9,685 research outputs found

    Onset of collective and cohesive motion

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    We study the onset of collective motion, with and without cohesion, of groups of noisy self-propelled particles interacting locally. We find that this phase transition, in two space dimensions, is always discontinuous, including for the minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for which a non-trivial critical point was previously advocated. We also show that cohesion is always lost near onset, as a result of the interplay of density, velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let

    A Few Steps Towards a More Quantitative Understanding of Contrast in the Scanning Electron Microscope

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    The interaction volume of the electron beam with the specimen in a scanning electron microscope (SEM) is a highly complex function of the surface structure of the specimen, its chemical composition and the energy of the scanning electron beam.· The video signals formed by secondary electrons (SE) or backscattered electrons (BSE) reflect this complexity insofar as they may contain not only information of the interior of the pixel which has just been scanned and its neighborhood, but may depend on surface details hundreds of microns apart from the impinging point of the electron beam. This leads to artifacts in scanning electron micrographs, e.g., edge brightening. The knowledge of the spatial distribution of the current density of the BSE and SE released by the impinging beam are the key for a more quantitative understanding of contrasts in scanning electron micrographs. In a first step, our emission microscopic method to visualize these distributions has been improved by substituting a photographic registration method by a charged couple device (CCD) densitometer. The resolution of our present densitometer (256 grey levels) is not sufficient to record the full dynamic range of the SE current density distributions. However, this will be possible in the near future with a state of the art CCD-camera and a 14 bit image processing system

    Off-resonance field enhancement by spherical nanoshells

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    We study light scattering by spherical nanoshells consistent of metal/dielectric composites. We consider two geometries of metallic nanoshell with dielectric core, and dielectric coated metallic nanoparticle. We demonstrate that for both geometries the local field enhancement takes place out of resonance regions ("dark states"), which, nevertheless, can be understood in terms of the Fano resonance. At optimal conditions the light is stronger enhanced inside the dielectric material. By using nonlinear dielectric materials it will lead to a variety nonlinear phenomena applicable for photonics applications

    Combined SIMS-SPM Instrument For High Sensitivity And High Resolution Elemental 3D Analysis

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    Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 - August 2, 201

    Ocean response to greenhouse warming

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    Changes in surface air temperature resulting from a doubling in atmospheric carbon dioxide drive changes in ocean circulation. Results from an ocean general circulation model project a global mean sea level rise from thermal expansion alone to be 19cm in 50 years. Regional values, however, can vary: a rise of 40cm is projected in the North Atlantic (owing to reduction of deep-water formation), whereas the level of the Ross Sea actually falls through changes in ocean circulation
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