Artigos
URI permanente para esta coleçãohttps://locus.ufv.br/handle/123456789/11797
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Item Roughness exponent in the Domany-Kinzel cellular automaton(Journal of Physics A: Mathematical and General, 1999-02-12) Martins, Marcelo Lobato; Sales, J. A. de; Moreira, J. G.We propose a method to detect phase transitions in discrete lattice models based on the roughness exponent. This approach is applied to the one-dimensional Domany-Kinzel cellular automaton (CA). Our results, obtained by numerical simulations, show that the roughness exponent method detects the frozen-active phase transition directly from the CA spatio-temporal configurations without any reference to thermodynamical potentials, order parameters or response functions.Item Off-equilibrium dynamics of the frustrated ising lattice gas(Physical Review E., 1999-05-01) Stariolo, Daniel A.; Arenzon, Jeferson J.We study by means of Monte Carlo simulations the off-equilibrium properties of a model glass, the frustrated Ising lattice gas in three dimensions. We have computed typical two times quantities, such as density-density autocorrelations and the autocorrelation of internal degrees of freedom. We find an aging scenario particularly interesting in the case of the density autocorrelations in real space that is very reminiscent of spin glass phenomenology. While this model captures the essential features of structural glass dynamics, its analogy with spin glasses may make possible its complete description using the tools developed in spin glass theory.Item Analytical verification of scaling laws for the Ising model with external field in fractal lattices(Physical Review E, 1999-09-01) Redinz, José ArnaldoWe use an exact recursion procedure to verify analytically, without any intermediary numerical calculation, the validity of the hyperscaling (Josephson) law extended to fractals, the Rushbrooke and Griffiths scaling laws for the Ising ferromagnet with external magnetic field in the whole family of Migdal-Kadanoff-like hierarchical lattices.Item Cellular automata model for citrus variegated chlorosis(Physical Review E., 2000-06-14) Martins, M. L.; Ceotto, G.; Alves, S. G.; Bufon, C. C. B.; Silva, J. M.; Laranjeira, F. F.A cellular automata model is proposed to analyze the progress of citrus variegated chlorosis epidemics in São Paulo orange plantations. In this model epidemiological and environmental features, such as motility of sharpshooter vectors that perform Lévy flights, level of plant hydric and nutritional stress, and seasonal climatic effects, are included. The observed epidemic data were quantitatively reproduced by the proposed model on varying the parameters controlling vector motility, plant stress, and initial population of diseased plants.Item Failure regimes in (1+1) dimensions in fibrous materials(Physical Review E, 2001-01-26) Menezes-Sobrinho, I. L.; Bernardes, A. T.; Moreira, J. G.In this paper, we introduce a model for fracture in fibrous materials that takes into account the rupture height of the fibers, in contrast with previous models. Thus, we obtain the profile of the fracture and calculate its roughness, defined as the variance around the mean height. We investigate the relationship between the fracture roughness and the fracture toughness.Item Fracture toughness in fibrous materials(Physical Review E, 2001-12-14) Menezes-Sobrinho, I. L.In the present paper, a fiber bundle model in (1+1) dimensions that simulates the rupture process of a fibrous material pulled by an uniaxial force F is analyzed. In this model the load of a broken fiber is shifted in equal portions onto the nearest unbroken fibers. The force-displacement diagram is obtained for several traction velocities v and temperatures t. Also, it is shown how the fracture toughness Kc changes with the traction velocity v and with the temperature t. In this paper it is shown that the rupture process is strongly dependent on temperature t and on velocity v.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 Absorbing state transition in a one-dimensional contact replication process(Physical Review E, 2004-09-29) Ferreira Jr., S. C.In this work, the contact process (CP) is modified in order to model a contact replication process (CRP) for monoclonal reproduction. The occupation rates of an empty site depend on the nearest-neighbor and next- nearest-neighbor sites. The CRP exhibits an absorbing state transition studied through cluster approximations and Monte Carlo simulations. The critical rate obtained from simulations,λc = 2.0263(4), is smaller than that for CP. However, the CRP critical exponents are in agreement with those for CP and, consequently, the model belongs to the directed percolation universality class.Item Roughness of CdTe thin films grown on glass by hot wall epitaxy(Journal of Physics, 2004-12-10) Leal, F.F.; Ferreira, S.O.; Menezes-Sobrinho, I.L.; Faria, T.E.Cadmium telluride films were grown on glass substrates using the hot wall epitaxy (HWE) technique. The samples were polycrystalline with a preferential (111) orientation. Scanning electron micrographs reveal a grain size between 0.1 and 0.5 μm. The surface morphology of the samples was studied by measuring the roughness profile using a stylus profiler. The roughness as a function of growth time and scale size were investigated to determine the growth and roughness exponents, β and α, respectively. From the results we can conclude that the growth surface has a self-affine character with a roughness exponent α equal to 0.69 ± 0.03 and almost independent of growth time. The growth exponent β was equal to 0.38 ± 0.06. These values agree with that determined previously for CdTe(111) films grown on GaAs(100).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 Field-induced nonmagnetic impurities interaction in the quantum ising chain(Solid State Communications, 2005-05) Silva, Ricardo L.; Guimaraes, Paulo R. C.; Pereira, Afranio R.We study static vacancies on a ferromagnetic spin-1/2 chain described by the transverse Ising model with second neighbor interactions at zero temperature. Using exact diagonalization techniques and applying a finite-size scaling approach, it is found that a strong magnetic field induces an effective potential of interaction between two vacancies that is attractive.