambiguously unravel the intriguing but complex contributions to the spin relaxation at the metal-to-insulator transition (MIT). Just below the MIT, the interplay between hopping induced loss of spin coherence and hyperfine interaction yields a maximum spin lifetime exceeding 800 ns. At slightly higher doping concentrations, however, the spin relaxation...

Ultrahigh bandwidth spin noise spectroscopy: Detection of large g-factor fluctuations in highly-n-doped GaAs

Fabian Berski, Hendrik Kuhn, Jan G. Lonnemann, Jens Hübner & Michael Oestreich
We advance all optical spin noise spectroscopy (SNS) in semiconductors to detection bandwidths of several hundred gigahertz by employing a sophisticated scheme of pulse trains from ultrafast laser oscillators as an optical probe. The ultrafast SNS technique avoids the need for optical pumping and enables nearly perturbation free measurements of extremely short spin dephasing times. We apply the technique to highly-n-doped bulk GaAs where magnetic field dependent measurements show unexpected large g-factor fluctuations. Calculations suggest...

Electron-spin relaxation in bulk GaAs for doping densities close to the metal-to-insulator transition

M. Römer, H. Bernien, G. Müller, D. Schuh, Jens Hübner & Michael Oestreich
We have measured the electron-spin-relaxation rate and the integrated spin noise power in n-doped GaAs for temperatures between 4 and 80 K and for doping concentrations ranging from 2.7×1015 to 8.8×1016 cm−3 using spin noise spectroscopy. The temperature-dependent measurements show a clear transition from localized to free electrons for the lower doped samples and confirm mainly free electrons at all temperatures for the highest doped sample. While the sample at the metal-to-insulator transition shows the longest...

Optical orientation of electron spins in GaAs quantum wells

S. Pfalz, R. Winkler, T. Nowitzki, D. Reuter, A.D. Wieck, D. Hägele & Michael Oestreich
We present a detailed experimental and theoretical analysis of the optical orientation of electron spins in GaAs/AlAs quantum wells. Using time and polarization resolved photoluminescence excitation spectroscopy, the initial degree of electron-spin polarization is measured as a function of excitation energy for a sequence of quantum wells with well widths between 63 and 198 Å. The experimental results are compared with an accurate theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and...

A practical synthesis of Rho-Kinase inhibitor Y-27632 and fluoro derivatives and their evaluation in human pluripotent stem cells

Jiri Paleek, Robert Zweigerdt, Ruth Olmer, Ulrich Martin, Andreas Kirschning & Gerald Dräger
A practical synthesis of the Rho-Kinase inhibitor Y-27632 and two new fluoro derivatives was achieved in seven steps and with a good overall yield of 45% starting from commercially available (R)-1-phenylethylamine. Compared to Y-27632 the new fluoro derivatives showed reduced or no effect on hPSC vitality and expansion after dissociation in human pluripotent stem cells. © 2011 The Royal Society of Chemistry.

AsS2Cl - An arsenic(V) compound? Formation, stability and structure of gaseous AsSCl and AsS2Cl - A combined experimental and theoretical study

E. Milke, R. Köppe & Michael Binnewies
By reaction of solid As4S4 with gaseous Cl2 at a temperature of 410 K gaseous AsSCl and AsS2Cl are formed. Unexpectedly in AsS2Cl the arsenic is not of formal oxidation state +V but +III: the molecular structure of AsS2Cl is arranged as a 1-chloro-dithia-arsirane and comprises an hitherto unknown AsS2 three-membered ring. Thermodynamic data on AsSCl and AsS2Cl are obtained by mass spectrometry (MS). The experimental data are extended and confirmed by ab initio quantum...

Brookite versus anatase TiO2 photocatalysts: Phase transformations and photocatalytic activities

Tarek A. Kandiel, Lars Robben, Ayad Alkaim & Detlef W. Bahnemann
Titanium dioxide nanoparticles consisting of pure anatase, anatase-rich, brookite-rich, and pure brookite modifications were synthesized and characterized by X-ray diffraction, field emission-scanning electron microscopy and nitrogen adsorption. The phase transformations among the three modifications of TiO2 (anatase, brookite, and rutile) and the photocatalytic activities of these nanoparticles were investigated by heat treatment over the temperature range from 400 to 800 °C and by the photooxidation of methanol, respectively. Direct transformation of anatase and brookite to...

