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W. A. Moura-Melo

Publications and source records attributed to W. A. Moura-Melo.

44 records · Page 3Linked to original sources
arXiv2007

On Dirac-like Monopoles in a Lorentz- and CPT-violating Electrodynamics

We study magnetic monopoles in a Lorentz- and CPT-odd electrodynamical framework in (3+1) dimensions. This is the standard Maxwell model extended by means of a Chern-Simons-like term, $b_μ\tilde{F}^{μν}A_ν$ ($b_μ$ constant), which respects gauge invariance but violates both Lorentz and CPT symmetries (as a consequence, duality is also lost). Our main interest concerns the analysis of the model in the presence of Dirac monopoles, so that the Bianchi identity no longer holds, which naively yields the non-conservation of electric charge. Since gauge symmetry is respected, the issue of charge conservation is more involved. Actually, the inconsistency may be circumvented, if we assume that the appearance of a monopole induces an extra electric current. The reduction of the model to (2+1) dimensions in the presence of both the magnetic sources and Lorentz-violating terms is presented. There, a quantization condition involving the scalar remnant of $b_μ$, say, the mass parameter, is obtained. We also point out that the breaking of duality may be associated with an asymmetry between electric and magnetic sources in this background, so that the electromagnetic force experienced by a magnetic pole is supplemented by an extra term proportional to $b_μ$, whenever compared to the one acting on an electric charge.

hep-th↗
arXiv2006

Heisenberg model on a space with negative curvature: topological spin textures on the pseudosphere

Heisenberg-like spins lying on the pseudosphere (a 2-dimensional infinite space with constant negative curvature) cannot give rise to stable soliton solutions. Only fractional solutions can be stabilized on this surface provided that at least a hole is incorporated. We also address the issue of `in-plane' vortices, in the XY regime. Interestingly, the energy of a single vortex no longer blows up as the excitation spreads to infinity. This yields a non-confining potential between a vortex and a antivortex at large distances so that the pair may dissociate at arbitrarily low temperature.

cond-mat.str-el↗
arXiv2006

Analytical and Monte Carlo study of two antidots in magnetic nanodisks with vortex-like magnetization

How stable vortex-like magnetization in magnetic nanodisks with small aspect ratio ($L/R<<1$) is affected by two antidots is investigated analytically and by Monte Carlo simulations. For suitable ranges of the physical parameters this vortex presents bistable states when pinned around the antidots. The hysteresis loop obtained shows a central loop associated to the pinning mechanism of the vortex at the antidots. Our results agree qualitatively well with those provided by experiments and micromagnetic simulations.

cond-mat.mtrl-sci↗
arXiv2006

Geometrical pinning of magnetic vortices induced by a deficit angle on a surface: anisotropic spins on a conic space background

We study magnetic vortex-like excitations lying on a conic space background. Two types of them are obtained. Their energies appear to be linearly dependent on the conical aperture parameter, besides of being logarithmically divergent with the sample size. In addition, we realize a geometrical-like pinning of the vortex, say, it is energetically favorable for it to nucleate around the conical apex. We also study the problem of two vortices on the cone and obtain an interesting effect on such a geometry: excitations of the same charge, then repealing each other, may nucleate around the apex for suitable cone apertures. We also pay attention to the problem of the vortex pair and how its dissociation temperature depends upon conical geometry.

cond-mat.mtrl-sci↗
arXiv2004

Heisenberg spins on a cone: an interplay between geometry and magnetism

This work is devoted to the study of how spin texture excitations are affected by the presence of a static nonmagnetic impurity whenever they lie on a conical support. We realize a number of novelties as compared to the flat plane case. Indeed, by virtue of the conical shape, the interaction potential between a soliton and an impurity appears to become stronger as long as the cone is tighten. As a consequence, a new kind of solitonic excitation shows up exhibiting lower energy than in the absence of such impurity. In addition, we conclude that such an energy is also dependent upon conical aperture, getting lower values as the latter is decreased. We also discuss how an external magnetic field (Zeeman coupling) affects static solitonic textures, providing instability to their structure.

cond-mat.str-el↗
arXiv2002

Thermodynamics of the one-dimensional s=1 XXZ Heisenberg model: analytical results

We apply the results recently derived by Rojas et al. to derive the beta-expansion of the Helmholtz free energy of the spin-1 XXZ Heisenberg model up to 5th order in beta. The analytical expansion obtained is valid for all phases of this model. Our curves of the specific heat fit well Bloete's numerical results in the high temperature regime.

cond-mat↗
arXiv2002

Dirac-like Monopoles in Three Dimensions and Their Possible Influences on the Dynamics of Particles

Dirac-like monopoles are studied in three-dimensional Abelian Maxwell and Maxwell-Chern-Simons models. Their scalar nature is highlighted and discussed through a dimensional reduction of four-dimensional electrodynamics with electric and magnetic sources. Some general properties and similarities of them when are considered in Minkowski or Euclidian space are mentioned. However, by virtue of the structure of the space-time in which they are considered a number of differences among them take place. Furthermore, we pay attention to some consequences of these objects when acting upon usual particles. Among other subjects, special attention is given to the study of a Lorentz-violating non-minimal coupling between neutral fermions and the field generated by a monopole alone. In addition, an analogue of the Aharonov-Casher effect is discussed in this framework.

hep-th↗