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Joás Venâncio

Publications and source records attributed to Joás Venâncio.

8 recordsLinked to original sources
arXiv2025

Finite temperature Casimir effect for a spinor field in cosmic dispiration spacetime

This study explores the finite temperature Casimir effect for a massive spinor field in cosmic dispiration spacetime, formed by the combination of a cosmic string and a screw dislocation using the generalized zeta function regularization method. First, we examine the cosmic string spacetime with a quasi-antiperiodic boundary condition, where the Casimir energy and its corrections depend on two nonzero heat kernel coefficients, one associated with the Euclidean divergence and the other with the nontrivial topology, both vanishing when renormalized. Interestingly, for specific choice of parameters the quasi-antiperiodicity effect can entirely cancel out the topological contribution, leaving only the Euclidean divergence. We then extend this analysis to cosmic dispiration spacetime. This configuration alters the spacetime topology, modifying the structure of the heat kernel coefficient related to the new nontrivial topology. In this case, the renormalized Casimir energy density can take positive or negative values and decreases exponentially as the field mass increases. Additionally, we examine the asymptotic behavior of the renormalized temperature correction term in the massless regime, showing that the spinor vacuum free energy vanishes at very high temperatures. At very low temperatures, it is dominated by the zero-temperature Casimir energy density.

hep-th↗
arXiv2024

Thermal Casimir effect for a Dirac field on flat space with a nontrivial circular boundary condition

This work investigates the thermal Casimir effect associated with a massive spinor field defined on a four-dimensional flat space with a circularly compactified spatial dimension whose periodicity is oriented along a vector in $xy$-plane. We employ the generalized zeta function method to establish a finite definition for the vacuum free energy density. This definition conveniently separates into the zero-temperature Casimir energy density and additional terms accounting for temperature corrections. The structure of existing divergences is analyzed from the asymptotic behavior of the spinor heat kernel function and removed in the renormalization by subtracting scheme. The only non-null heat coefficient is the one associated with the Euclidean divergence. We also address the need for a finite renormalization to treat the ambiguity in the zeta function regularization prescription \text{associated} with this Euclidean heat kernel coefficient and ensure that the renormalization procedure is unique. The high- and low-temperature asymptotic limits are also explored. In particular, we explicitly show that free energy density lacks a classical limit at high temperatures, and the entropy density agrees with the Nernst heat theorem at low temperatures.

hep-th↗
arXiv2023

Integrability of the Dirac Equation in the Presence of Fluxes on Product Manifolds

This paper aims to show that the Dirac equation coupled to an arbitrary inhomogeneous flux field admits separation in manifolds formed from the direct product of bidimensional spaces. As a direct application of these results, we study a spin-$1/2$ charged particle propagating in a background conformally related to a novel and complex string-inspired model in $D = 10$ spacetime dimension whose base manifold is a product of four two-dimensional unit spheres, $S^{2}$, in the presence of $1$- and $3$-form fluxes.

hep-th↗
arXiv2023

On the quasinormal modes in generalized Nariai spacetimes

Quasinormal modes are eigenmodes of dissipative systems. For instance, if a spacetime with an event or cosmological horizon is perturbed from its equilibrium state, quasinormal modes arise as damped oscillations with a spectrum of complex frequencies, called quasinormal frequencies, that depends just on the charges which define the geometry of the spacetime in which the perturbation is propagating, such as the mass, electric charge, and angular momentum. Mathematically, this discrete spectrum of quasinormal modes stems from the fact that certain boundary conditions must be imposed to the physical fields propagating in such a spacetime. In this thesis, we shall consider a higher-dimensional generalization of the charged Nariai spacetime that is comprised of the direct product of the two-dimensional de Sitter space, $dS_2$, with an arbitrary number of two-spheres, $S^2$, and investigate the dynamics of spin-$s$ field perturbations for $s = 0, 1/2, 1$ and $2$. The boundary conditions leading to quasinormal modes are analyzed and the quasinormal frequencies are analytically obtained.

gr-qc↗
arXiv2020

Two-Component Spinorial Formalism using Quaternions for Six-dimensional Spacetimes

In this article we construct and discuss several aspects of the two-component spinorial formalism for six-dimensional spacetimes, in which chiral spinors are represented by objects with two quaternionic components and the spin group is identified with $SL(2;\mathbb{H})$, which is a double covering for the Lorentz group in six dimensions. We present the fundamental representations of this group and show how vectors, bivectors, and 3-vectors are represented in such spinorial formalism. We also complexify the spacetime, so that other signatures can be tackled. We argue that, in general, objects built from the tensor products of the fundamental representations of $SL(2;\mathbb{H})$ do not carry a representation of the group, due to the non-commutativity of the quaternions. The Lie algebra of the spin group is obtained and its connection with the Lie algebra of $SO(5,1)$ is presented, providing a physical interpretation for the elements of $SL(2;\mathbb{H})$. Finally, we present a bridge between this quaternionic spinorial formalism for six-dimensional spacetimes and the four-component spinorial formalism over the complex field that comes from the fact that the spin group in six-dimensional Euclidean spaces is given by $SU(4)$.

hep-th↗
arXiv2020

Spin 2 Quasinormal Modes in Generalized Nariai Spacetimes

In this work we analytically obtain the quasinormal spectrum for the gravitational perturbation on a higher-dimensional generalization of the Nariai spacetime that is comprised of the direct product of the two-dimensional de Sitter space with several two-spheres. A key step in order to attain this result is to use a suitable basis for the angular functions depending on the rank of the tensorial degree of freedom that one needs to describe. Here we define such a basis, which is a generalization of the tensor spherical harmonics that is suited for spaces that are the product of several spaces of constant curvature.

gr-qc↗
arXiv2018

Quasinormal Modes in Generalized Nariai Spacetimes

The perturbations of fields with spin 0, 1/2, and 1 propagating in a higher-dimensional generalization of the charged Nariai spacetime are investigated. The boundary conditions leading to quasinormal modes are analyzed and the quasinormal frequencies are analytically obtained.

gr-qc↗