Magnetization reversals in a disk-shaped small magnet with an interface

We consider a nanodisk possessing two coupled materials with different ferromagnetic exchange constant. The common border line of the two media passes at the disk center dividing the system exactly in two similar half-disks. The vortex core motion crossing the interface is investigated with a simple description based on a two-dimensional model which mimics a very thin real material with such a line defect. The main result of this study is that, depending on the magnetic coupling which connects the media, the vortex core can be dramatically and repeatedly flipped from up to down and vice versa by the interface. This phenomenon produces burst-like emission of spin waves each time the switching process takes place.Comment: 11 pages, 10 figure

Inert scalar dark matter in an extra dimension inspired model

In this paper we analyze a dark matter model inspired by theories with extra dimensions. The dark matter candidate corresponds to the first Kaluza-Klein mode of a real scalar added to the Standard Model. The tower of new particles enriches the calculation of the relic abundance. For large mass splitting, the model converges to the predictions of the inert singlet dark matter model. For nearly degenerate mass spectrum, coannihilations increase the cross-sections used for direct and indirect dark matter searches. Moreover, the Kaluza-Klein zero mode can mix with the SM higgs and further constraints can be applied.Comment: 21 pages, 6 figures. We have improved the text and added references. Version published in JCA

On the elimination of infinitesimal Gribov ambiguities in non-Abelian gauge theories

An alternative method to account for the Gribov ambiguities in gauge theories is presented. It is shown that, to eliminate Gribov ambiguities, at infinitesimal level, it is required to break the BRST symmetry in a soft manner. This can be done by introducing a suitable extra constraint that eliminates the infinitesimal Gribov copies. It is shown that the present approach is consistent with the well established known cases in the literature, i.e., the Landau and maximal Abelian gauges. The method is valid for gauges depending exclusively on the gauge field and is restricted to classical level. However, occasionally, we deal with quantum aspects of the technique, which are used to improve the results.Comment: 29 pp. No figures. Discussions added. Final version to appear in EPJ

Gribov ambiguities at the Landau -- maximal Abelian interpolating gauge

In a previous work, we presented a new method to account for the Gribov ambiguities in non-Abelian gauge theories. The method consists on the introduction of an extra constraint which directly eliminates the infinitesimal Gribov copies without the usual geometric approach. Such strategy allows to treat gauges with non-hermitian Faddeev-Popov operator. In this work, we apply this method to a gauge which interpolates among the Landau and maximal Abelian gauges. The result is a local and power counting renormalizable action, free of infinitesimal Gribov copies. Moreover, the interpolating tree-level gluon propagator is derived.Comment: Several changes: figures removed, typos corrected and discussions included. 24 pages, to appear in EPJ

A model for structural defects in nanomagnets

A model for describing structural pointlike defects in nanoscaled ferromagnetic materials is presented. Its details are explicitly developed whenever interacting with a vortex-like state comprised in a thin nanodisk. Among others, our model yields results for the vortex equilibrium position under the influence of several defects along with an external magnetic field in good qualitative agreement with experiments. We also discuss how such defects may affect the vortex motion, like its gyrotropic oscillation and dynamical polarization reversal.Comment: 8 pages, resubmitted to Journal of Applied Physic

Collapsing and Expanding Cylindrically Symmetric Fields with Ligh-tlike Wave-Fronts in General Relativity

The dynamics of collapsing and expanding cylindrically symmetric gravitational and matter fields with lightlike wave-fronts is studied in General Relativity, using the Barrabes-Israel method. As an application of the general formulae developed, the collapse of a matter field that satisfies the condition R_{AB}g^{AB} = 0, (A, B = z, phi), in an otherwise flat spacetime background is studied. In particular, it is found that the gravitational collapse of a purely gravitational wave or a null dust fluid cannot be realized in a flat spacetime background. The studies are further specified to the collapse of purely gravitational waves and the general conditions for such collapse are found. It is shown that after the waves arrive at the axis, in general, part of them is reflected to spacelike infinity along the future light cone, and part of it is focused to form spacetime singularities on the symmetry axis. The cases where the collapse does not result in the formation of spacetime singularities are also identified.Comment: 3 figures, prepared in Latex. Inter. J. Mod. Phys. D11, 561-579 (2002

Extending spin ice concepts to another geometry: the artificial triangular spin ice

In this work we propose and study a realization of an artificial spin ice-like system, not based on any real material, in a triangular geometry. At each vertex of the lattice, the "ice-like rule" dictates that three spins must point inward while the other three must point outward. We have studied the system's ground-state and the lowest energy excitations as well as the thermodynamic properties of the system. Our results show that, despite fundamental differences in the vertices topologies as compared to the artificial square spin ice, in the triangular array the lowest energy excitations also behave as a kind of Nambu monopoles (two opposite monopoles connected by an energetic string). Indeed, our results suggest that the monopoles charge value may have a universal value while the string tension could be tuned by changing the system's geometry, probably allowing the design of systems with different string tensions. Our Monte Carlo results suggest a phase transition in the Ising universality class where the mean distance between monopoles and anti-monopoles increases considerably at the critical temperature. The differences on the vertices topologies seem to facilitate the experimental achievement of the system's ground-state, thereby allowing a more detailed experimental study of the system's properties.Comment: This new version of the paper includes all changes described in the erratum published at PRB 86, 219902(E) (2012) (http://dx.doi.org/10.1103/PhysRevB.86.219902

How hole defects modify vortex dynamics in ferromagnetic nanodisks

Defects introduced in ferromagnetic nanodisks may deeply affect the structure and dynamics of stable vortex-like magnetization. Here, analytical techniques are used for studying, among other dynamical aspects, how a small cylindrical cavity modify the oscillatory modes of the vortex. For instance, we have realized that if the vortex is nucleated out from the hole its gyrotropic frequencies are shifted below. Modifications become even more pronounced when the vortex core is partially or completely captured by the hole. In these cases, the gyrovector can be partially or completely suppressed, so that the associated frequencies increase considerably, say, from some times to several powers. Possible relevance of our results for understanding other aspects of vortex dynamics in the presence of cavities and/or structural defects are also discussed.Comment: 9 pages, 4 page