29,050 research outputs found

    A mass-transportation approach to a one dimensional fluid mechanics model with nonlocal velocity

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    We consider a one dimensional transport model with nonlocal velocity given by the Hilbert transform and develop a global well-posedness theory of probability measure solutions. Both the viscous and non-viscous cases are analyzed. Both in original and in self-similar variables, we express the corresponding equations as gradient flows with respect to a free energy functional including a singular logarithmic interaction potential. Existence, uniqueness, self-similar asymptotic behavior and inviscid limit of solutions are obtained in the space P2(R)\mathcal{P}_{2}(\mathbb{R}) of probability measures with finite second moments, without any smallness condition. Our results are based on the abstract gradient flow theory developed in \cite{Ambrosio}. An important byproduct of our results is that there is a unique, up to invariance and translations, global in time self-similar solution with initial data in P2(R)\mathcal{P}_{2}(\mathbb{R}), which was already obtained in \textrm{\cite{Deslippe,Biler-Karch}} by different methods. Moreover, this self-similar solution attracts all the dynamics in self-similar variables. The crucial monotonicity property of the transport between measures in one dimension allows to show that the singular logarithmic potential energy is displacement convex. We also extend the results to gradient flow equations with negative power-law locally integrable interaction potentials

    Mean-field analysis of the majority-vote model broken-ergodicity steady state

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    We study analytically a variant of the one-dimensional majority-vote model in which the individual retains its opinion in case there is a tie among the neighbors' opinions. The individuals are fixed in the sites of a ring of size LL and can interact with their nearest neighbors only. The interesting feature of this model is that it exhibits an infinity of spatially heterogeneous absorbing configurations for LL \to \infty whose statistical properties we probe analytically using a mean-field framework based on the decomposition of the LL-site joint probability distribution into the nn-contiguous-site joint distributions, the so-called nn-site approximation. To describe the broken-ergodicity steady state of the model we solve analytically the mean-field dynamic equations for arbitrary time tt in the cases n=3 and 4. The asymptotic limit tt \to \infty reveals the mapping between the statistical properties of the random initial configurations and those of the final absorbing configurations. For the pair approximation (n=2n=2) we derive that mapping using a trick that avoids solving the full dynamics. Most remarkably, we find that the predictions of the 4-site approximation reduce to those of the 3-site in the case of expectations involving three contiguous sites. In addition, those expectations fit the Monte Carlo data perfectly and so we conjecture that they are in fact the exact expectations for the one-dimensional majority-vote model

    Upper bound for the conductivity of nanotube networks

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    Films composed of nanotube networks have their conductivities regulated by the junction resistances formed between tubes. Conductivity values are enhanced by lower junction resistances but should reach a maximum that is limited by the network morphology. By considering ideal ballistic-like contacts between nanotubes we use the Kubo formalism to calculate the upper bound for the conductivity of such films and show how it depends on the nanotube concentration as well as on their aspect ratio. Highest measured conductivities reported so far are approaching this limiting value, suggesting that further progress lies with nanowires other than nanotubes.Comment: 3 pages, 1 figure. Minor changes. Accepted for publication in Applied Physics Letter

    Electrochemical and STM Study of α,ω-alkanedithiols Self-assembled Monolayers

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    Self-assembled monolayers (SAMs), prepared by the immersion method, from ethanolic solutions containing α,ω-alkanedithiol, n-alkanethiol or mixed thiol/dithiol solutions, with 6, 9 and 10 carbon atoms in the alkyl chain, have been investigated. The amount of adsorbate and the SAM stability in alkaline medium is evaluated by reductive desorption of the prepared monolayers by cyclic voltammetry. An upright orientation of the dithiol self-assembled molecules and disulfide bonding at the SAM/solution interface are suggested by the higher reductive desorption charge of the dithiol monolayers (relative to thiol SAMs) for n = 6 and 9. The results show that an improvement on the stability of these dithiol SAMs is obtained by the presence of monothiols, resulting in mixed monolayers. Mixed SAMs prepared from longer alkane chain thiols, n = 10, allow to overcome the increased possibility of loop formation and therefore lower surface coverage is obtained for the 1,10-decanedithiol monolayers. Morphological characterisation of the modified electrodes is performed by scanning tunnelling microscopy (STM) ex situ, in air. Typical one atom deep thiol induced depressions are observed in the STM images of the dithiol and mixed SAMs

    Resistivity study of the pseudogap phase for (Hg,Re) - 1223 superconductors

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    The pseudogap phase above the critical temperature of high TcT_{c} superconductors (HTSC) presents different energy scales and it is currently a matter of intense study. The complexity of the HTSC normal state requires very accurate measurements with the purpose of distinguishing different types of phenomena. Here we have performed systematically studies through electrical resistivity (ρ\rho) measurements by several different current densities in order to obtain an optimal current for each sample. This approach allows to determine reliable values of the pseudogap temperature T(n)T^{*}(n), the layer coupling temperature between the superconductor layers TLD(n)T_{LD}(n), the fluctuation temperature Tscf(n)T_{scf}(n) and the critical temperature Tc(n)T_{c}(n) as function of the doping nn. The interpretation of these different temperature scales allows to characterize possible scenarios for the (Hg,Re) - 1223 normal state. This method, described in detail here, and used to derive the (Hg,Re)-1223 phase diagram is general and can be applied to any HTSC.Comment: 31 pages, 12 figures, Latex; 25 pages, LaTeX; 11 figures; rewrited section II and III; added 18 reference; rewrited title, added discussion sectio

    Teste de um modelo agrometeorológico para estudo da influência da variabilidade climática na cultura da soja.

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    Com o objetivo de verificar o efeito do clima sobre a produtividade da cultura da soja em Minas Gerais, foi desenvolvido um modelo de simulação dinâmico, mecanístico e deterrninístico. A principal característica do modelo foi a busca do equilíbrio entre a simplicidade no seu manuseio e o rigor científico necessário. Como variáveis de entrada, o modelo utiliza os elementos climáticos precipitação, temperatura e insolação. Os resultados mostraram que o modelo simulou satisfatoriamente a produtividade da soja para os anos agrícolas de 1995/96 e 1996/97, evídenciando as diferenças morfológicas e fisiológicas da cultura, em resposta às variações climáticas, o que evidencia o seu alto potencial de aplicação no entendimento das relações entre clima e cultura

    Caracterização molecular de isolados de Fusarium oxysporum f. sp. cubense no Brasil.

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    Segundo a FAO o Brasil foi em 2010 o quinto produtor de banana do mundo e o maior do hemisfério sul. A produção brasileira em 2010 foi de 7,0 milhões de toneladas em 487 mil hectares
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