arXiv subjects

C. A. S. Almeida

Publications and source records attributed to C. A. S. Almeida.

At least 19 recordsLinked to original sources

arXiv2026

Aharonov-Casher effect from a supersymmetric N=1 D=4 model with Kalb-Ramond Lorentz-violating background: a SUSY-preserving mechanism via the Fayet-Iliopoulos term

The Aharonov-Casher (AC) effect describes the geometric phase acquired by a neutral particle carrying a magnetic dipole moment moving in an external electric field. In supersymmetric gauge theories it is often argued that exact supersymmetry enforces the vanishing of anomalous magnetic dipole moments, suggesting that the AC interaction may be incompatible with unbroken supersymmetry in four dimensions. In this work we show that this conclusion is model-dependent. We construct an $N=1$, $D=4$ supersymmetric gauge model in which a Lorentz-violating Kalb-Ramond background induces dynamically the dipole interaction responsible for the AC effect while leaving the supersymmetry algebra intact. The model couples a chiral superfield to an Abelian gauge superfield through a Chern-Simons-type interaction supplemented by a Fayet--Iliopoulos term. A duality identification between the symmetric combination $S+S^\dagger$ of the chiral superfield and the Kalb-Ramond superfield strength allows the antisymmetric tensor background to enter the supersymmetric dynamics without breaking supersymmetry. Integrating out the auxiliary $D$ field generates dynamically an effective dipole interaction of the form $\barψσ^{μν}F_{μν}ψ$. In the nonrelativistic limit the resulting equation of motion reproduces the Aharonov-Casher Hamiltonian for a neutral fermion. The effective magnetic dipole moment is expressed in terms of the parameters of the model and can be mapped onto tensor coefficients of the fermion sector of the Standard Model Extension. Our results therefore provide an explicit realization of a four-dimensional supersymmetric theory in which the Aharonov--Casher interaction emerges dynamically while supersymmetry remains exact in the presence of Lorentz violation.

hep-th↗

arXiv2026

Five-dimensional thick branes in the scalar representation of $f(Q,B)$ gravity

One examines a codimension-one thick braneworld in the scalar representation of $f(Q,B)$ gravity, where $Q$ is the nonmetricity scalar, and $B$ is the boundary term satisfying $R=Q-B$. We write the gravitational sector in terms of two auxiliary scalar fields, $φ=f_Q$ and $ψ=f_B$, and an interaction potential $U(φ,ψ)$ obtained by a Legendre transformation from the $f(Q,B)$ function. Working in the coincident gauge and assuming a five-dimensional warped geometry with four-dimensional Poincaré symmetry, we derive the complete set of brane equations sourced by a canonical bulk scalar field. To accomplish our purpose, we address two models of smooth asymptotically AdS thick brane with localized scalar profiles. Furthermore, we also analyze tensor perturbations and demonstrate the localization and stability of the massless graviton.

gr-qc↗

arXiv2026

Kalb-Ramond Topological Term in Majorana Superspace and Kaluza-Klein Spectrum Deformation in Five Dimensions

