Resumo em Inglês:

In this paper we present an inversion procedure for electromagnetic scattering, based on the coupled-dipole method (CDM) combined with an inversion algorithm making use of the singular value decomposition procedure associated with a regularization factor. This method permits to obtain images of non-homogeneous dielectric objects whose dimensions are comparable to the incident wavelength. The feasibility of this method is showed in two synthetic examples using the CDM with 257 and 515 dipoles, for spherical objects with a spherical inclusion. The method also works if the scattered electric field, which is the input data, is corrupted with Gaussian noise.

Resumo em Inglês:

The van Hemmen model with a random field is studied to analyze the tricritical behavior in the Ising spin glass phase. The free energy and phase diagram (T versus H and T versus Jo/J, where H is the root mean square deviation of the magnetic field, Jo and J are the ferromagnetic and root mean square deviation exchange, respectively) are calculated for the model with discrete (or bimodal) and Gaussian distributions. For the case of the bimodal probability distribution (random field and exchange), we have the presence of three ordered phases, namely: spin glass (SG), mixed (pi) and ferromagnetic (F). The root mean square deviation of the random field H destroys the spin glass and mixed phases. The mixed phase doesn't appear with Gaussian distribution. In the plane T versus H, we analyze the tricritical behavior for the case of the bimodal distribution, and we compare it with the results obtained by using the Gaussian distribution that presents only second-order phase transition.

Resumo em Inglês:

Wavelengths from radiation of plasmas produced by a Nd:YAG/glass laser focused on target of Co and Ni have been recorded photographically in the region 240-600 Å with a 3m normal incidence spectrograph. For this sequence (Co XIII and Ni XIV) we have identified 13 new lines belonging to the array 3s²3p³-3s3p4 and derived 7 new levels for the 3s3p4 configuration. The classification was established by comparison of the relative intensities for the lines along the isoelectronic sequence, extrapolation, and Hartree-Fock calculation.

Resumo em Inglês:

Effects of incomplete surface accommodation in rarefied gas flow have been studied by using the Direct Simulation Monte Carlo (DSMC) method in conjunction with the Cercignani-Lampis-Lord gas surface interaction model. The DSMC calculations examine differences in predictions of aerodynamic forces and heat transfer between full and partial surface accommodation for hypersonic flow past round leading edges at zero incidence. The aerodynamic performance of round leading edges is assessed by using the heat transfer rate, the total drag and the shock wave standoff distance. Twenty-five combinations of nose radius, normal and tangential accommodation coefficients were used in the simulation. For the flow conditions considered, the analysis showed that stagnation point heating, total drag and shock standoff distance are sensitive to changes on either the normal or tangential accommodation coefficient. The results substantiate that it becomes imperative to take surface accommodation into account in order to make accurate predictions of the aerodynamic forces on, and heat transfer rates to, bodies in rarefied hypersonic flow.

Resumo em Inglês:

We use the black hole entropy function to study the effect of Born-Infeld terms on the entropy of extremal black holes in heterotic string theory in four dimensions. We find that after adding a set of higher curvature terms to the effective action, attractor mechanism works and Born-Infeld terms contribute to the stretching of near horizon geometry. In the alpha' -> 0 limit, the solutions of attractor equations for moduli fields and the resulting entropy, are in conformity with the ones for standard two charge black holes.

Resumo em Inglês:

We discuss some formal and practical aspects related to the replacement of Integral Dispersion Relations (IDR) by derivative forms, without high-energy approximations. We first demonstrate that, for a class of functions with physical interest as forward scattering amplitudes, this replacement can be analytically performed, leading to novel Extended Derivative Dispersion Relations (EDDR), which, in principle, are valid for any energy above the physical threshold. We then verify the equivalence between the IDR and EDDR by means of a popular parametrization for total cross sections from proton-proton and antiproton-proton scattering and compare the results with those obtained through other representations for the derivative relations. Critical aspects on the limitations of the whole analysis, from both formal and practical points of view, are also discussed in some detail.

Resumo em Inglês:

We examine the evolution of perturbations on the kink configuration in <FONT FACE=Symbol>l f</FONT>4 theory and of the Nielsen-Olesen vortex in scalar electrodynamics through the Galerkin method. The problem is reduced to a finite dynamical system for which the linear and nonlinear regimes are studied. The linear stability of both is associated to a motion in a stable torus present in phase space, whereas the nonlinear evolution of perturbations can be viewed as a consequence of the breakdown of the tori structure and the onset of chaos. We discuss this regime in connection with the stability of the configurations. Also, the Galerkin method is used to obtain approximate analytical expressions for the vortex profile.

