arXiv subjects

R. R. Landim

Publications and source records attributed to R. R. Landim.

At least 19 recordsLinked to original sources

arXiv2026

Dymnikova Black Hole Tidal Forces

In this work we investigate the tidal properties of the Dymnikova regular black hole and their ef fects on massive particles in radial free-fall. Starting from Dymnikova static, spherically symmetric solution, we derive the equations governing timelike radial geodesics and construct an orthonormal tetrad adapted to a free-falling observer. We then obtain the radial and angular components of the tidal tensor and analyze their dependence on the black hole mass and the characteristic length scale of the de Sitter core. At large radial distances, tidal forces recover Schwarzschild behavior, whereas near the regular center, both components remain finite, reflecting the non-singular nature of the spacetime. We show that the radial and angular tidal forces vanish and change sign at characteris tic radii inside the event horizon, indicating transitions between stretching and compression regimes. A particle released from rest outside the event horizon reaches a turnaround point located inside the Cauchy horizon, rather than reaching the regular center. We also solve the geodesic deviation equations for two sets of initial conditions and examine the evolution of the radial and transverse components of the deviation vector. Although the solutions asymptotically reproduce Schwarzschild behavior, they differ significantly in the inner region: the deviation vector components remain finite up to the turnaround point, whereas the corresponding radial component in the Schwarzschild case diverges at the singularity. These results demonstrate how the de Sitter core regularizes the tidal dynamics of extended falling bodies.

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arXiv2026

Topological Thermodynamics of Generalized Bardeen Black Hole

Neves and Saa introduced a two parameter spacetime that includes the Hayward, Bardeen, and Simpson-Visser geometries as particular cases. In this work, we employ the generalized off-shell Helmholtz free energy method to investigate the thermodynamic properties of the generalized Bardeen black hole within a topological framework. We construct the associated vector field and analyze its zeros, whose winding numbers allow us to classify the thermodynamic branches and identify critical points associated with phase transitions. The regular black hole configurations exhibit two topological defects with opposite winding numbers, resulting in a vanishing total topological charge, while the Schwarzschild case contains a single unstable branch. Our results demonstrate how the regularization parameters affect the thermodynamic stability and phase structure of the spacetime.

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arXiv2026

Topological Thermodynamics of Black Holes: Revisiting the methods of winding numbers calculation

In this paper, the equivalence between two methods for computing winding numbers is established: the approach of $ϕ$-mapping topological current and the residue method. The methods are shown to be equivalent when the condition $M'' S' - S'' M' \neq 0$ holds, while deviations appear when this relation fails, signaling subtle connections between mass $M(r_h)$, entropy $S(r_h)$, and topological structure, with $r_h$ being the horizon radius. We first verify this equivalence to Schwarzschild and Reissner-Nordstr"om black holes, recovering known classifications and confirming the consistency of our approach with respect to the validity of the above condition. We then extend the analysis to four-dimensional black strings, regarded as cylindrically symmetric black hole solutions in asymptotically AdS spacetimes. Our results show that both neutral and charged black strings possess the same global topological number, $W = +1$, implying that electric charge does not influence their topological classification. This insensitivity to charge mirrors earlier findings for BTZ black holes in three dimensions, suggesting that it may represent a universal property of cylindrically symmetric black holes in AdS backgrounds.

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arXiv2026

Cylindrically Symmetric Black Holes Sourced by Dekel-Zhao Dark Matter

In this work, we obtain analytical solutions for a $(3+1)$-dimensional black string and a $(2+1)$-dimensional black hole, both sourced by the Dekel-Zhao dark matter (DM) density profile. Our results indicate that the event horizon radius is sensitive to the inner slope parameter $a$; specifically, beyond a critical threshold, the horizon vanishes, leading to the formation of naked singularities. We find that the DM environment induces curvature singularities in the Ricci and Kretschmann scalars, which are absent in the vacuum BTZ case. Furthermore, an analysis of the effective energy-momentum tensor shows that while the null, weak, and strong energy conditions are strictly satisfied, the dominant energy condition is violated in the lower-dimensional scenario due to the high tangential pressure gradient. We also observe that DM modifies the Hawking temperature and free energy without compromising local or global stability. Notably, the DM distribution transforms the originally constant-curvature BTZ spacetime into a singular one, suggesting that a inherent stiffness of the DM profile is a determinant factor in the causal structure of these solutions.