We construct an intrinsic $N=1$, $D=5$ superspace whose Grassmann coordinate is a five-dimensional Dirac spinor split into two Majorana spinors, $Θ=θ+i\tildeθ$, and use it to analyze the supersymmetric structure of the five-dimensional Kalb-Ramond (KR) topological term. The algebra closes on the fifth momentum through anticommutators of opposite-sector generators, realizing $\partial_5$ as a central charge; this assignment is the unique covariant one, and we give a dictionary to the $N=1/2$ superspace of Linch, Luty and Phillips and to the $N=1$ superform hierarchies of Becker et al. First, a tensor multiplet obeying both same-side chirality conditions cannot propagate along $x^5$; the KR multiplet is therefore a singly chiral spinor superfield with field strength $T_{5α}=\partial_5 B_α+W_α(V_T)$, and the supersymmetric completion of the term is a $BF$-type invariant agreeing with the pseudo-supersymmetric action. Second, we correct the six-dimensional origin of the term, $S_6=\int dϕ\wedge B\wedge H$, and show that for the KR field alone it is a total derivative and leaves the Kaluza-Klein tower undeformed. After the identification $B_{m5}\to A_m$, however, it becomes a genuine $BF$ coupling between independent multiplets, and the spectrum splits exactly into $m_{n,\pm}^2=(\sqrt{n^2/R^2+M^2}\pm M)^2$: the zero mode acquires a topological mass $2M$, and the excited towers are shifted by $\pm M$ relative to uncoupled bulk fields, a shift that cannot be absorbed into $R$. On the orbifold $S^1/\mathbb{Z}_2$, supersymmetry correlates the parities of the two multiplets, the topological term is parity-odd, and a constant coupling is projected out; the deformation is governed by a kink-type coupling, the one generated by the six-dimensional parent action, and becomes second order, with brane-localized contributions at the fixed points.

hep-th↗

arXiv2026

Operator ordering as an emergent gauge field in twisted bilayer graphene: singular spectral signatures at the magic angle

Scanning-tunnelling spectroscopy at the AB and BA stacking points of magic-angle twisted bilayer graphene should reveal two asymmetric Van~Hove peaks separated by $Δ_{\mathrm{split}}\approx 171$~meV -- a splitting absent from the standard Bistritzer--MacDonald spectrum. We show this signature arises naturally from the Hermitian ordering correction of the Dirac Hamiltonian with spatially varying mass, which generates an emergent Aharonov--Bohm flux of $h/(2e)$ at each zero of the effective mass $m_{\mathrm{eff}}(\mathbf{r})=w|f(\mathbf{r})|$. In the chiral limit, the interlayer coupling is locally diagonalised by a spatially dependent unitary transformation; the ordering term $H_{\mathrm{ord}}=-\tfrac{i}{2}\boldsymbolσ\cdot \nabla\ln m_{\mathrm{eff}}$ then develops a $1/r$ singularity at the AB/BA stacking points, where $m_{\mathrm{eff}}$ vanishes. The splitting scales as $\sqrtθ$ -- distinguishing it from correlation-driven gaps ($\proptoθ$ or $\propto 1/θ$) -- is gate-voltage independent, and is spatially localised within $r_c\approx 2.1$~nm of each AB/BA point. Within the local asymptotic theory near the zeros of $\mathrm{eff}$, the ordering-corrected zero mode acquires parabolic-cylinder character $D_{-1/2}$. The spatially resolved AB/BA spectrum reported in recent STM studies of magic-angle TBG has not been analysed for two-peak structure and constitutes an immediate experimental test; the predicted cell-averaged broadening of $\approx 14$~meV is consistent with the $16$~meV discrepancy between existing STM data and the BM tight-binding prediction.

cond-mat.mes-hall↗

arXiv2026

Influence of the Electron's Anomalous Magnetic Dipole Moment on High-Atomic-Number Atoms

Super-heavy atoms ($Z > 100$) are usually studied in the context of the so-called ``Quantum Electrodynamics of Strong Fields''. In this theory the problem of the singularity in the electron energy whenever $Z > 137$ is overcome. This is done by considering the finite size of the nucleus and leads to interesting phenomena, such as the spontaneous production of positrons. Here, we show that taking into account the contribution from the Anomalous Magnetic Dipole Moment of the electron (by means of an effective theory), within a point-nucleus model, is a sufficient condition to obtain regular wave functions and physically acceptable energy values for $Z > 137$.

quant-ph↗

arXiv2026

Fidelity susceptibility and geometric response in flux-tuned Dirac systems: exact results from a low-energy two-level reduction

