Institute For Angewandte Physik

Hamburg, Germany

Institute For Angewandte Physik

Hamburg, Germany
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Purwins H.-G.,Institute For Angewandte Physik | Bodeker H.U.,Institute For Angewandte Physik | Amiranashvili S.,Institute For Angewandte Physik | Amiranashvili S.,Weierstrass Institute for Applied Analysis And Stochastics
Advances in Physics | Year: 2010

The present review summarizes experimental and theoretical work dealing with self-organized solitary localized structures (LSs) that are observed in spatially extended nonlinear dissipative systems otherwise exhibiting translational and rotational symmetry. Thereby we focus on those LSs that essentially behave like particles and that we call dissipative solitons (DSs). Such objects are also solutions of corresponding nonlinear evolution equations and it turns out that they are rather robust with respect to interaction with each other, with impurities, and with the boundary; alternatively they are generated or annihilated as a whole. By reviewing the experimental results it turns out that the richest variety of DS phenomena has been observed in electrical transport systems and optical devices. Nevertheless, DSs show up also in many other systems, among which nerve pulses in living beings are of uppermost importance in practice. In most of these systems DSs behave very similarly. The experimental results strongly suggest that phenomenon of DSs is universal. On the background of the experimental findings models for a theoretical understanding are discussed. It turns out that in a limited number of cases a straightforward quantitative description of DS patterns can be carried out. However, for the overwhelming number of systems only a qualitative approach has been successful so far. In the present review particular emphasis is laid on reaction-diffusion systems for which a kind of 'normal form' can be written down that defines a relatively large universality class comprising e.g. important electrical transport, chemical, and biological systems. For the other large class of DS carrying systems, namely optical devices, the variety of model equations is much larger and one is far away, even from a universal qualitative description. Because of this, and due to the existence of several extensive reviews on optical systems, their theoretical treatment has been mentioned only shortly. Finally, it is demonstrated that in terms of a singular perturbation approach the interaction of DSs and important aspects of their bifurcation behaviour, under certain conditions, can be described by rather simple equations. This is also true when deriving from the underlying field equations a set of ordinary differential equations containing the position coordinates of the individual DSs. Such equations represent a theoretical foundation of the experimentally observed particle-like behaviour of DSs. Though at present there is little real practical application of DSs and related patterns in an outlook we point out in which respects this might change in future. A systematic summary of a large amount of experimental and theoretical results on reaction-diffusion systems, being rather close to the subject of the present review, can also be found on the website Summary. © 2010 Taylor & Francis.

Ostermann R.,Justus Liebig University | Zieba R.,BASF | Rudolph M.,Institute For Angewandte Physik | Schlettwein D.,Institute For Angewandte Physik | Smarsly B.M.,Justus Liebig University
Chemical Communications | Year: 2011

Nanoparticles of ATO (antimony doped tin oxide) were used to produce thick conductive, free standing mats of nanofibers via electrospinning. These fibrous mats were incorporated into polymer films to produce a transparent conducting polymer foil. Moreover, the fiber mats can serve as porous electrodes for electrodeposition of Prussian Blue and TiO2 and were tested in dye-sensitized solar cells. © The Royal Society of Chemistry 2011.

Klinkhammer J.,University of Cologne | Forster D.F.,University of Cologne | Schumacher S.,University of Cologne | Oepen H.P.,Institute For Angewandte Physik | And 2 more authors.
Applied Physics Letters | Year: 2013

We present a straightforward and reproducible method to grow stoichiometric and single phase (100) textured EuO thin films on epitaxial graphene. Depending on coverage, either separated EuO grains or fully closed layers can be prepared. Room temperature preparation followed by annealing in Eu vapor leads to a random distribution of the in-plane orientation, whereas growth under distillation conditions at 720 K induces a fixed orientation with respect to the substrate. Magneto-optical Kerr effect (MOKE) shows that the films are ferromagnetic with an enhanced Curie temperature. © 2013 AIP Publishing LLC.

Vogl U.,Institute For Angewandte Physik | Vogl U.,U.S. National Institute of Standards and Technology | Sa A.,Institute For Angewandte Physik | Haelmann S.,Institute For Angewandte Physik | Weitz M.,Institute For Angewandte Physik
Journal of Modern Optics | Year: 2011

We describe measurements demonstrating laser cooling of an atomic gas by means of collisional redistribution of radiation. The experiment uses rubidium atoms in the presence of several hundred bar of argon buffer gas pressure. Frequent collisions in the dense gas transiently shift a far-red detuned optical field into resonance, while spontaneous emission occurs close to the unperturbed atomic transition frequency. Evidence for the cooling is obtained via both thermographic imaging and thermographic deflection spectroscopy. The cooled gas has a density above 1021 atoms/cm3, yielding evidence for the laser cooling of a macroscopic ensemble of gas atoms. © 2011 Taylor & Francis.

