arXiv subjects

L. M. Abreu

Publications and source records attributed to L. M. Abreu.

47 records · Page 3Linked to original sources

arXiv2013

Magnetic effects on spontaneous symmetry breaking/restoration in a toroidal topology

We study temperature and finite-size effects on the spontaneous symmetry breaking/restoration for a scalar field model under the influence of an external magnetic field, at finite chemical potential. We use the 2PI formalism and consider the large-$N$ limit. We find that there is a minimal size of the system to sustain the broken phase, which diminishes as the applied field increases but is independent of the chemical potential. We analyze the critical curves and show that the magnetic field enhances the broken-phase regions, while increasing the chemical potential leads to a diminishement of the critical temperature.

hep-th↗

arXiv2012

Exotic Behavior of Heavy-Flavored Meson Matter

In this work, we study the thermodynamic behavior of heavy-flavored meson matter in the framework of $(σ,ω)$-meson-exchange model in relativistic mean field theory. We find a decreasing of the effective masses of $D$ and $B$ mesons as the temperature increases. By using the effective mass and maximum value of dissociation temperatures available from lattice QCD, the masses of the bound states $D \bar{D}$ and $B \bar{B}$ are estimated in 2 MeV for both molecules. For the $B$-meson matter, the pressure presents an exotic behavior, being negative for temperatures above 6.6 times the deconfinement transition temperature $T_c$. In addition, the ratio of pressure to energy density is similar to the value predicted for systems that behave as dark energy matter.

hep-ph↗

arXiv2011

Finite-size effects on the phase diagram of difermion condensates in two-dimensional four-fermion interaction models

We investigate finite-size effects on the phase structure of chiral and difermion condensates at finite temperature and density in the framework of the two-dimensional large-$N$ Nambu-Jona-Lasinio model. We take into account size-dependent effects by making use of $zeta$-function and compactification methods. The thermodynamic potential and the gap equations for the chiral and difermion condensed phases are then derived in the mean-field approximation. Size-dependent critical lines separating the different phases are obtained considering anti-periodic boundary conditions for the spatial coordinate.

hep-th↗

arXiv2009

Finite-size effects on the chiral phase diagram of four-fermion models in four dimensions

We study the size dependence of the dynamical symmetry breaking in the four-dimensional Nambu-Jona-Lasinio model. We show that the presence of boundaries reduces the chiral breaking region, and this effect is strengthened for a larger number of compactified dimensions. A critical value for the length of the compactified dimensions exists, below which the dynamical symmetry breaking is not possible. Considering finite temperature and chemical potential, the chiral phase structure for the system with compactified dimensions is obtained. A gradual decreasing of the chiral breaking region with increasing of chemical potential is found. Also, at fixed chemical potential, the decreasing of the size of the system changes the order of the chiral phase transition.

hep-th↗

arXiv2006

Finite-size effects on the phase structure of the Nambu-Jona-Lasinio model

The Nambu-Jona-Lasinio (NJL) model is one of the most frequently used four-fermion models in the study of dynamical symmetry breaking. In particular, the NJL model is convenient for that analysis at finite temperature, chemical potential and size effects, as has been explored in the last decade. With this motivation, we investigate the finite-size effects on the phase structure of the NJL model in $D = 3$ Euclidean dimensions, in the situations that one, two and three dimensions are compactified. In this context, we employ the zeta-function and compactification methods to calculate the effective potential and gap equation. The critical lines that separate trivial and non-trivial fermion mass phases in a second order transition are obtained. We also analyze the system at finite temperature, considering the inverse of temperature as the size of one of the compactified dimensions.

hep-th↗

arXiv2005

Critical behaviour of the compactified $λϕ^4$ theory

We investigate the critical behaviour of the $N$-component Euclidean $λϕ^4$ model at leading order in $\frac{1}{N}$-expansion. We consider it in three situations: confined between two parallel planes a distance $L$ apart from one another, confined to an infinitely long cylinder having a square cross-section of area $A$ and to a cubic box of volume $V$. Taking the mass term in the form $m_{0}^2=α(T - T_{0})$, we retrieve Ginzburg-Landau models which are supposed to describe samples of a material undergoing a phase transition, respectively in the form of a film, a wire and of a grain, whose bulk transition temperature ($T_{0}$) is known. We obtain equations for the critical temperature as functions of $L$ (film), $A$ (wire), $V$ (grain) and of $T_{0}$, and determine the limiting sizes sustaining the transition.

math-ph↗

arXiv2004

Halperin-Lubensky-Ma effect in type-I superconducting films

In this note we employ concurrently techniques of generalized $zeta$-functions and compactification methods introduced in previous publications, to study the Halperin-Lubensky-Ma theory of induced weak first-order phase transitions applied to type-I superconducting films. We obtain closed formulas to the critical temperature and to the size temperature as functions of the film thickness.

cond-mat.supr-con↗

arXiv2003

A minimal size for granular superconductors

We investigate the minimal size of small superconducting grains by means of a Ginzburg-Landau model confined to a sphere of radius $R$. This model is supposed to describe a material in the form of a ball, whose transition temperature when presented in bulk form, $T_{0}$, is known. We obtain an equation for the critical temperature as a function of $R$ and of $T_{0}$, allowing us to arrive at the minimal radius of the sphere below which no superconducting transition exists.

cond-mat.supr-con↗

arXiv2003

Gauge Fluctuations in Superconducting Films

In this paper we consider a superconducting film modeled by the Ginzburg-Landau model, confined between two parallel planes a distance $L$ apart from one another. Our approach is based on the Gaussian effective potential in the transverse unitarity gauge, which allows to treat gauge contributions in a compact form. Using techniques from dimensional and $zeta$-function regularizations, modified by the confinement conditions, we investigate the critical temperature as a function of the film thickness $L$. The contributions from the scalar self-interaction and from the gauge fluctuations are clearly identified. The model suggests the existence of a minimal critical thickness below which superconductivity is suppressed.

cond-mat.supr-con↗

arXiv2003

Large-N transition temperature for superconducting films in a magnetic field

We consider the $N$-component Ginzburg-Landau model in the large $N$ limit, the system being embedded in an external constant magnetic field and confined between two parallel planes a distance $L$ apart from one another. On physical grounds, this corresponds to a material in the form of a film in the presence of an external magnetic field. Using techniques from dimensional and $zeta$-function regularization, modified by the external field and the confinement conditions, we investigate the behavior of the system as a function of the film thickness $L$. This behavior suggests the existence of a minimal critical thickness below which superconductivity is suppressed.

cond-mat.supr-con↗

arXiv2002

The Cangemi-Jackiw manifold in high dimensions and symplectic structure

The notion of Poincare gauge manifold ($G$), proposed in the context of an (1+1) gravitational theory by Cangemi and Jackiw (D. Cangemi and R. Jackiw, Ann. Phys. (N.Y.) 225 (1993) 229), is explored from a geometrical point of view. First $G$ is defined for arbitrary dimensions, and in the sequence a symplectic structure is attached to $T*G$. Treating the case of five dimensions, a (4,1)-de Sitter space, aplications are presented studing representations of the Poincare group in association with kinetic theory and the Weyl operators in phase space. The central extension in the Aghassi-Roman-Santilli group (J. J. Aghassi, P. Roman and R. M. Santilli, Phys. Rev. D 1(1970) 2573) is derived as a subgroup of linear transformations in $G$ with six dimensions.

hep-th↗