We study the parametric sensitivity of two-dimensional massive Dirac fermions subject to Aharonov-Bohm flux insertion, using the Bures metric (fidelity susceptibility) as the central statistical-mechanical response function. Near integer values of the reduced flux, the low-energy spectrum undergoes a flux-induced avoided crossing whose structure is controlled by the Dirac mass. Through a controlled low-energy projection of the full Dirac=Aharonov-Bohm operator onto an effective two-level subspace, valid near integer flux, we derive an exact closed-form expression for the ground-state Bures metric, which takes a universal Lorentzian profile centered at integer flux with width set by the mass parameter. The peak value scales as $g^{\max}_{λλ}\sim m^{-2}$, diverging in the chiral limit in direct analogy with the divergence of thermodynamic susceptibilities near critical points. We introduce an integrated geometric susceptibility $χ(m) = π/(8m)$, whose inverse-mass scaling is the information-geometric counterpart of power-law critical behavior, with the Dirac mass playing the role of a relevant coupling controlling the distance from the chiral fixed point. The Lorentzian profile is shown to arise from the curvature of the ground-state manifold on the Bloch sphere, requiring no dynamical input beyond the spectral structure. Importantly, this geometric response is independent of Berry curvature and topological invariants, emerging instead from a universal local spectral mechanism. Through its spectral representation, the Bures metric is identified as the geometric (paramagnetic) contribution to the persistent current susceptibility, encoding the sensitivity of persistent currents and orbital magnetization to flux variations and connecting information geometry to measurable response functions in mesoscopic Dirac systems.

cond-mat.mes-hall↗

arXiv2026

Chirality loss during brane merging: a universal power law from the Jackiw-Rebbi index

We investigate the rate at which chiral fermion localisation is lost when two domain walls merge in extra-dimensional braneworld scenarios, using the $(1+1)$-dimensional Jackiw-Rebbi framework as a controlled analytical laboratory. As the inter-brane separation $d$ decreases, left- and right-handed zero modes hybridise and chiral asymmetry is progressively lost. We show that the spatial separation between the chiral zero modes follows a universal power law $|Δ_{\mathrm{abs}}|\propto d^γ$ in the merging limit $d\to 0^{+}$, with the critical exponent $γ$ determined solely by the Jackiw-Rebbi topological index $N_{\mathrm{JR}}$, and independent of the fermionic mass gap, the integrability of the scalar sector, and the detailed shape of the domain wall profile. Comparing the integrable sine-Gordon model with four members of the non-integrable double sine-Gordon family, all sharing $N_{\mathrm{JR}}=1$, we find $γ\in[0.930,0.985]$. For the sine-Gordon model we derive the closed-form overlap integral $I(d)=2d/\sinh(2d)$, from which the exact chiral separation follows as a ratio of hyperbolic functions without free parameters. This result identifies $γ$ as the crossover plateau of a local effective exponent $γ_{\mathrm{eff}}(d)$, explaining the sub-unit value analytically and tracing the universality to the Pöschl-Teller structure of the $N_{\mathrm{JR}}=1$ zero mode. The universality of $γ$ implies that the rate of four-dimensional Yukawa coupling collapse during brane merging is a topological invariant, insensitive to the microscopic scalar dynamics generating the walls.

hep-th↗

arXiv2026

Operator ordering as an emergent geometric background in Dirac systems with spatially varying mass

We investigate the spectral consequences of the uniquely determined Hermitian ordering of the Dirac Hamiltonian with spatially varying mass. In contrast to the nonrelativistic case, where continuous families of admissible prescriptions exist, the relativistic Dirac operator admits a single consistent ordering compatible with probability-current conservation. This requirement generates an additional logarithmic-gradient term proportional to the spatial variation of the mass profile. We show that this contribution modifies the effective kinetic operator and induces a universal deformation of the spectral quantization condition. In compact geometry, an explicit analytic computation reveals a mode-dependent second-order spectral shift that becomes strongly enhanced near the mass-inversion threshold. These results demonstrate that the consistent relativistic ordering of the Dirac operator leads to observable modifications of discrete spectra in spatially inhomogeneous scalar backgrounds.