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arXiv2026

Tidal forces around the Letelier-Alencar cloud of strings black hole

In this work, we investigate relativistic tidal forces around a black hole sourced by a cloud of strings, described by the generalized Letelier-Alencar solution. We first review the original Letelier spacetime and its recent generalization, computing the Kretschmann scalar and showing that the generalized model exhibits a stronger curvature divergence at $r \to 0$ than both Letelier and Schwarzschild cases. We then analyze geodesic motion in this background. For massless particles, we focus on circular photon orbits, while for massive particles, we consider both radial infall and circular motion. We find that the radii of the photon sphere and of the innermost stable circular orbit increase with the cloud of strings parameter $g_s$ and decrease with the length scale $l_s$, and circular orbits cease to exist in certain regions of the parameter space. For radial motion, we compute the radial acceleration and the corresponding tidal forces. In this case, we show that an inversion between stretching and compression may occur, although this regime is typically hidden inside the event horizon. Once the tidal forces are known, we computed the behavior of the displacement vector in order to verify whether the usual stretching behavior induced by tidal forces is preserved. Finally, we study tidal forces for observers in circular motion, showing that the cloud of strings modifies the Keplerian frequency and the tidal force profile even at large distances, and that in this case there is no sign change of the tidal components.

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arXiv2025

Generalized black-bounces solutions in f(R) gravity and their field sources

In this work, following our recent findings in [1], we extend our analysis to explore the generalization of spherically symmetric and static black-bounce solutions, known from General Relativity, within the framework of the $f(R)$ theory in the metric formalism. We develop a general approach to determine the sources for any model where $f(R) = R + H(R)$, provided that the corresponding source for the bounce metric in General Relativity is known. As a result, we demonstrate that black-bounce solutions can emerge from this theory when considering the coupling of $f(R)$ gravity with nonlinear electrodynamics and a partially phantom scalar field. We also analyzed the energy conditions of these solutions and found that, unlike in General Relativity, it is possible to satisfy all energy conditions in certain regions of space-time.

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arXiv2025

Dymnikova Black Hole Surrounded by Quintessence

The Dymnikova black hole (BH) is a regular solution that interpolates between a de Sitter core near the origin and a Schwarzschild-like behavior at large distances. In this work, we investigate the properties of a Dymnikova BH immersed in a quintessential field, characterized by the state parameter $ω$ and a normalization constant $c$. We explore the thermodynamic behavior, null geodesics, scalar quasinormal modes and shadow profiles for this model. Our analysis shows that the presence of quintessence alters the Hawking temperature and specific heat, leading to parameter-dependent phase transitions. The null geodesics and corresponding black hole shadows are also found to be sensitive to the model parameters, especially $ω$ and $c$. This sensitivity influences light deflection and shadow size. Furthermore, we compute the scalar quasinormal modes and observe that quintessence tends to enhance the damping of the modes, indicating greater stability under perturbations.

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arXiv2025

A New Cloud of Strings

In this work, we present a generalization of the cloud of strings model originally proposed by Letelier, by introducing a magnetic-like component, $Σ_{23}$, in addition to the electric-like component, $Σ_{01}$, considered in the original formulation. This extension leads to a new black hole solution of the form \begin{equation} f(r) = 1 - \frac{2M}{r} + \frac{g_s^2 \ell_s^2}{r^2} \, {}_2F_1\left(-\frac{1}{2}, -\frac{1}{4}, \frac{3}{4}, -\frac{r^4}{\ell_s^4}\right), \end{equation} where ${}_2F_1$ denotes the Gaussian hypergeometric function. The solution is characterized by the string length $\ell_s$ and the string coupling constant $g_s$ yields the condition $0 < a < 1$, which now emerges from the model itself, ensuring both energy positivity and the existence of horizons-rather than being imposed ad hoc. We also investigate the thermodynamic properties of the resulting black hole, and find that the entropy remains consistent with the Bekenstein-Hawking formula, $S = A/4$, in agreement with several string-theoretic derivations.

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arXiv2024

Thermodynamics and Quasinormal Modes of the Dymnikova Black Hole in Higher Dimensions

In this study, we investigate the thermodynamic properties and quasinormal modes of Dymnikova black holes within the context of higher dimensions in Einstein's general theory of relativity. We calculate the thermodynamic parameters, including the Hawking temperature and heat capacity, which allowed us to investigate the black hole's stability. Lastly the quasinormal modes with the WKB formula were calculated.

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arXiv2023

Charged black string bounce and its field source

This work builds upon the previous article [1] and explores the solution of the charged black string introduced in [2]. The black bounce regularization method, based on the Simpson-Visser solution, is employed by transforming the radial variable using $r\rightarrow \sqrt{r^2+a^2}$. The regular charged black string metric is defined, and the properties of event horizons, surface gravity, and Hawking temperature are investigated. The behavior of curvature quantities, including curvature invariants and tensors, is examined to verify the absence of singularities when $a\neq 0$. The Einstein equation for the energy-momentum tensor is solved, and the null energy condition is analyzed for the obtained solution. The sources of this solution are evaluated, combining a scalar field with nonlinear electrodynamics. However, unlike other works, an electric field is considered instead of a magnetic field. Finally, the study calculates the possibility of stable or unstable circular orbits for massive and massless particles.

