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We consider the modification of the Cahn-Hilliard equation when a time delay process through a memory function is taken into account. We then study the process of spinodal decomposition in fast phase transitions associated with a conserved order parameter. The introduced memory effect plays an important role to obtain a finite group velocity. Then, we discuss the constraint for the parameters to satisfy causality. The memory effect is seen to affect the dynamics of phase transition at short times and have the effect of delaying, in a significant way, the process of rapid growth of the order parameter that follows a quench into the spinodal region.

961 downloads

Numerical simulation of Ginzburg-Landau-Langevin equations
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Cassol-Seewald, N. C.

; Krein, G.

Brazilian Journal of Physics

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mar 2007, Volume 37 N. 1a Pages 37 - 40

Abstract: En | Text: En | PDF: En

This work is concerned with non-equilibrium phenomena, with focus on the numerical simulation of the relaxation of non-conserved order parameters described by stochastic kinetic equations known as Ginzburg-Landau-Langevin (GLL) equations. We propose methods for solving numerically these type of equations, with additive and multiplicative noises. Illustrative applications of the methods are presented for different GLL equations, with emphasis on equations incorporating memory effects.

1886 downloads

Charmed-meson scattering on nucleons in a QCD Coulomb gauge quark model
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Antunes, S. M.

; Krein, G.

; Vizcarra, V.E.

Brazilian Journal of Physics

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mar 2007, Volume 37 N. 1b Pages 265 - 269

Abstract: En | Text: En | PDF: En

The scattering of charmed mesons on nucleons is investigated within a chiral quark model inspired on the QCD Hamiltonian in Coulomb gauge. The microscopic model incorporates a longitudinal Coulomb confining interaction derived from a self-consistent quasi-particle approximation to the QCD vacuum, and a traverse hyperfine interaction motivated from lattice simulations of QCD in Coulomb gauge. From the microscopic interactions at the quark level, effective meson-baryon interactions are derived using a mapping formalism that leads to quark-Born diagrams. As an application, the total cross-section of heavy-light D-mesons scattering on nucleons is estimated.

985 downloads

Dynamic behavior of cluster observables for the 2d Ising model
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Wanzeller, Wanderson G.

; Mendes, Tereza

; Krein, Gastão

Brazilian Journal of Physics

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sep 2006, Volume 36 N. 3a Pages 657 - 659

Abstract: En | Text: En | PDF: En

We present results of our numerical study of the critical dynamics of percolation observables for the two-dimensional Ising model. We consider the (Monte Carlo) short-time evolution of the system with small initial magnetization and heat-bath dynamics. We find qualitatively different dynamic behaviors for the magnetization M and for omega, the so-called strength of the percolating cluster, which is the order parameter of the percolation transition. More precisely, we obtain a (leading) exponential form for omega as a function of the Monte Carlo time t, to be compared with the power-law increase encountered for M at short times. Our results suggest that, although the descriptions in terms of magnetic or percolation order parameters may be equivalent in the equilibrium regime, greater care must be taken to interpret percolation observables at short times.

1400 downloads

Quark matter in a QCD Coulomb gauge quark model
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Antunes, S. M.

; Krein, G.

; Vizcarra, V. E.

; Panda, P. K.

Brazilian Journal of Physics

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sep 2005, Volume 35 N. 3b Pages 877 - 879

Abstract: En | Text: En | PDF: En

In this communication we present results of a study of chiral symmetry in quark matter using an effective Coulomb gauge QCD Hamiltonian. QCD in Coulomb gauge is convenient for a variational approach based on a quasiparticle picture for the transverse gluons, in which a confining Coulomb potential arises naturally. We show that such an effective Hamiltonian predicts chiral restoration at too low quark densities. Possible reasons for such deficiency are discussed.

836 downloads

Percolation of Monte Carlo clusters
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Wanzeller, W.G.

; Cucchieri, A.

; Mendes, T.

; Krein, G.

Brazilian Journal of Physics

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mar 2004, Volume 34 N. 1a Pages 247 - 250

Abstract: En | Text: En | PDF: En

Percolation theory is of interest in problems of phase transitions in condensed matter physics, and in biology and chemistry. More recently, concepts of percolation theory have been invoked in studies of color deconfinement at high temperatures in Quantum Chromodynamics. In the present paper we briefly review the basic concept of percolation theory, exemplify its application to the Ising model, and present the arguments for a possible relevance of percolation theory to the problem of color deconfinement.