cond-mat.mes-hall↗

arXiv2026

Helicity-dependent corrections to black-hole shadows from the gravitational spin Hall effect

Black-hole shadows are purely geometric in the leading-order geometric-optics approximation: their boundary is set by null geodesics and carries no information about the polarization of the probing radiation. At subleading order, the gravitational spin Hall effect of light introduces helicity-dependent corrections to photon propagation. We show that, in any static spherically symmetric spacetime, an exact equatorial reflection symmetry of the full spin Hall equations forces these corrections to cancel at the capture threshold: the critical impact parameter remains identical for opposite helicities, and no polarization-dependent shadow splitting occurs. Rotation breaks this symmetry. Using a double perturbative expansion in the black-hole spin $χ= a/M$ and in the inverse frequency $1/ω$, we derive the first non-vanishing helicity-dependent shift of the critical impact parameter for slowly rotating (Kerr) black holes. The effect is linear in $χ$, scales as $1/ω$, and appears as a $\cosϕ$ modulation of the shadow boundary, with a sign reversal on one side of the image for spins $χ\gtrsim 0.21$. Although parametrically small for astrophysical sources, the splitting is a robust, model-independent signature of spin-optical dynamics in strong fields. Our analysis also identifies a methodological pitfall: a naive radial projection that suppresses transverse motion can produce a spurious splitting even in spherical symmetry, a lesson of general relevance for future studies of spin-optical effects.

gr-qc↗

arXiv2026

Gravitationally induced wave-function collapse from dynamical bifurcation

We propose an effective non-relativistic framework in which wave-function collapse emerges as a deterministic dynamical instability induced by gravitational self-interaction and regulated by short-distance repulsion. The dynamics is described by a nonlinear Schrödinger equation supplemented by a phenomenological repulsive sector ensuring regularity at high densities. Using a variational Gaussian ansatz, we derive an explicit effective energy functional and show that extended quantum states lose stability beyond a critical mass scale. This loss of stability is associated with a bifurcation in the reduced dynamical system governing the wave-function width, leading to the emergence of stable localized configurations. Within this picture, collapse corresponds to the dynamical selection of one of these localized attractors, driven by infinitesimal asymmetries in the initial state and occurring without stochastic noise or environmental coupling. The mechanism provides a controlled and quantitative realization of gravity-induced localization, extending Schrödinger--Newton-type models while avoiding their pathological short-distance behavior. Possible implications for mesoscopic systems probing the quantum-to-classical transition are briefly discussed.

quant-ph↗

arXiv2026

Chiral Fermion Localization in Two-Kink Scalar Backgrounds: Tunable Brane Positioning and Universal Divergence at the Single-Kink Limit

The localization of chiral fermionic zero modes in scalar field backgrounds with domain wall structure is a central mechanism in brane-world scenarios. We investigate this mechanism in a system that provides an effective realization of the $(1+1)$-dimensional Jackiw--Rebbi model, using a two-kink scalar background generated by the deformation method applied to the $φ^4$ model. The two-kink profile introduces two physically distinct parameters: an asymmetry parameter $a_2$ controlling the left-right symmetry of the scalar background, and an inter-kink separation parameter $b$ controlling the distance between the constituent domain walls. We establish two independent scaling laws. First, the collective center-of-mass position of the chiral zero modes responds linearly to $a_2$, providing a mechanism for continuously tuning the effective brane position in the extra dimension. Second, the differential spatial separation between the two chiral modes diverges as the two-kink background collapses into a simple kink, following a power law in $(b-1)$ with exponent statistically consistent with $-1$. These two results are physically independent and each admits a precise interpretation in the language of brane-world scenarios. The mechanism is realized concretely in bilayer graphene under an asymmetric two-kink electrostatic potential, providing a tunable platform for probing extra-dimensional localization physics.