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arXiv2023

Thermodynamic properties of the noncommutative quantum Hall effect with anomalous magnetic moment

In this paper, we study the thermodynamic properties of the noncommutative quantum Hall effect (NCQHE) with anomalous magnetic moment (AMM) for both relativistic and nonrelativistic cases in the high temperatures regime. Thus, we use the canonical ensemble for a set of $N$-particles in contact with a thermal bath. Next, we explicitly determine the thermodynamic properties of our interest, namely: the Helmholtz free energy, the entropy, the mean energy, and the heat capacity. In order to perform the calculations, we work with the Euler-MacLaurin formula to construct the partition function of the system. In that way, we plotted the graphs of thermodynamic properties as a function of temperature for six different values of the magnetic field and of the NC parameters. As a result, we note that the Helmholtz free energy decreases with the temperature, increases with the NC parameters, and can decrease or increase for certain values of the magnetic field, white that the entropy increases with the temperature, decreases with the NC parameters, and can decrease or increase for certain values of the magnetic field. Besides, the mean energy increases linearly with the temperature and its values for the relativistic case are twice of the nonrelativistic case, consequently, the heat capacity for the relativistic case is twice of the nonrelativistic case, where both are constants, and therefore, satisfying the so-called Dulong-Petit law. Finally, we also verify that there is no influence of the AMM on the thermodynamic properties of the system.

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arXiv2023

A Gravity-Consistent Confinement of Fermions in Braneworld

In this manuscript, we discuss the confinement of the spin $\frac{1}{2}$ field on a plethora of branewords models. Recently, in (Eur.Phys.J.C 80 (2020) 5, 432), we studied the consistency of the Standard Model (SM) fields localization on braneworlds with the Einstein equation. In that paper, we discussed the consistency of the spinor field confinement and, by using a Yukawa-like interaction given by $\mathcal{L}_{int}\propto f(y)\barΨΨ$, we obtained that the function must be defined as $f(y)\propto e^{-A}A'$. This shape of the scalar function emerge from the requirement that the spin $\frac{1}{2}$ (zero-mode) localization cannot modify the metric on bulk. This ensures that the confinement of gravity on the brane is preserved. In the present manuscript, we find a covariant scalar-coupling function that can generate this interaction. This provide a new mechanism for localizing fermion fields over the brane. We also discuss massive modes and we found some gravitational configuration where there are confined and discretized massive modes.

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arXiv2022

On the traversable Yukawa-Casimir wormholes

Wormholes (WH) require negative energy, and therefore an exotic matter source. Since Casimir energy is negative, it has been speculated as a good candidate to source that objects a long time ago. However only very recently a full solution for $D = 4$ has been found by Garattini, thus the Casimir energy can be a source of traversable WHs. In the manuscript published by the authors, we show that this solution is possible for all space-time of dimension $D > 3$. Recently, Garattini sought to analyze the effects of Yukawa-type terms on shape functions and obtained promising results. However, it assumes reasonably questionable premises when establishing a non-homogeneity in the Equation of State, when assuming a fixation between the constants that is only observed in the usual model and when considering the Zero Tidal Condition, not observed in Casimir's wormholes. In this work, we study the effects generated by Yukawa-type corrective factors on traversable wormhole solutions generated by Casimir energy without assuming such premises. We observe that, in addition to being possible to build traversable wormholes that satisfy all the necessary conditions, it is possible to obtain an adequate fixation of the constants in order to recover the standard case without a double limit. It was observed that, in the global and constant cases, it is possible to set a value in the shielding parameter that makes the wormhole generate a repulsive gravitational force. Finally, we observed that in the constant case there was a limitation for the shielding factor, something not observed in the original article.

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arXiv2022

Thermodynamic properties of the noncommutative Dirac oscillator with a permanent electric dipole moment

In this paper, we investigate the thermodynamic properties of the noncommutative Dirac oscillator with a permanent electric dipole moment in the presence of an electromagnetic field in contact with a heat bath. Using the canonical ensemble, we determine the properties for both relativistic and nonrelativistic cases through the \textit{Euler-MacLaurin} formula in the high temperatures regime. In particular, the main properties are: the Helmholtz free energy, the entropy, the mean energy, and the heat capacity. Next, we analyze via 2D graphs the behavior of the properties as a function of temperature. As a result, we note that the Helmholtz free energy decreases with the temperature and $ω_θ$, and increases with $ω$, $\Tildeω$, $ω_η$, where $ω$ is the frequency of the oscillator, $\Tildeω$ is a type of cyclotron frequency, and $ω_θ$ and $ω_η$ are the noncommutative frequencies of position and momentum. With respect to entropy, we note an increase with the temperature and $ω_θ$, and a decrease with $ω$, $\Tildeω$, $ω_η$. Now, with respect to mean energy, we note that such property increases linearly with the temperature, and their values for the relativistic case are twice that of the nonrelativistic case. As a direct consequence of this, the value of the heat capacity for the relativistic case is also twice that of the nonrelativistic case, and both are constants, thus satisfying the \textit{Dulong-Petit} law. Lastly, we also note that the electric field does not influence the properties in any way.

