arXiv subjects

R. N. Costa Filho

Publications and source records attributed to R. N. Costa Filho.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical Schwinger Production of Dipole--Antidipole Pairs in a Fractonic Lattice

We investigate field-induced excitation production from the unit-filled Mott background of a one-dimensional dipole-conserving Bose--Hubbard chain driven by a periodic quadratic potential, realizing a time-dependent rank-two electric field. Unlike protocols that manipulate pre-existing dipolar or fractonic excitations, we address their production from an initially Mott-like state without prepared dipolar or fractonic excitations. A single correlated hop directly nucleates the compact $|030\rangle$ configuration at the atomic energy $3U$, while a lower manifold derived from $2U$ contains spatially resolved dipole--antidipole configurations. Exact diagonalization shows that finite hopping turns this lower manifold into a dispersive band whose finite-chain edge lies substantially below the compact $3U$ scale. Despite this lower energetic edge, its normalized odd-harmonic spectral weight is perturbatively suppressed as $(J/U)^2$, reflecting virtual hybridization of the initial and final eigenstates. A consistent second-order treatment of the $|1111\rangle\leftrightarrow|0220\rangle$ channel includes diagonal self-energy shifts and predicts a weak-field resonance displacement confirmed by exact time evolution. Extended-chain dynamics shows genuine dipole--antidipole separation together with redistribution into higher-energy many-body sectors. The resulting mechanism provides a condensed-matter analogue of multiphoton dynamical Schwinger production in which exact dipole conservation replaces mobile opposite charges by mobile opposite dipoles, while isolated fractonic charges remain immobile.

hep-th↗

arXiv2026

Electrically Charged Non-Abelian Black String in Anti-de Sitter Space

We construct a new family of static, electrically charged, cylindrically symmetric black-string solutions of four-dimensional Einstein--Yang--Mills theory with a negative cosmological constant, supported by a genuinely non-Abelian $SU(2)$ vortex field. The coexistence of electric and vortex sectors renders the standard single-function Lemos metric inconsistent with the Einstein equations, requiring a three-function metric compatible with the anisotropic Yang--Mills stress tensor. Regular near-horizon and asymptotic AdS expansions provide boundary data for numerical integration of the complete field equations. Two limiting sectors are recovered: vanishing electric horizon datum yields the neutral Lemos black string with zero Yang--Mills field strength, while switching off the vortex component gives the embedded $U(1)\subset SU(2)$ charged Lemos--Zanchin solution. Perturbations about the latter show that genuinely non-Abelian hair appears at linear order through the commutator field strength, whereas its backreaction on the electric profile and geometry begins at quadratic order. The horizon equations yield the analytic local non-degeneracy bound $R_1^{\rm crit}=s_0α\sqrt{3/(4πG)}$. The asymptotic geometry exhibits a quadratic correction absorbable into an effective Abelian charge and a cubic coefficient giving the first irreducible asymptotic signature of the non-Abelian hair. The entropy obeys the Bekenstein--Hawking area law, while the temperature difference from the embedded Abelian solution arises through the asymptotic lapse normalization. The fully backreacted numerical solutions are nonlinear realizations of this transverse non-Abelian deformation.

gr-qc↗

arXiv2026

The End of the Road for Bulk Fields in Warped Randall-Sundrum Braneworlds

In this manuscript we generalize Ref. [1] and derive a complete set of local consistency conditions for bulk fields in braneworld scenarios with an arbitrary number of dimensions. This provides the first fully local and dimension-independent generalization of all known criteria for bulk fields. Within this framework, we show that a free scalar field is consistent and localized, whereas minimally and non-minimally coupled Maxwell fields violate the conditions, leading to a no-go theorem valid in any dimension. For nonlinear electrodynamics, we find that only the model $L(F)=b\sqrt{F}$ admits a consistent and normalizable zero mode, and that among p-forms, consistency occurs solely for the free 0-form. We also demonstrate that Dirac fermions, with or without Yukawa terms, are inconsistent within this framework and therefore cannot propagate in the bulk. Our local approach makes explicit that these conclusions do not depend on any particular internal geometry or warp factor: previously known results arise merely as special cases of a broader and strictly local structure, highlighting the universality of the constraints derived here.