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 Anisotropy in rupture lines of paper sheets(Physical Review E, 2005-06-22) Menezes-Sobrinho, I. L.; Couto, M. S.; Ribeiro, I. R. B.We have experimentally investigated the fracture process in paper samples submitted to a uniaxial force. Five types of paper sheets (newsprint, towel, sulfite, silk, and couche papers) were fractured along two orthogonal orientations. In order to characterize the rupture lines of the paper sheets we utilized the Hurst exponent. Our results indicate a dependence of the Hurst exponent on the orientation of the paper sheets for samples of newsprint and, probably, towel and silk papers. For the other types of paper the Hurst exponent does not depend on the direction of crack propagation.Item Study of optical properties of electropolymerized melanin films by photopyroelectric spectroscopy(European Biophysics Journal, 2005-11-12) Albuquerque, José Eduardo de; Giacomantonio, Clare; White, Andrew G.; Meredith, PaulPhotopyroelectric (PPE) spectroscopy, in the 350–1,075 nm wavelength range, was used to study the optical properties of electropolymerized melanin films on indium tin oxide (ITO) coated glass. The PPE intensity signal as a function of the wavelength λ, V n(λ) and its phase F n(λ) were independently measured. Using the PPE signal intensity and the thermal and optical properties of the pyroelectric detector, we were able to calculate the optical absorption coefficient β of melanin in the solid-state. We believe this to be the first such measurement of its kind on this material. Additionally, we found an optical gap in these melanin films at 1.70 eV.Item Ensino de física no nível médio: tópicos de Física Moderna e experimentação(Revista Ponto de Vista, 2006-01) Pereira, Denis Rafael de Oliveira; Aguiar, OderliO ensino fundamental e médio no Brasil, como se sabe, é de baixa qualidade em muitas regiões. Muitos fatores concorrem para esse resultado. O desinteresse dos alunos em relação às aulas e as dificuldades que muitos professores enfrentam no exercício da docência são fatores determinantes desse quadro. Este artigo discute o ensino da física no nível médio, ressaltando a necessidade de abordagens experimentais e o estudo de tópicos de Física Moderna e Contemporânea, nesse nível de ensino. Uma proposta de atividade prática, em anexo, que envolve o efeito fotoelétrico, já experimentada no nível médio de ensino, enfatiza uma estratégia pedagógica, de experimentação como forma de inovar a prática docente e envolver efetivamente os alunos no processo ensino-aprendizagem melhorando efetivamente a qualidade do ensino.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 Experimental realization of the fuse model of crack formation(Physical Review Letters, 2006-03-07) Otomar, D. R.; Menezes-Sobrinho, I. L.; Couto, M. S.In this work, we present an experimental investigation of the fuse model. Our main goal was to study the influence of the disorder on the fracture process. The experimental apparatus used consisted of an L×L square lattice with fuses placed on each bond of the lattice. Two types of materials were used as fuses: copper and steel wool wires. The lattice composed only of copper wires varied from a weakly disordered system to a strongly disordered one. The lattice formed only by steel wool wires corresponded to a strongly disordered one. The experimental procedure consisted of applying a potential difference V to the lattice and measuring the respective current I. The characteristic function I(V) obtained was investigated in order to find the scaling law dependence of the voltage and the current on the system size L when the disorder was changed. Our results show that the scaling laws are only verified for the disordered regime.Item Exact solution for the An–Liu–Johnson–Lovett model related to the dynamics of allelochemicals in the environment(Ecological Modelling, 2006-03-15) Martins, M.L.General solutions for a model proposed by An et al. [An, M., Liu, D.L., Johnson, I.R., Lovett, J.V., 2003. Mathematical modelling of allelopathy. II. The dynamics of allelochemicals from living plants in the environment. Ecol. Model. 161, 53–66] to describe the dynamics of allelochemicals in the environment are presented. The particular forms of such solutions in two of the main existing cases in living plants are illustrated.Item A method based on a nonlinear generalized Heisenberg algebra to study the molecular vibrational spectrum(The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics, 2006-06-30) Oliveira-Neto, N. M.; Souza, J. de; Ribeiro-Silva, C. I.We propose a method, based on a generalized Heisenberg algebra (GHA), to reproduce the anharmonic spectrum of diatomic molecules. The theoretical spectrum generated by GHA allows us to fit the experimental data and to obtain the dissociation energy for the carbon monoxide molecule. Our outcomes are more accurate than the standard models used to study molecular vibrations, namely the Morse and the q-oscillator models and comparable to the perturbed Morse model proposed by Huffaker [CITE], for the first experimental levels. The dissociation energy obtained here is more accurate than all previous models.Item Dynamic scaling of polymer gels comprising nanoparticles(The Journal of Physical Chemistry B, 2006-09-22) Teixeira, A. V.; Geissler, E.; Licinio, P.We present dynamic light scattering (DLS) measurements of soft poly(methyl-methacrylate) (PMMA) and polyacrylamide (PA) polymer gels prepared with trapped bodies (latex spheres or magnetic nanoparticles). We show that the anomalous diffusivity of the trapped particles can be analyzed in terms of a fractal Gaussian network gel model for the entire time range probed by DLS technique. This model is a generalization of the Rouse model for linear chains extended for structures with power law network connectivity scaling, which includes both percolating and uniform bulk gel limits. For a dilute dispersion of strongly scattering particles trapped in a gel, the scattered electric field correlation function at small wavevector ideally probes self-diffusion of gel portions imprisoning the particles. Our results show that the time-dependent diffusion coefficients calculated from the correlation functions change from a free diffusion regime at short times to an anomalous subdiffusive regime at long times (increasingly arrested displacement). The characteristic time of transition between these regimes depends on scattering vector as ∼q-2, while the time decay power exponent tends to the value expected for a bulk network at small q. The diffusion curves for all scattering vectors and all samples were scaled to a single master curve.