Resumo em Inglês:

Current ophthalmic technology allows the manipulation of eye components, such as anterior cornea and lens, of the human eye with a considerable precision and customization. This technology opens up the possibility of exploiting some characteristics of the eye in order to improve the methods of correcting optical aberrations. Moreover, product development and research for the eye-care professional has reached very high standards, since there is nowadays software available to design and simulate practically any mechanical or optical characteristic of the product, even before it is thrown into production line. Although quite similar in the general form, different human eye models simulate the image formation by considering different property combinations in the constitutive elements of the eye structure (such as refraction index and surface curvatures), producing retinal images that resemble very closely those of the biological eye. Using optical design software, we have implemented a simulation of 5 well-known schematic eyes available in the literature. These models were the Helmholtz-Laurance, Gullstrand, Emsley, Greivenkamp and Liou & Brennan. The optical performance of these different models was compared using different quantitative optical quality parameters. The model of Liou and Brennan, contains features of the biological eye that were not considered in previous models, as the distribution of a gradient refraction index and a decentered pupil. Furthermore, it has great reliability since it takes into account the mean value of empirical measurements of the in vivo eye in order to define size and parameters such as anterior and posterior curvature of cornea, lens, axial length, etc. Comparisons between the MTF (Modulation Transfer Function), spot diagrams and ray fan showed the difference in image quality between eye models, and the Strehl Ratio was also used as a parameter of comparison. A careful comparison between the different models showed that the first four schematic eyes have better optical quality than what is expected for the general and healthy emmetropic in vivo eye. Liou and Brennan schematic eye is the one that most closely resembles the in vivo biological eye. Therefore, in applications, such as research or product development for customized vision correction, which must consider optical properties intrinsic to the biological eye, we recommend this latter model; for applications that do not require refraction-limited performance, most of the other models should be a good approximation.

Resumo em Inglês:

Elastic scattering of positive and/or negative pions (pi+ and/or pi-) by nuclei has been studied by means of Glauber's multiple scattering theory (GMST). The differential, total and total elastic cross sections using both the full series expansion and the first order correction (called optical limit results, OLA) of the Glauber amplitude, have been calculated. The in-medium pN amplitude (sigmaeff., sigmaeff.) as well as the free piN one (sigmafree, sigmafree) are invoked into the full series calculations of the Glauber amplitude and our results were compared with the corresponding experimental data. These calculations are carried out for the elastic collisions, pi-- 2He4 at incident energy 180 MeV, pi±- 3Li6 at 240 MeV, pi-- 6C12 at incident energies 180, 200 and 260 MeV, pi+- 8O16 at 240 and 270 MeV and pi±- 20Ca40 at energy 292.5 MeV. The comparisons reflect that, except around the minima, the full series calculations with the in-medium pN amplitude are better in describing the scattering data than those using the free piN one and both are better than the optical limit results.

Resumo em Inglês:

We study the non integrability of the Friedmann-Robertson-Walker cosmological model, in continuation of the work [5] of Coehlo, Skea and Stuchi. Using Morales-Ramis theorem ([10]) and applying a practical non-integrability criterion deduced from it, we find that the system is not completely integrable for almost all values of the parameters lambda and lambda, which was already proved by the authors of [5] applying Kovacic's algorithm. Working on a level surface H = h with h <FONT FACE=Symbol>¹</FONT> 0 and h <FONT FACE=Symbol>¹</FONT> -1/4\lambda and using the Morales-Ramis-Simo ''higher variational'' theory ([11]), we prove that the hamiltonian system cannot be integrable for particular values of lambda among the exceptional values and that it is completely integrable in two special cases (lambda = lambda = -m² and lambda = lambda = m^2/3). We conjecture that there is no other case of complete integrability and give detailed arguments towards this.

Resumo em Inglês:

We investigate a dynamical mass generation mechanism for the off-diagonal gluons and ghosts in SU(N) Yang-Mills theories, quantized in the maximal Abelian gauge. Such a mass can be seen as evidence for the Abelian dominance in that gauge. It originates from the condensation of a mixed gluon-ghost operator of mass dimension two, which lowers the vacuum energy. We construct an effective potential for this operator by a combined use of the local composite operators technique with algebraic renormalization and we discuss the gauge parameter independence of the results. We also show that it is possible to connect the vacuum energy, due to the mass dimension two condensate discussed here, with the non-trivial vacuum energy originating from the condensate <FONT FACE=Symbol>á</FONT>Aµ²<FONT FACE=Symbol>ñ</FONT>, which has attracted much attention in the Landau gauge.

Resumo em Inglês:

Hadron energy spectra induced by one-single nucleon are obtained solving diffusion equations by the method of characteristics. Our solutions are a generalization of earlier papers that allow us to calculate hadron fluxes including the energy dependence of the interaction lengths and inelasticities. A comparison with the integral hadron spectra of the so-called ''halo events" detected by the Brazil-Japan Collaboration at Mt. Chacaltaya is made, in order to test our solutions. A reasonable agreement between then is obtained, considering the rising with energy of the nucleon inelasticity coefficient.