gr-qc↗

arXiv2026

Chiral states induced by symmetry-breaking in $α-T_3$ lattices: Magnetic field effect

The sublattice-symmetry breaking in the $α-T_3$ lattice leads to a bandgap opening. A defect line in the substrate on which the $α-T_3$ lattice is deposited can be viewed as a topological change in the substrate that induces translational in-plane symmetry breaking, resulting in mid-gap states. These topologically protected states are confined along the defect line and exhibit preferential directional motion, with different signs for the different Dirac valleys. Within this context, we investigate how these unidirectional interface chiral states are affected in the presence of a perpendicular magnetic field and how they can be tuned by varying the controlling system parameter $α$. The latter tunes the $α-T_3$ structure from a honeycomb-like lattice ($α=0$) to a dice lattice ($α=1$). Our theoretical framework is based on the continuum approximation described by a $3\times 3$ matrix Hamiltonian with a sublattice symmetry-breaking term given by $Δ(x) diag(1,\quad -1,\quad 1)$, assuming $Δ(x)$ as a kink-like mass potential profile. Results for dispersion relations and wavefunction distributions for different $α$ parameters and magnetic field amplitudes are discussed. We demonstrate lifting of Landau levels degeneracy and of valley degeneracy. Our findings pave the way for proposing valley filter devices based on any evolutionary stage between the honeycomb-like and dice lattice structures of the $α-T_3$ phase, controlled by external fields.

cond-mat.mes-hall↗

arXiv2025

Mechanism of the electrochemical hydrogenation of graphene

The electrochemical hydrogenation of graphene induces a robust and reversible conductor-insulator transition, of strong interest in logic-and-memory applications. However, its mechanism remains unknown. Here we show that it proceeds as a reduction reaction in which proton adsorption competes with the formation of H2 molecules via an Eley-Rideal process. Graphene's electrochemical hydrogenation is up to $10^6$ times faster than alternative hydrogenation methods and is fully reversible via the oxidative desorption of protons. We demonstrate that the proton reduction rate in defect-free graphene can be enhanced by an order of magnitude by the introduction of nanoscale corrugations in its lattice, and that the substitution of protons for deuterons results both in lower potentials for the hydrogenation process and in a more stable compound. Our results pave the way to investigating the chemisorption of ions in 2D materials at high electric fields, opening a new avenue to control these materials' electronic properties.

physics.chem-ph↗

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.

gr-qc↗

arXiv2025

Constraints on Bulk Fields: No-Go Conjectures for Braneworld Models

This work establishes a series of no-go conjectures that impose rigorous constraints on the localization of bulk fields in braneworld scenarios, specifically affecting gauge and spinor fields within five-dimensional spacetimes. Our approach differs from traditional methods as it does not rely on specific equations of motion, making our results broadly applicable across various braneworld models. These no-go conditions reveal fundamental limitations in field localization, challenging the feasibility of embedding fields on the brane. For instance, our analysis demonstrates that existing models fail to achieve consistent localization for gauge and spinor fields. Additionally, one of our conditions indicates that the effective Lagrangian on the brane cannot exhibit conformal invariance.

hep-ph↗

arXiv2025

Traversable Wormholes Sourced by Dark Matter in Loop Quantum Cosmology

In this work, we investigate the existence of wormholes within the framework of Loop Quantum Cosmology, using isotropic dark matter as the source. We analyze three distinct density profiles and solve the modified gravity field equations alongside the stress-energy tensor conservation, applying appropriate boundary conditions to obtain traversable wormhole solutions. Each solution is shown to satisfy the geometric criteria for wormholes, and their regularity is verified by computing the Kretschmann scalar to ensure the absence of singularities under determined conditions. Additionally, we examine the stress-energy tensor to identify scenarios in which energy conditions are violated within this model. The wormhole geometry is further explored through embedding diagrams, and the amount of exotic matter required to sustain these structures is computed using the Volume Integral Quantifier. Finally, we study the shadow produced by our wormhole solution, considering one of the dark matter density profiles, and compare it with observations of the M87 galaxy.