Investigation of ion-ion-recombination at atmospheric pressure with a pulsed electron gun

Andre Heptner, Philipp Cochems, Jens Langejuergen, Frank Gunzer & Stefan Zimmermann
For future development of simple miniaturized sensors based on pulsed atmospheric pressure ionization as known from ion mobility spectrometry, we investigated the reaction kinetics of ion-ion-recombination to establish selective ion suppression as an easy to apply separation technique for otherwise non-selective ion detectors. Therefore, the recombination rates of different positive ion species, such as protonated water clusters H +(H 2O) n (positive reactant ions), acetone, ammonia and dimethyl-methylphosphonate ions, all recombining with negative oxygen clusters...

High-resolution 27Al MAS NMR spectroscopic studies of the response of spinel aluminates to mechanical action

Vladimir Šepelák, Ingo Bergmann, Sylvio Indris, Armin Feldhoff, Horst Hahn, Klaus-Dieter Becker, Clare P. Grey & Paul Heitjans
The response of the local structure of various types of spinel aluminates, ZnAl2O4 (normal spinel), MgAl2O4 (partly inverse spinel), and Li0.5Al2.5O4 (fully inverse spinel), to mechanical action through high-energy milling is investigated by means of 27Al MAS NMR. Due to the ability of this nuclear spectroscopic technique to probe the local environment of Al nuclei, valuable quantitative insight into the mechanically induced changes in the spinel structure, such as the local cation disorder and the...

Skepsis als Weg zum Glück? Eine Untersuchung zum Zusammenhang von Urteilsenthaltung und Seelenruhe in der Antike und der Aufklärung

Anneke Meyer
Wezel stimmte 1776 ein Lob des Zweifels an, obwohl zu dieser Zeit der frühe neuzeitliche Zweifel längst vom Fortschrittsoptimismus der Wissenschaft abgelöst war. Zugleich behauptete er, dass eine skeptische Lebenshaltung zu einem ruhigen und gelassenen, womöglich sogar glücklichen Leben führen könne. (www.wehrhahn-verlag.de)

Hydraulic performance of elastomeric bonded permeable revetments and subsoil response to wave loads

Hocine Oumeraci, T. Staal, S. Pförtner, M. Kudella, Stefan Schimmels & H.J. Verhagen
Elastomeric bonded permeable revetments, also called PBA (Polyurethane bonded aggregate) revetments, are highly porous structures made of mineral aggregates (e.g. crushed stones) which are durably and elastically bonded by polyurethane (PU). Despite their numerous advantages as compared to conventional revetments and the large experience available from more than 25 pilot projects, physically-based design formulae to predict their hydraulic performance, wave loading and response are still lacking. Therefore, the present study aims at improving the understanding...

Risk-based life cycle mim strategy for coastal structures - Effect of pre-existing damages on failure probability

Nannina Horstmann, Kerstin Hinze, Stefan Schimmels & Hocine Oumeraci
A risk-based strategy for monitoring, inspection and maintenance (MIM) is described as a key component of an overall framework for life-cycle engineering and management. Its application for coastal structures is exemplarily outlined for sea/estuary dikes and harbor quay walls. The necessity of the incorporation of pre-existing damages into this strategy is exemplarily illustrated for quay walls, showing how this will affect the failure probability in comparison to a quay wall without any pre-existing damages.

Modelling wave-induced residual pore pressure and deformation of sand foundations underneath caisson breakwaters

Hisham El Safti, Matthias Kudella & Hocine Oumeraci
A finite volume model is developed for modelling the behaviour of the seabed underneath monolithic breakwaters. The fully coupled and fully dynamic Biot's governing equations are solved in a segregated approach. Two simplifications to the governing equations are presented and tested: (i) the pore fluid acceleration is completely neglected (the u-p approximation) and (ii) only the convective part is neglected. It is found that neglecting the pore fluid convection does not reduce the computational time...