hep-th↗

arXiv2026

Codimension-controlled universality of quantum Fisher information singularities at topological band-touching defects

Topological phase transitions in generic multiband systems are mediated by band-touching defects whose codimension -- the number of momentum directions along which the gap closes linearly -- varies across universality classes. Although singular behavior of fidelity susceptibilities and quantum Fisher information (QFI) has been computed for specific models, no unifying principle connecting these results has been identified: it has remained unclear whether the controlling variable is spatial dimensionality, band structure, or an intrinsic geometric property of the defect. We resolve this question by showing that the singular contribution to the QFI with respect to the tuning parameter $m$ obeys a universal power-law scaling $\sim |m|^{p-2}$ for $p \neq 2$, with a logarithmic divergence $\sim \ln(1/|m|)$ at the marginal codimension $p = 2$, where $p$ denotes the codimension of the band-touching defect. This exponent is independent of spatial dimensionality, anisotropies, ultraviolet regularization, and additional gapped bands, and is protected by renormalization-group arguments at the linearized fixed point. The result unifies previously isolated observations for SSH chains ($p=1$), Chern insulators ($p=2$), and Weyl semimetals ($p=3$) as instances of a single codimension-dependent universality class, and reveals that only defects with $p \leq 2$ generate divergent information-geometric responses. This establishes a direct and previously missing link between topological classification in momentum space and quantum distinguishability in parameter space, with implications for metrological sensitivity near topological transitions and for the experimental detection of topological criticality via quantum geometric observables.

quant-ph↗

arXiv2026

Scalaron excitation by topological vortices in quadratic $f(R)$ gravity on a BTZ black hole background

In three spacetime dimensions, pure Einstein gravity admits no local propagating degrees of freedom, yet nontrivial gravitational backgrounds such as the BTZ black hole provide a natural arena to probe dynamical extensions of the theory. In quadratic $f(R)$ gravity the Ricci scalar becomes a propagating degree of freedom - the scalaron. We investigate how localized Maxwell-Higgs vortices excite this scalar mode in a static BTZ black-hole background. Working in the perturbative regime $α\ll \ell^2$, the trace equation reduces to a massive Klein-Gordon equation for the curvature scalar sourced by the trace of the vortex energy-momentum tensor. Using the Sturm-Liouville structure of the radial operator, we construct the corresponding Green function and obtain the curvature profile generated by an arbitrary localized source. The induced excitation exhibits a universal asymptotic decay $R(r) \sim r^{-(1+ν)}$, independent of the detailed vortex structure. The scalar excitation is linearly stable, carries finite energy, and produces parametrically suppressed backreaction, ensuring the smooth recovery of the Einstein limit. These results provide a concrete realization of how higher-curvature corrections activate the unique local gravitational degree of freedom in three dimensions and how localized sources excite this scalar mode in black-hole spacetimes.

gr-qc↗

arXiv2026

Dirac fermions on a surface with localized strain

We study the influence of a localized Gaussian deformation on massless Dirac fermions confined to a two-dimensional curved surface. Both in-plane and out-of-plane displacements are considered within the framework of elasticity theory. These deformations couple to the Dirac spinors via the spin connection and the vielbeins, leading to a position-dependent Fermi velocity and an effective geometric potential. We show that the spin connection contributes an attractive potential centered on the deformation and explore how this influences the fermionic density of states. Analytical and numerical solutions reveal the emergence of bound states near the deformation and demonstrate how the Lamé coefficients affect curvature and state localization. Upon introducing an external magnetic field, the effective potential becomes confining at large distances, producing localized Landau levels that concentrate near the deformation. A geometric Aharonov-Bohm phase is identified through the spinor holonomy. These results contribute to the understanding of strain-induced electronic effects in Dirac materials, such as graphene.

cond-mat.mes-hall↗

arXiv2026

Velocity effects slightly mitigating the quantumness degradation of an Unruh-DeWitt detector