gr-qc↗

arXiv2024

Energy levels and Aharonov-Bohm oscillations in twisted bilayer graphene quantum dots and rings

We present a systematic study of the energy levels of twisted bilayer graphene (tBLG) quantum dots (QD) and rings (QR) under an external perpendicular magnetic field. The confinement structures are modeled by a circular dot-like- and ring-like-shaped site-dependent staggered potential, which prevents edge effects and leads to an energy gap between the electron and hole states. Results are obtained within the tight-binding model with interlayer hopping parameters defined by the Slater-Koster form for different interlayer twist angles $θ$. Our findings show that, for $θ$ around 0$^\circ$ or $60^\circ$, the energy spectra exhibit features resulting from the interplay between characteristics of the AA and AB/BA stacking orders that compose the moiré pattern of such tBLG, while the low-energy levels are shown to be nearly independent on the rotation angle for $10^\circ\lesssim θ\lesssim 50^\circ$. In the absence of a magnetic field, the energy levels of the QR scale with its width $W$ according to a power law $W^{-α}$, whose exponent $1 \lessapproxα\lessapprox 2$ depends on the twist angle. Most interestingly, the lowest energy states of tBLG QRs oscillate as a function of its average radius, with the oscillation period matching half of the moiré period. In the presence of an intense magnetic field, the lowest energy levels for the tBLG QDs and QRs match almost perfectly, regardless of whether the external radius of the quantum confinement structure is smaller or on the order of the moiré period, which is due to the interplay of the trigonal warping effect and the preferential localization of the eigenstates. Our results reveal relevant information about the moiré pattern in tBLG and its role in charge particle confinement.

cond-mat.mes-hall↗

arXiv2024

Control of proton transport and hydrogenation in double-gated graphene

The basal plane of graphene can function as a selective barrier that is permeable to protons but impermeable to all ions and gases, stimulating its use in applications such as membranes, catalysis and isotope separation. Protons can chemically adsorb on graphene and hydrogenate it, inducing a conductor-insulator transition that has been explored intensively in graphene electronic devices. However, both processes face energy barriers and various strategies have been proposed to accelerate proton transport, for example by introducing vacancies, incorporating catalytic metals or chemically functionalizing the lattice. However, these techniques can compromise other properties, such as ion selectivity or mechanical stability. Here we show that independent control of the electric field, E, at around 1 V nm-1, and charge-carrier density, n, at around 1 x 10^14 cm-2, in double-gated graphene allows the decoupling of proton transport from lattice hydrogenation and can thereby accelerate proton transport such that it approaches the limiting electrolyte current for our devices. Proton transport and hydrogenation can be driven selectively with precision and robustness, enabling proton-based logic and memory graphene devices that have on-off ratios spanning orders of magnitude. Our results show that field effects can accelerate and decouple electrochemical processes in double-gated 2D crystals and demonstrate the possibility of mapping such processes as a function of E and n, which is a new technique for the study of 2D electrode-electrolyte interfaces.

cond-mat.mes-hall↗

arXiv2024

A New Braneworld with Conformal Symmetry Breaking

We explore the conformal 5D braneworld, where warping emerges through conformal symmetry breaking. Our scenario seamlessly aligns with conventional brane approaches if conformal symmetry remains unbroken. It is shown that a model with a single conformal breaking parameter effectively localizes gravity on the brane, but it falls short in trapping gauge bosons. However, in scenarios with two parameters, gravity is localized, and the model also achieves the localization of zero modes for both gauge and Dirac fields.

hep-th↗

arXiv2023

Charged vacancy in graphene: interplay between Landau levels and atomic collapse resonances

The interplay between a magnetic field and the Coulomb potential from a charged vacancy on the electron states in graphene is investigated within the tight-binding model. The Coulomb potential removes locally Landau level degeneracy, while the vacancy introduces a satellite level next to the normal Landau level. These satellite levels are found throughout the positive energy region, but in the negative energy region they turn into atomic collapse resonances. Crossings between Landau levels with different angular quantum number $m$ are found. Unlike the point impurity system in which an anticrossing occurs between Landau levels of the same $m$, in this work anticrossing is found between the normal Landau level and the vacancy induced level. The atomic collapse resonance hybridize with the Landau levels. The charge at which the lowest Landau level $m = -1, N = 1$ crosses increases $E = 0$ with enhancing magnetic field. Landau level scaling anomaly occurs when the charge is larger than the critical charge $β\approx0.6$ and this critical charge is independent of the magnetic field.