Process regulation in the problem-solving processes of fifth graders [Usmerjanje procesov resevanja problemov petosolcev]

Benjamin Rott
It is well known that the regulation of processes is an important factor in problem solving from Grade 7 to university level (cf. Mevarech & Kramarski, 1997; Schoenfeld, 1985). We do not, however, know much about the problem-solving competencies of younger children (cf. Heinze, 2007, p. 15). Do the results of studies also hold true for students below Grade 7? The study presented here strongly suggests that metacognition and process regulation is important in Grade...

Longitudinal spin relaxation in simple stochastic models for disordered systems

P. Borgs, K.W. Kehr & Paul Heitjans
The relaxation of single probe spins was investigated for simple models of systems with quenched disorder. The spin relaxation was calculated for a two-site model with arbitrarily oriented magnetic fields and the result was averaged over various distributions of the fields, and of the hopping rates of the spin. On an intermediate time scale, a modified Kubo-Toyabe behavior is obtained for large hopping rates, in agreement with recent SR experiments. A stretched-exponential decay of the...

Fast dynamics of H2O in hydrous aluminosilicate glasses studied with quasielastic neutron scattering

Sylvio Indris, Paul Heitjans, Harald Behrens, Reiner Zorn & Bernhard Frick
We studied the dynamics of dissolved water in aluminosilicate glasses with the compositions NaAlSi3O8⋅0.3H2O, NaAlSi3O8⋅1.3H2O and Ca0.5AlSi3O8⋅1.3H2O using quasielastic neutron scattering. As shown by near-infrared spectroscopy on these samples, H2O molecules are the predominant hydrous species in the water-rich glasses whereas OH groups bound to tetrahedrally coordinated cations are predominant at low water contents. Backscattering and time-of-flight methods were combined to investigate motional correlation times in the range between 0.2ps and 2ns. For the water-rich...

Solid-State NMR Spectroscopy Study of Cation Dynamics in Layered Na2Ti3O7 and Li2Ti3O7

Kai Volgmann, Vanessa Werth, Suliman Nakhal, Martin Lerch, Thomas Bredow & Paul Heitjans
Ti-based materials exhibit suitable properties for usage in secondary Li-and Na-ion batteries and were in the focus of several electrochemical and ion conductivity studies. A material of such interest is layer-structured, monoclinic Na2Ti3O7. Additionally, the sodium in Na2Ti3O7 can be replaced completely with lithium to achieve monoclinic Li2Ti3O7, whose electrochemical properties were already investigated as well. Both materials exhibit interesting properties such as zero-strain behavior upon intercalation and high cycling stability. However, there is still...

High-resolution vehicle headlamps: Technologies and scanning prototype

Gerolf Kloppenburg, Alexander Wolf & Roland Lachmayer
The introduction of adaptive front lighting systems for vehicles has increased road safety and drivers' comfort significantly within the last years. A next step in this development is the realization of higher resolution systems to further increase the functionality of vehicle headlamps toward fully adaptive front lighting systems. In this paper, we present a short overview on highly dynamic front lighting systems and the essential technologies for their realization. Different approaches are compared and evaluated...

Lithium motion in the anode material LiC6 as seen via time-domain 7Li NMR

J. Langer, Viktor Epp, Paul Heitjans, F.A. Mautner & Martin Wilkening
Since the commercialization of rechargeable lithium-ion energy storage systems in the early 1990s, graphite intercalation compounds (GICs) have served as the number one negative electrode material in most of today's batteries. During charging the performance of a battery is closely tied with facile Li insertion into the graphite host structure. So far, only occasionally time-domain nuclear magnetic resonance (NMR) measurements have been reported to study Li self-diffusion parameters in GICs. Here, we used several NMR...

Studying Li dynamics in a gas-phase synthesized amorphous oxide by NMR and impedance spectroscopy

Viktor Epp, Christian Brünig, Michael Binnewies, Paul Heitjans & Martin Wilkening
Li diffusion parameters of a structurally disordered Li-Al-Si-oxide prepared by gas-phase synthesis were complementarily investigated by both time-domain NMR techniques and impedance spectroscopy. The first include 7Li NMR spin-lattice relaxation (SLR) measurements in the laboratory as well as in the rotating frame of reference. An analysis of variable-temperature NMR line widths point to an activation energy E a of approximately 0.6 eV. The value is confirmed by rotating-frame SLR NMR data recorded at approximately 11...

Registration Year

  • 2017

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  • Collection