In this work, we investigate the velocity effects on information degradation due to the Unruh effect in accelerated quantum systems (with finite interaction time). We consider a detector moving along a spatial trajectory within a two-dimensional plane. The quantum systems studied were: accelerated single-qubit, quantum interferometric circuit, and which-path distinguishability circuit. Thus, for non-relativistic velocity regime, we obtained analytical expressions such as transition rates, quantum coherence, visibility, distinguishability, and the complementarity relation. On the other hand, for the ultra-relativistic velocity regime, we saw that the Unruh effect is suppressed and therefore the detector does not respond in this case. Our findings revealed that velocity effects imply mitigation of information degradation, this interesting behaviors happen because of the composite effect of both velocity and acceleration. The results obtained show that the addition of the non-relativistic, transverse and constant motion of an accelerated detector can play a protective role in quantumness in systems at high accelerations, although the effects are very small.

quant-ph↗

arXiv2025

Soft breaking of the $\mathbb{Z}_2$ symmetry in the $ϕ^4$ theory

We consider a two-dimensional scalar field theory that modifies the standard $ϕ^4$ model by introducing a smooth breaking of translational invariance through a hyperbolic generalizing function. This function explicitly breaks the $\mathbb{Z}_2$ symmetry; however, it also introduces a mechanism capable of generating new energy minima (vacua) and of localizing or delocalizing field fluctuations around these vacua. Thus, this mechanism enables the continuous transformation of the kink/antikink-like configurations into compacted asymmetric double-kink/antikink structures. Accordingly, this transformation gives rise to new classes of configurations resembling asymmetric non-topological solitons, characterized by a soft breaking of the $\mathbb{Z}_2$ symmetry. These findings are particularly compelling, as they are consistent with results describing Su-Schrieffer-Heeger (SSH) domain walls in dimerized polymer systems.

hep-th↗

arXiv2025

Properties of a Majorana fermion ensemble with exciton-like mass

Considering the relativistic scenario, we dedicate our study to the relativistic quantum description of one-dimensional Majorana fermions. Thus, we focus on aspects related to exciton-like particles. Seeking to reach our purpose, one analyzes the relativistic quantum mechanical system characterized by an effective mass distribution. In this context, we adopt an exciton-like position-dependent mass without impurity, i.e., without electromagnetic interactions. From this perspective, one notes results of noteworthy interest as consequences of the theory adopted. For instance, we highlight that, even without interaction, exciton-like Majorana fermions manifest theoretically bound states. Also, we construct a Majorana fermion ensemble with effective mass immersed in a thermal reservoir. That allows for a thorough investigation of the thermodynamic properties of the system. Among the thermodynamic characteristics studied in the canonical ensemble, we focus on the Helmholtz free energy, mean energy, entropy, and heat capacity. The numerical results obtained for these thermodynamic properties corroborate the validity of the Dulong-Petit law for our system.

cond-mat.mes-hall↗

arXiv2025

Mitigating the information degradation in a massive Unruh-DeWitt theory

We investigated the influence of the massive scalar field on the information degradation concerning the Unruh-DeWitt (UDW) detectors. In this conjecture, we adopted a system with a finite and large interaction time. To accomplish our purpose, one examines the quantum coherence of a uniformly accelerated qubit and the probability of finding the detector in the ground state. In this framework, we consider a quantum interferometric circuit to obtain the probability, visibility, and coherence. Naturally, these measurements provide us with wave-like information. Besides, one modifies the circuit to describe the path distinguishability and the particle-like information. These results are promising, as they allow us to understand the influence of the Unruh effect on the wave-particle duality. Thus, our findings announce that the increase in the scalar field mass induces a decrease in information degradation. Finally, we noted that the information concerning the Unruh effect remains preserved when $m \geq Ω$. Therefore, the detector cannot absorb particles with mass equal to or greater than its energy gap. These results indicate that the scalar field mass is a protective factor against information degradation for systems under high acceleration conditions.

hep-th↗