cond-mat.mes-hall↗

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.

gr-qc↗

arXiv2023

Black Strings in Asymptotically Safe Gravity

In this paper, we study black strings in asymptotic safety gravity (ASG) scenario. The ASG approach is introduced by implementing gravitational and cosmological running coupling constants directly in the black string metric. We calculate the Hawking temperature, entropy, and heat capacity of the improved black string metric in two cases: considering the cosmological constant fixed in some fixed point and the general case where both Newton's constant and cosmological constant are improved. For the identification of the scale moment we used an general inverse law setting $k(r)\sim 1/r^{n}$. We show that improving only the Newton's constant the problem of singularity is solved for the identifications with $n>1$. However, if the cosmological constant is also running the singularity persists in the solution. Also, we show that the ASG effects predicts the presence of a remnant mass in the final evaporation process. Besides that, a logarithmic correction is observed in the entropy. However, a running cosmological constant introduces new correction terms to the entropy beyond that. We show that the improved black string solution remains stable, as in the usual case. Phase transitions are not observed in both cases studied here.

gr-qc↗

arXiv2022

Generalized Ellis-Bronnikov graphene wormhole

In this paper, we investigate the spinless stationary Schrödinger equation for the electron when it is permanently bound to a generalized Ellis-Bronnikov graphene wormhole-like surface. The curvature gives rise to a geometric potential affecting thus the electronic dynamics. The geometry of the wormhole's shape is controlled by the parameter $n$ which assumes even values. We discuss the role played by the parameter $n$ and the orbital angular momentum on bound states and probability density for the electron.

gr-qc↗

arXiv2021

Universal Mass Scale for Bosonic Fields in Multi-Brane Worlds

In this paper we find an universal mass scale for all $q-$ forms in multi-brane worlds model. It is known that this model provides an ultralight mode for the fields. However, to get this, the Lagrangians considered in the literature are not covariant. In order to solve this, we propose a covariant version to multi-localize $q-$ form fields. As a consequence of the covariance, we show that all the $q$-form fields have an ultralight mode with the same mass as the gravitational one. That way we show that there is an universal mass scale for the ultralight modes of the bosonic fields. This suggests that a new physics must emerge, for all these fields, at the same scale.

hep-th↗

arXiv2020

Two dimensional electron gas in a non-Euclidean space

A charged particle in the presence of a magnetic field is studied in the position dependent operator formalism. Instead of a quantum harmonic oscillator, the solution of the resulting Schrödinger-like equation is the one for the Morse oscillator. The anharmonicity that shows up naturally from the theory is analogous to the corrections introduced by relativistic ones.The degeneracy of the spin up and down levels is lifted due to the non-Euclidean space. It is shown that the Fermi energy and the total ground state energy of the 2DEG is also modified.

cond-mat.mes-hall↗

arXiv2019

Rainbow's Gravity Corrections to the Black Hole Global Casimir Effect

In this manuscript we compute corrections to the global Casimir effect at zero and finite temperature due to Rainbow's Gravity (parametrized by $ξ$). For this we use the solutions for the scalar field with mass $m$ in the deformed Schwarzschild background and the corresponding quantized energies of the system, which represent the stationary states of the field and yield the stable part of the quantum vacuum energy. The analysis is made here by considering the limit for which the source mass, $M$, approaches zero, in order to verify the effects on the global Casimir effect in mini black holes near to the Planck scale, $ω_P$. We find a singular behavior for the regularized vacuum energy at zero temperature and for all the corresponding thermodynamic quantities when $m^2=ω^2_P/ξ$, what can be seen as the limit of validity of the model. Furthermore, we show that the remnant Casimir tension over the event horizon in the limit $M\to 0$ is finite for any temperature and all the space of parameters. In fact we show that the remnant tension receives no corrections from Rainbow's Gravity. This points to the fact that such a behavior may be an universal property of this kind of system.

hep-th↗