Horn W.,Institute For Angewandte Physik | Bassewitz J.V.,Institute For Angewandte Physik | Denz C.,Institute For Angewandte Physik
Journal of Optics | Year: 2010

We demonstrate slow and fast light at room temperature by dispersive phase-coupling in a photorefractive SBN:60 crystal. Non-degenerated wave-mixing is used to obtain delayed and amplified pulses with reduced or increased group velocity of vg = 0.2 cm s-1 and vg =-1.3 cm s-1, respectively. The gain spectrum is modulated by using multiple frequency shifted pumps simultaneously. The complete dispersion of the spectrum is determined via the phase modulation technique. We compare the experimental results to numerical simulations based on coupled-wave equations. © 2010 IOP Publishing Ltd.

Heyn C.,Institute For Angewandte Physik | Schnull S.,Institute For Angewandte Physik | Hansen W.,Institute For Angewandte Physik
Journal of Applied Physics | Year: 2014

We study the tuneability of nanoholes created by local droplet etching of AlGaAs surfaces with Al droplets at varied coverage θ of the deposited droplet material and process temperature T. For the contact angle of the as-grown droplets a mean value of 66° is determined, which depends neither on θ nor on T. The experimental results on the hole structural characteristics are interpreted in terms of scaling models yielding a general form f (θ, T) θ q exp (E / [k B T]), with constants q and E and Boltzmanns constant kB. In detail, the hole density varies from N 4.0 × 106 up to 1.5 × 108 cm-2, and the scaling parameters are q 0 and E EN 2.46 eV. The hole diameter varies from 50 up to 190 nm with scaling parameters q 1/3 and E -E N/3. Moreover, the hole depth varies from 9 up to 125 nm with q 2/3 and E -1.73 eV. Furthermore, a threshold coverage of at least 0.2 monolayers (ML) must be deposited before hole formation takes place. In situ electron diffraction indicates that these 0.2 ML are consumed for a surface reconstruction change from (3 × 1) to (2 × 1). For coverages above 2.0 ML holes with a bimodal depth distribution are observed. © 2014 AIP Publishing LLC.

Horn W.,Institute For Angewandte Physik | Kroesen S.,Institute For Angewandte Physik | Denz C.,Institute For Angewandte Physik
Applied Physics Letters | Year: 2011

We demonstrate group-velocity control with an optically reconfigurable narrow-band filter at 1550 nm based on a volume-holographic Bragg grating. The filter is dynamically addressed to obtain different tunable magnitude and phase function by modification of the length, coupling strength, apodization, and phase discontinuities. We characterize the switching behavior, group-delay, electro-optical tuning, and Gaussian pulse propagation. © 2011 American Institute of Physics.

Heyn C.,Institute For Angewandte Physik | Schmidt M.,Institute For Angewandte Physik | Schwaiger S.,Institute For Angewandte Physik | Stemmann A.,Institute For Angewandte Physik | And 2 more authors.
Applied Physics Letters | Year: 2011

We demonstrate the fabrication of thin GaAs layers which quasi hover above the underlying GaAs substrate. The hovering layers have a perfect epitaxial relationship to the substrate crystal lattice and are connected to the substrate surface only by lattice matched nanopillars of low density. These air-gap heterostructures are created by combining in situ molecular beam epitaxy compatible self-assembled droplet-etching and ex situ selective wet-chemical etching. © 2011 American Institute of Physics.

Heyn C.,Institute For Angewandte Physik
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The self-organized in situ drilling of nanoholes into semiconductor surfaces by using liquid metallic droplets during conventional molecular beam epitaxy represents a new degree of freedom for the design of heterostructure devices. A model of this local droplet etching is presented that is based on a core-shell droplet structure. With the model, the evolution of the droplet and substrate morphology is calculated. We demonstrate quantitative agreement between model results and measured morphologies. Furthermore, also the influence of the process temperature is correctly reproduced by the model. © 2011 American Physical Society.

Bartsch Th.,Institute For Angewandte Physik | Schmidt M.,Institute For Angewandte Physik | Heyn Ch.,Institute For Angewandte Physik | Hansen W.,Institute For Angewandte Physik
Physical Review Letters | Year: 2012

We study the thermal conductance of ballistic point contacts. These contacts are realized as few nanometer long pillars in so-called air-gap heterostructures (AGHs). The pillar length is orders of magnitude smaller than the mean free path of the phonons up to room temperature. Because of the small dimension and the low density of the pillars, the thermal conductance of the AGHs is several orders of magnitude reduced in comparison to bulk structures. The measurement results are in quantitative agreement with a simple model that is based on the Boltzmann transport equation. © 2012 American Physical Society.

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