20,370 research outputs found

    Domain wall network evolution in (N+1)-dimensional FRW universes

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    We develop a velocity-dependent one-scale model for the evolution of domain wall networks in flat expanding or collapsing homogeneous and isotropic universes with an arbitrary number of spatial dimensions, finding the corresponding scaling laws in frictionless and friction dominated regimes. We also determine the allowed range of values of the curvature parameter and the expansion exponent for which a linear scaling solution is possible in the frictionless regime.Comment: 5 pages, 2 figure

    Scaling laws for weakly interacting cosmic (super)string and p-brane networks

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    In this paper we find new scaling laws for the evolution of pp-brane networks in N+1N+1-dimensional Friedmann-Robertson-Walker universes in the weakly-interacting limit, giving particular emphasis to the case of cosmic superstrings (p=1p=1) living in a universe with three spatial dimensions (N=3). In particular, we show that, during the radiation era, the root-mean-square velocity is vˉ=1/2{\bar v} =1/{\sqrt 2} and the characteristic length of non-interacting cosmic string networks scales as La3/2L \propto a^{3/2} (aa is the scale factor), thus leading to string domination even when gravitational backreaction is taken into account. We demonstrate, however, that a small non-vanishing constant loop chopping efficiency parameter c~\tilde c leads to a linear scaling solution with constant LH1L H \ll 1 (HH is the Hubble parameter) and vˉ1/2{\bar v} \sim 1/{\sqrt 2} in the radiation era, which may allow for a cosmologically relevant cosmic string role even in the case of light strings. We also determine the impact that the radiation-matter transition has on the dynamics of weakly interacting cosmic superstring networks.Comment: 5 pages, 2 figure

    Evolution of domain wall networks: the PRS algorithm

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    The Press-Ryden-Spergel (PRS) algorithm is a modification to the field theory equations of motion, parametrized by two parameters (α\alpha and β\beta), implemented in numerical simulations of cosmological domain wall networks, in order to ensure a fixed comoving resolution. In this paper we explicitly demonstrate that the PRS algorithm provides the correct domain wall dynamics in N+1N+1-dimensional Friedmann-Robertson-Walker (FRW) universes if α+β/2=N\alpha+\beta/2=N, fully validating its use in numerical studies of cosmic domain evolution. We further show that this result is valid for generic thin featureless domain walls, independently of the Lagrangian of the model.Comment: 4 page

    Exponential Distributions in a Mechanical Model for Earthquakes

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    We study statistical distributions in a mechanical model for an earthquake fault introduced by Burridge and Knopoff [R. Burridge and L. Knopoff, {\sl Bull. Seismol. Soc. Am.} {\bf 57}, 341 (1967)]. Our investigations on the size (moment), time duration and number of blocks involved in an event show that exponential distributions are found in a given range of the paramenter space. This occurs when the two kinds of springs present in the model have the same, or approximately the same, value for the elastic constants. Exponential distributions have also been seen recently in an experimental system to model earthquake-like dynamics [M. A. Rubio and J. Galeano, {\sl Phys. Rev. E} {\bf 50}, 1000 (1994)].Comment: 11 pages, uuencoded (submitted to Phys. Rev. E

    CVD of CrO2 Thin Films: Influence of the Deposition Parameters on their Structural and Magnetic Properties

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    This work reports on the synthesis of CrO2 thin films by atmospheric pressure CVD using chromium trioxide (CrO3) and oxygen. Highly oriented (100) CrO2 films containing highly oriented (0001) Cr2O3 were grown onto Al2O3(0001) substrates. Films display a sharp magnetic transition at 375 K and a saturation magnetization of 1.92 Bohr magnetons per f.u., close to the bulk value of 2 Bohr magnetons per f.u. for the CrO2. Keywords: Chromium dioxide (CrO2), Atmospheric pressure CVD, Spintronics.Comment: 5 pages, 6 figure

    We are all School

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    The project “We are all school” aimed to break down barriers between the school and the community, to convey the idea that we all have something to add to the school, as we are all educational agents. Our intention was to give more visibility to the activities that the school develops with the community in general. As this community is made up of people from different parts of the world, with diverse cultures and knowledge, we set out to look for  their experiences and knowledge so that the school could benefit from the richness of this diversity. We also set out to involve the whole community more actively, valuing all the knowledge that could be carried to benefit the full learning path of students, regardless of their origin or social position

    Site-dependent hydrogenation on graphdiyne

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    Graphene is one of the most important materials in science today due to its unique and remarkable electronic, thermal and mechanical properties. However in its pristine state, graphene is a gapless semiconductor, what limits its use in transistor electronics. In part due to the revolution created by graphene in materials science, there is a renewed interest in other possible graphene-like two-dimensional structures. Examples of these structures are graphynes and graphdiynes, which are two-dimensional structures, composed of carbon atoms in sp2 and sp-hybridized states. Graphdiynes (benzenoid rings connecting two acetylenic groups) were recently synthesized and some of them are intrinsically nonzero gap systems. These systems can be easily hydrogenated and the relative level of hydrogenation can be used to tune the band gap values. We have investigated, using fully reactive molecular dynamics (ReaxFF), the structural and dynamics aspects of the hydrogenation mechanisms of graphdiyne membranes. Our results showed that the hydrogen bindings have different atom incorporation rates and that the hydrogenation patterns change in time in a very complex way. The formation of correlated domains reported to hydrogenated graphene is no longer observed in graphdiyne cases.Comment: Submitted to Carbo
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