2377 downloads

Hadron-hadron interactions in Coulomb gauge QCD
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Szpigel, Sérgio

; Krein, G.

; Marques de Carvalho, R. S.

Brazilian Journal of Physics

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mar 2004, Volume 34 N. 1a Pages 279 - 282

Abstract: En | Text: En | PDF: En

We describe the derivation of an effective Hamiltonian which involves explicit hadron degrees of freedom and consistently combines chiral symmetry and color confinement. We use a method known as Fock-Tani (FT) representation and a quark model formulated in the context of Coulomb gauge QCD. Using this Hamiltonian, we evaluate the dissociation cross section of J/psi in collision with rho.

987 downloads

A QCD sum rule approach to the charmonium - pion cross section
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Nielsen, M.

; Navarra, F.S.

; Krein, G.

; Marques de Carvalho, R.S.

Brazilian Journal of Physics

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jun 2003, Volume 33 N. 2 Pages 316 - 319

Abstract: En | Text: En | PDF: En

We study the Borel sum rule for the tensor amplitude of the processes J/y p ® <IMG SRC="/img/revistas/bjp/v33n2/a30img01.gif" >D*,<IMG SRC="/img/revistas/bjp/v33n2/a30img01.gif"> D, <IMG SRC="/img/revistas/bjp/v33n2/a30img01.gif" >*D* and <IMG SRC="/img/revistas/bjp/v33n2/a30img01.gif" >D*. We also evaluate the cross sections as a function of <IMG SRC="/img/revistas/bjp/v33n2/a30img02.gif" >. We find that our results are smaller than the J/y p ® charmed mesons cross sections obtained with models based on meson exchange, but are close to those obtained with quark exchange models.

985 downloads

10.

Transport properties and equation of state of quark-nuclear matter
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Graça, C.O. da

; Krein, G.

Brazilian Journal of Physics

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jun 2003, Volume 33 N. 2 Pages 311 - 315

Abstract: En | Text: En | PDF: En

Transport equations for composite nucleons and deconfined quarks in quark-nuclear matter (QNM) are derived. QNM is a many-body system containing hadrons and deconfined quarks. The starting point is a microscopic quark-meson coupling (QMC) Hamiltonian with a density-dependent quark-quark interaction. An effective quark-hadron Hamiltonian containing canonical hadron and quark field operators is constructed using a mapping procedure. For high densities, the effective Hamiltonian contains interactions that lead to quark decon- finement. Transport equations of the Ueling-Ullenbeck-Vlasov type for quarks and nucleons are obtained using standard many-body techniques with the effective quark-hadron Hamiltonian.

1112 downloads

11.

The Gerasimov-Drell-Hearn Sum Rule and the Spin Structure of the Proton
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Krein, G.

Brazilian Journal of Physics

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jun 2001, Volume 31 N. 2 Pages 179 - 182

Abstract: En | Text: En | PDF: En

We present an overview of the present theoretical status of the Gerasimov-Drell-Hearn (GDH) sum rule. It is emphasized the importance of near threshold s-wave photoproduction contributions to the sum rule. We also discuss the experimental verification of the sum rule, with particular emphasis on the recent experiment at the Mainz Microtron (MAMI). The spin polarizabilities and the generalized GDH sum rule, which are of interest to issues related to the spin of the proton as measured at high energies, are discussed.

945 downloads

12.

The d Expansion and the Principle of Minimal Sensitivity
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Krein, G.

; Menezes, D.P.

; Nielsen, M.

; Pinto, M.B.

Brazilian Journal of Physics

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mar 1998, Volume 28 N. 1 Pages 66 - 71

Abstract: En | Text: En | PDF: En

The d-expansion is a nonperturbative approach for field theoretic models wich combines the techniques of perturbation theory and the variational principle. Different ways of implemeting the principle of minimal sensitivity to the d-expansion produce in general different results for observables. For illustration we use the Nambu- Jona-Lasinio model for chiral symmetry restoration at finite density and compare results with those obtained with the Hartree-Fock approximation.

910 downloads

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