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arXiv2022

Generalized Ellis-Bronnikov wormholes in asymptotically safe gravity

In this paper we study a class of wormhole solutions called generalized Ellis-Bronnikov wormholes in the context of asymptotically safe gravity (ASG). These solutions are characterized by two parameters: an even number $n$ and the wormhole throat radius $r_t$. The particular case $n=2$ recovers the usual Ellis-Bronnikov spacetime, which has already been addressed in the literature. We analyzed the nature of matter in the wormhole's throat, and in nearby regions, of these generalized solutions with $n>2$, using three curvature scalars in the ASG approach, namely, the Ricci scalar, squared Ricci and the Kretschmann scalar. We have shown that the ASG leads to corrections in the matter at the wormhole's throat only for the $n=4$ case. For the squared Ricci and the Kretschmann we find that exotic matter is always necessary, as previously found for the usual Ellis-Bronnikov. However, for the Ricci scalar case, we found that ordinary matter is allowed at the throat. Therefore, the generalized Ellis-Bronnikov wormhole provides to the possibility of having ordinary matter at the throat in the context of asymptotically safe gravity.

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arXiv2022

The noncommutative Dirac oscillator with a permanent electric dipole moment in the presence of an electromagnetic field

In this paper, we investigate the bound-state solutions of the noncommutative Dirac oscillator with a permanent electric dipole moment in the presence of an electromagnetic field in (2+1)-dimensions. We consider a radial magnetic field generated by anti-Helmholtz coils, and the uniform electric field of the Stark effect. Next, we determine the bound-state solutions of the system, given by the two-component Dirac spinor and the relativistic energy spectrum. We note that this spinor is written in terms of the generalized Laguerre polynomials, and this spectrum is a linear function on the potential energy $U$, and depends explicitly on the quantum numbers $n$ and $m$, spin parameter $s$, and of four angular frequencies: $ω$, $\tildeω$, $ω_θ$, and $ω_η$, where $ω$ is the frequency of the oscillator, $\tildeω$ is a type of ``cyclotron frequency'', and $ω_θ$ and $ω_η$ are the noncommutative frequencies of position and momentum. Besides, we discussed some interesting features of such a spectrum, for example, its degeneracy, and then we graphically analyze the behavior of the spectrum as a function of the four frequencies for three different values of $n$, with and without the influence of $U$. Finally, we also analyze in detail the nonrelativistic limit of our results, and comparing our problem with other works, where we verified that our results generalize several particular cases of the literature.

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arXiv2022

Horizon Fractalization in Black Strings Ungravity

In this paper we study the scalar(tensor) and vector unparticle corrections for cosmic and black strings. Initially we have considered an static cosmic string ansatz from which we obtain the solution in terms of first and second kind Bessel functions. We have also obtained the solution for black string in the unparticle scenario. We could identify two regimes, namely, a gravity dominated regime and an ungravity dominated regime. In the gravity dominated regime the black string solution recovers the usual solution for black strings. The Hawking temperature was also studied in both regimes and in the ungravity dominated regime. As in the static and rotating black hole, we found a fractalization of the event horizon. This points to the fact that fractalization is a natural consequence of unparticles. Finally, we study the thermodynamic of the black string in the ungravity scenario by computing the entropy, heat capacity and free energy. For both cases we find that, depending on the region of the parameter $d_U$, we can have phase transitions.

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arXiv2022

The noncommutative quantum Hall effect with anomalous magnetic moment in three different relativistic scenarios

In the present paper, we investigate the bound-state solutions of the noncommutative quantum Hall effect (NCQHE) with anomalous magnetic moment (AMM) in three different relativistic scenarios, namely: the Minkowski spacetime (inertial flat case), the spinning cosmic string (CS) spacetime (inertial curved case), and the spinning CS spacetime with noninertial effects (noninertial curved case). In particular, in the first two scenarios, we have an inertial frame, while in the third, we have a rotating frame. With respect to bound-state solutions, we focus primarily on eigenfunctions (Dirac spinor and wave function) and on energy eigenvalues (Landau levels), where we use the flat and curved Dirac equation in polar coordinates to reach such solutions. However, unlike the literature, here we consider a CS with an angular momentum non-null and also the NC of the positions, and therefore, we seek a more general description for the QHE. Once the solutions are obtained, we discuss the influence of all parameters and physical quantities on relativistic energy levels. Finally, we analyze the nonrelativistic limit, and we also compared our problem with other works, where we verified that our results generalize some particular cases of the literature.

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