Resumo em Inglês:

We discuss the properties of an electromechanical oscillator whose operation is based upon the cyclic, quasi-conservative conversion between gravitational potential, kinetic, and magnetic energies. The system consists of a strong-pinning type-II superconductor square loop subjected to a constant external force and to magnetic fields. The loop oscillates in the upright position at a frequency that can be tuned in the range 10-1000 Hz, and has induced in it a rectified electrical current. The emphasis of this paper is on the evaluation of the major remaining sources of losses in the oscillations. We argue that such losses should be associated with the viscous vibration of pinned flux lines in the superconductor Nb-Ti wire, provided the oscillator is kept under vacuum and the magnetic field is sufficiently uniform. Losses of similar or greater magnitude would be associated with dragging of the loop against the He atmosphere remaining in the evacuated cryostat. We discuss how other different sources of loss would become negligible for such operational conditions, so that a very high quality factor Q exceeding 10(10) might in principle be reached by the oscillator. The prospective utilization of such oscillator as a low-frequency high-Q clock is analyzed.

Resumo em Inglês:

It is shown that conductivity and molar volume in binary rubidium and cesium silicate glasses, both measured at room temperature, obey a common cubic scaling relation due to increase in alkali content. The drastic drop in conductivity up to 15 orders of magnitude for so many ion-conducting binary alkali silicate glasses (in wide composition range) is mainly caused by the structure and the ion content. In particular, it is suggested that the glass network expansion, which is related to the available free volume, is a parameter that could explain the increase in ionic conductivity for these binary systems.

Resumo em Inglês:

We prove that a spherically symmetric exterior solution of the field equations of a metric nonsymmetric theory of gravitation coupled with the electromagnetic field is necessarily static.

Resumo em Inglês:

We define a coupling of two baker maps through a pi/2 rotation both in position and in momentum. The classical trajectories thus exhibit spiraling, or loxodromic motion, which is only possible for conservative maps of at least two degrees of freedom. This loxodromic baker map is still hyperbolic, that is, fully chaotic. Quantization of this map follows on similar lines to other generalized baker maps. It is found that the eigenvalue spectrum for quantum loxodromic baker map is far removed from those of the canonical random matrix ensembles. An investigation of the symmetries of the loxodromic baker map reveals the cause of this deviation from the Bohigas-Giannoni-Schmit conjecture.

Resumo em Inglês:

A digital system to characterize solid state nuclear track detectors(SSNTD) has been developed. It was used in the study of two detectors and its performance evaluated by comparing the obtained results with those determined by using an analog system, already in use for such purpose. The comparison of the results together with the feasibility and rapidness in data acquisition have demonstrated the viability of the digital system to characterize SSNTD.

Resumo em Inglês:

In this paper we show that with standard numerical methods it is possible to obtain highly precise results for quasi normal modes (QNMs). In particular, secondary modes are obtained by numerical integration done in the well-known null grid. We have compared such numerical results to the also well-known 6th-order WKB method and have found a striking degree of agreement, which could be as good as seven significant figures for the fundamental mode and three for the first overtone. We have chosen the Schwarzschild BH (black hole) to start with because it is the simplest and most well-known of all BHs, so it provides a very safe testing ground to the aforementioned numerical method.

Resumo em Inglês:

We use a global model (volume averaged) to study plasma discharges in molecular oxygen gas in the 1-100 mTorr pressure range. This model determines densities of positive ions O2+ and O+ , negative ion O- , electrons, ground state O2 and O atoms, and metastables O2(a¹deltag) and O(¹D), and electron temperature as function of gas pressure and input power, for a cylindrical discharge. We apply the model to O2 discharges and the results are compared to a particle-in-cell simulation (PIC), experimental data and a volume-averaged global model developed at the University of California at Berkeley. We find that the total positive ion density increases with pressure at low pressures (up to approximately 30 mTorr), and decreases at higher pressures. The electronegativity decreases with increased power and increased pressure as predicted by the global models presented in the literature. The predictions for electron temperature are also in agreement with these models. However, there is a discrepancy betweeen these global models and PIC simulations and experimental data, for 20 and 40 mTorr cases, concerning electronegativity calculations. PIC simulations yield much higher electronegativities. There are strong indications that this is due to the assumption of Maxwellian electron energy distribution functions in the global model, while in the PIC simulations this is clearly not the case.

Resumo em Inglês:

We consider a probabilistic cellular automaton to analyze the stochastic dynamics of a predator-prey system. The local rules are Markovian and are based in the Lotka-Volterra model. The individuals of each species reside on the sites of a lattice and interact with an unsymmetrical neighborhood. We look for the effect of the space anisotropy in the characterization of the oscillations of the species population densities. Our study of the probabilistic cellular automaton is based on simple and pair mean-field approximations and explicitly takes into account spatial anisotropy.