Artigos
URI permanente para esta coleçãohttps://locus.ufv.br/handle/123456789/11797
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Item Small amplitude oscillations of sine-gordon vortex states in planar magnets in three dimensions(Brazilian Journal of Physics, 2002-09) Pereira, A . R.We investigate the spectrum of small amplitude oscillations of the sine-Gordon vortex-antivortex pair in layered ferromagnetic systems described by the anisotropic three-dimensional XY-model. In the case of a small interlayer coupling constant, vortex-pairs can be formed independently in each plane. Analytical expressions for two discrete modes with zero frequency are obtained. These zero modes are associated with the two-dimensional motion of the excitation on an individual plane and are important because the interference between them and a vortex-pair gives rise to contributions to the central peak. The continuum states, which contribute to EPR linewidth, are derived using the Born approximation.Item On the series expansion of the general spin-S Ising chain(Physica A: statistical mechanics and its applications, 2007-01-01) Rojas, Onofre; Souza, Sérgio M. de; Moura-Melo, Winder A.Thermodynamical properties of spin-S Ising chains can nowadays be easily obtained using numerical calculation. However, from a mathematical point of view, its exact solution for arbitrary spin is still a challenge. Only limiting cases have been solved exactly, such as the infinite spin limit and lowest spin values. The present article addresses this issue. Using the high-temperature series expansion we obtain a new analytical series expansion of the partition function for the Ising chain, in the absence of magnetic field. Our general results cover all spins from 12 to infinite, in the high-temperature region, up to order β40(β=(kT)-1). In order to extend our results to finite-temperature we employ the method presented in the work of Ref. [Bernu and Misguich, Phys. Rev. B63 (2001) 134409]. We also present a matrix formulation of our series expansion and relate it to the transfer matrix technique.Item Electrocrystallization under magnetic fields: experiment and model(Physica A: Statistical Mechanics and its Applications, 2005-05-25) Mansur Filho, J. C.; Silva, A. G.; Carvalho, A. T. G.; Martins, M. L.We report some experimental results for quasi-two-dimensional electrocrystallization of copper under magnetic fields. Such results are theoretically investigated by large scale simulations of a DLA-like model in which random walkers can move along circular vortices enhanced by the Lorentz force. In addition, a sticking probability is used to take into account the complex reaction dynamics at the cathode surface. Our results indicate that the convective motion does not change the nature of the normal diffusive regime, but increases dramatically the diffusion constant by a factor of up to six. The characteristic features (morphology and scaling laws) of both random walks and growing electrodeposits under a perpendicular magnetic field are determined.Item Heisenberg spins on a cone: an interplay between geometry and magnetism(Physics Letters A, 2005-03-14) Freitas, Walter A.; Moura-Melo, Winder A.; Pereira, A. R.This work is devoted to the study of how spin texture excitations are affected by the presence of a static non-magnetic 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.Item Geometrical pinning of magnetic vortices induced by a deficit angle on a surface: anisotropic spins on a conic space background(Physics Letters A, 2007-01-01) Moura-Melo, W. A.; Pereira, A. R.; Mól, L. A. S.; Pires, A. S. T.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.Item Heisenberg model on a space with negative curvature: Topological spin textures on the pseudosphere(Physics Letters A, 2007-06-11) Belo, L. R. A.; Oliveira Neto, N. M.; Moura Melo, W. A.; Pereira, A. R.; Ercolessi, ElisaHeisenberg-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 an antivortex at large distances so that the pair may dissociate at arbitrarily low temperature.Item Magnetotunneling spectroscopy of a biased triple-barrier semiconductor heterostructure(Superlattices and Microstructures, 2006-02-17) Velásquez, RoberIn this work we describe a magnetotunneling spectroscopy technique for probing the localization degree of two-dimensional states and mapping the subbands in the active region of a resonant-tunneling semiconductor heterostructure. The reported experimental data consist of the low-temperature tunneling current traces of an asymmetric triple-barrier structure measured by sweeping an in-plane magnetic field up to 10 T. According to our interpretation model, the main features observed in the tunnel current traces are due to the field-induced resonant transitions between two-dimensional states at the crossing region between dispersion curves. The data reveals the highly localized nature of the quantum states in an asymmetric double-quantum-well structure even for those with very narrow middle barriers.Item Field-induced energy band and effective mass anisotropy in coupled double quantum wells(Superlattices and Microstructures, 2008-03-19) Velásquez, R.; Pankiewicz, C. G.The effects of the field-induced conduction band anisotropy on the electron effective mass and electrical transport properties of coupled double quantum well structures are discussed. This anisotropy is due to the formation of a partial energy gap in the dispersion curves of coupled double quantum wells by an in-plane magnetic field. The reported data show that the magnitude of this gap is mainly dependent on the interwell coupling, although it also depends on the magnitude of the applied magnetic field. A field-induced anisotropy of the electron effective mass and the in-plane electrical transport properties, due to the formation of a saddle point at the lower edge of this gap, is predicted. Further, a simple model to estimate the critical field at which a saddle point appears is also discussed.Item Nonlinear sigma model of a spin ladder containing a static single hole(Solid State Communications, 2008-01) Pereira, A. R.; Ercolessi, E.; Pires, A. S. T.In this letter we extend the nonlinear σ model describing pure spin ladders with an arbitrary number of legs to the case of ladders containing a single static hole. A simple immediate application of this approach to classical ladders is considered.Item Study of optical absorption differences of doped polyaniline films by photothermal spectroscopies(Applied Physics A, 2007-07-29) Albuquerque, J. E. de; Melo, W. L. B.; Faria, R. M.In this work, we studied the optical absorption spectral differences between doped and slightly doped polyaniline films and a derivative by photopyroelectric and photoacoustic spectroscopies. There exist great spectral differences between doped and slightly doped samples as shown by conventional optical absorption spectroscopy, but not greatly evidenced by photopyroelectric spectroscopy. The latter was applied to obtain thermal parameters such as thermal diffusivity and thermal conductivity, and as a result it showed that these thermal properties of polyaniline films are very similar for doped and slightly doped samples. Also it showed that for thicker films (about 20 μm), there are no significant optical absorption differences between them. However, for thin films both techniques showed greater optical absorption differences for doped and slightly doped samples, mainly detected by photoacoustic spectroscopy. These behaviors are in accordance with published results which is the granular metal model for polyaniline. This model explains the polyaniline polymeric matrix as formed by conductive islands in the insulating bulk material.