arXiv subjects

Afranio R. Pereira

Publications and source records attributed to Afranio R. Pereira.

10 recordsLinked to original sources

arXiv2025

Energetics and thermodynamics of bilayer rectangular artificial spin ices

Bilayer rectangular artificial spin ices (BRASIs) with distinct aspect ratios, $γ$, are considered. Namely, we investigate how the underlying geometry modifies the interaction between two rectangular artificial spin ice layers separated by a height offset, $h$, whenever compared to the square case. Actually, rectangular layers interact by means of a Buckingham-like potential, whereas in the square case, one has an algebraic (van der Walls-like) interaction. In addition, Moiré patterns for BRASIs are less definite than for the square bilayer. We also deal with their basic thermodynamics, showing the behavior of the specific heat as a function of temperature and $γ$ for a number of height offsets.

cond-mat.other↗

arXiv2021

Bilayer artificial spin ice: magnetic force switching and basic thermodynamics

We study an artificial spin ice system consisting of two identical layers separated by a height offset $h$. For small separation, the layers are shown to attract each other, provided the whole system is in the ground state. Such an attraction comes about by means of a power-law force that we compare to van der Waals forces. When magnetic monopoles occur in one (or both) layers, the scenario becomes even more interesting and these layers may also repel each other. By tuning parameters like $h$ and monopole distance, switching between attraction and repulsion may be accomplished in a feasible way. Regarding its thermodynamics, the specific heat peak shifts to lower temperature as $h$ increases.

cond-mat.mes-hall↗

arXiv2017

Klein Tunneling of Spin Polarized Currents in HgTe Quantum Wells

We investigate the behavior of spin polarized currents in two-dimensional topological insulators (TI). Stationary solutions inside a HgTe/CdTe quantum well (QW) were obtained by Bernevig-Hughes-Zhang (BHZ) model modified by a electric and magnetic barrier inside a non-completely insulating bulk. An attenuated quantum spin Hall (QSH) effect occurs in the gaped region with an apparent Klein-like paradox. Even more interesting, for strong potential regime, the interaction between the quasiparticles and the barriers allows spin inversion of this electronic states in a distinct channel conduction. Thus, our findings suggest a mechanism to manipulated spin polarized currents in this system.

cond-mat.mes-hall↗

arXiv2015

Topological cutoff frequency in a slab waveguide: Penetration length in topological insulator walls

We study the propagation of electromagnetic (EM) waves in a slab-type waveguide which walls consist of three-dimensional topological insulator (3D TI). The results show that a cutoff frequency with topological stability limits the spectrum that propagates along the waveguide and are in agreement with experimental observations. Our approach also provides a way to measure the penetration length of surface metallic states in 3D TI.

cond-mat.mes-hall↗

arXiv2013

Bound-states and polarized charged zero modes in three-dimensional topological insulators induced by a magnetic vortex

By coating a three-dimensional topological insulator (TI) with a ferromagnetic film supporting an in-plane magnetic vortex, one breaks the time-reversal symmetry (TRS) without generating a mass gap. It rather yields electronic states bound to the vortex center which have different probabilities associated with each spin mode. In addition, its associate current (around the vortex center) is partially polarized with an energy gap separating the most excited bound state from the scattered ones. Charged zero-modes also appear as fully polarized modes localized near the vortex center. From the magnetic point of view, the observation of such a special current in a TI-magnet sandwich comes about as an alternative technique for detecting magnetic vortices in magnetic thin films.

cond-mat.mes-hall↗

arXiv2011

Geometrically induced electric polarization in conical topological insulators

We study the topological magnetoelectric effect on a conical topological insulator when a point charge $q$ is near the cone apex. The Hall current induced on the cone surface and the image charge configuration are determined. We also study a kind of gravitational Aharonov-Bohm effect in this geometry and realize a phase diference betwen the components of the wavefunctions (spinors) upon closed parallel transport around the (singular) cone tip. Concretely, a net current flowing towards cone apex (or botton) shows up, yielding electric polarization of the conical topological insulator. Such an effect may be detected, for instance, by means of the net accumulated Hall charge near the apex. Once it depends only on the geometry of the material (essetially, the cone apperture angle) this may be faced as a microscopic scale realization of (2+1)-dimensional Einstein gravity.

cond-mat.str-el↗

arXiv2011

Scattering of charge carriers in graphene induced by topological defects

We study the scattering of graphene quasiparticles by topological defects, represented by holes, pentagons and heptagons. For holes, we found that at low concentration they give a negligible contribution to the resistivity. Whenever pentagons or heptagons are introduced we realize that a fermionic current is scattered by defects.

cond-mat.str-el↗

arXiv2010

Effects of lattice defects in graphene on the scattering of Charge Carriers

We study the scattering of graphene quasiparticles by topological defects, represented by holes, pentagons and heptagons. For the case of holes, we obtain the phase shift and found that at low concentration they appear to be irrelevant for the electron transport, giving a negligible contribution to the resistivity. Whenever pentagons are introduced into the lattice and the fermionic current is constrained to move near one of them we realize that such a current is scattered with an angle that depends only on the number of pentagons and on the side the current taken. Such a deviation may be determined by means of a Young-type experiment, through the interference pattern between the two current branches scattered by a pentagon. In the case of a heptagon such a current is also scattered but it diverges from the defect, preventing a interference between two beams of current for the same heptagon.

cond-mat.str-el↗

arXiv2010

Ground-state configurations in ferromagnetic nanotori

Magnetization ground states are studied in toroidal nanomagnets. The energetics associated to the ferromagnetic, vortex and onion-like configurations are explicitly computed. The analysis reveals that the vortex appears to be the most prominent of such states, minimizing total energy in every torus with internal radius $r\gtrsim10\,{\rm nm}$ (for Permalloy). For $r\lesssim10\,{\rm nm}$ the vortex remains the most favorable pattern whenever $R/\ell_{ex}\gtrsim1.5$ ($R$ is the torus external radius and $\ell_{ex}$ is the exchange length), being substituted by the ferromagnetic state whenever $R/\ell_{ex}\lesssim1.5$.

cond-mat.mes-hall↗

arXiv2004

Diluted planar ferromagnets: nonlinear excitations on a non-simply connected manifold

We study the behavior of magnetic vortices on a two-dimensional support manifold being not simply connected. It is done by considering the continuum approach of the XY-model on a plane with two disks removed from it. We argue that an effective attractive interaction between the two disks may exist due to the presence of a vortex. The results can be applied to diluted planar ferromagnets with easy-plane anisotropy, where the disks can be seen as nonmagnetic impurities. Simulations are also used to test the predictions of the continuum limit.

cond-mat.str-el↗