19,600 research outputs found

    TWO-PION EXCHANGE NUCLEAR POTENTIAL - CHIRAL CANCELLATIONS

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    We show that chiral symmetry is responsible for large cancellations in the two-pion exchange nucleon-nucleon interaction, which are similar to those occuring in free pion-nucleon scattering.Comment: REVTEX style, 5 pages, 3 PostScrip figures compressed, tarred and uuencode

    Using zeros of the canonical partition function map to detect signatures of a Berezinskii-Kosterlitz-Thouless transition

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    Using the two dimensional XY−(S(O(3))XY-(S(O(3)) model as a test case, we show that analysis of the Fisher zeros of the canonical partition function can provide signatures of a transition in the Berezinskii-Kosterlitz-Thouless (BKTBKT) universality class. Studying the internal border of zeros in the complex temperature plane, we found a scenario in complete agreement with theoretical expectations which allow one to uniquely classify a phase transition as in the BKTBKT class of universality. We obtain TBKTT_{BKT} in excellent accordance with previous results. A careful analysis of the behavior of the zeros for both regions Re(T)≤TBKT\mathfrak{Re}(T) \leq T_{BKT} and Re(T)>TBKT\mathfrak{Re}(T) > T_{BKT} in the thermodynamic limit show that Im(T)\mathfrak{Im}(T) goes to zero in the former case and is finite in the last one

    Superinflation, quintessence, and the avoidance of the initial singularity

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    We consider the dynamics of a spatially flat universe dominated by a self-interacting nonminimally coupled scalar field. The structure of the phase space and complete phase portraits for the conformal coupling case are given. It is shown that the non-minimal coupling modifies drastically the dynamics of the universe. New cosmological behaviors are identified, including superinflation (HË™>0\dot{H}>0), avoidance of big bang singularities through classical birth of the universe from empty Minkowski space, and spontaneous entry into and exit from inflation. The relevance of this model to the description of quintessence is discussed.Comment: RevTex, 10 pages, 4 figures, To appear in the proceedings of the 5th Peyresq meetin

    Stability properties and asymptotics for N non-minimally coupled scalar fields cosmology

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    We consider here the dynamics of some homogeneous and isotropic cosmological models with NN interacting classical scalar fields non-minimally coupled to the spacetime curvature, as an attempt to generalize some recent results obtained for one and two scalar fields. We show that a Lyapunov function can be constructed under certain conditions for a large class of models, suggesting that chaotic behavior is ruled out for them. Typical solutions tend generically to the empty de Sitter (or Minkowski) fixed points, and the previous asymptotic results obtained for the one field model remain valid. In particular, we confirm that, for large times and a vanishing cosmological constant, even in the presence of the extra scalar fields, the universe tends to an infinite diluted matter dominated era.Comment: 10 page

    Evaluation of corrosion resistance of multi-layered Ti/gass-ceramic interfaces by electrochemical impedance spectroscopy

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    Practical applications of metal/ceramic joints can be found in the biomedical field regarding the encapsulation of implantable telemetric devices, the fabrication of crowns and bridges for dental restoration, or in the production of drug delivery systems, biomedical sensors and electrodes. Most of metal/ceramic joints are produced by the active metal brazing technique, which originates a multi-layered interface which should be able of accommodating the abrupt electronic, crystallographic, chemical, mechanical and thermo-mechanical discontinuity that characterize these systems. Additionally, when considering biomedical applications, corrosion resistance becomes of prime importance. In this work, the corrosion resistance of Ti/glass-ceramic interfaces obtained by active metal brazing was evaluated by electrochemical impedance spectroscopy (EIS) tests. The electrochemical behaviour of the interface was monitored, as a function of time, in a simulated physiological solution at room temperature. In order to evaluate the contribution of each layer and galvanic interactions between them, to the degradation mechanism of the interface, individual samples, representative of reaction layers present at the interface, were fabricated and electrochemically ested. Results show that the corrosion behaviour, of the whole interface was strongly influenced by the chemical composition of its constitutive layers. Thus, layers containing high contents of both titanium and silver showed a polarisation resistance increase with the immersion time, as a result of the formation of a thermodynamically stable passive film. On the other hand, the copper rich layer, appears to be the main responsible for the interface degradation. In fact, for high immersion times, an instable passive film is formed and, as a consequence, large amounts of copper are released. Galvanic interactions between the copper and the silver rich layers where also dentified.Fundação para a Ciência e a Tecnologia (FCT), Portugal Portugal (projects POCTI/CTM/33384/2000 and SFRH/BPD/5518/2001)
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