Repositório RCAAP

The two-dimensional spin-1 Baxter-Wu model is studied by using Monte Carlo simulations. The standard single-spin-flip Metropolis algorithm is used to generate the configurations from which the order parameter, specific heat and magnetic susceptibility are measured. The finite-size scaling procedure is employed in order to get the critical behavior. Extensive simulations show that the critical exponents are different from those of the spin-1/2 model suggesting that the spin-1 model is in a different universality class.

In this work we studied a ferromagnetic mixed-spin Ising model including a single ion crystal-field term. The model system consists of two interpenetrating sublattices with spins sigma = 1/2 and S = 1. The spins sigma = 1/2 occupy the sites of one sublattice, their nearest-neighbours are spins S on the other sublattice, and vice versa. The system is in contact with a heat bath, the spins flipping according to the Metropolis transition rate and, at the same time, subject to an external flow of energy, which is simulated by a two-spin flip process. The model is studied via the dynamical pair approximation and through Monte Carlo simulations. We have determined the phase diagram of the model in the plane crystal-field D versus competition parameter p. The parameter p accounts for the competition between the one- and two-spin flip processes. In the pair approximation, the phase diagram, at high temperatures, present three phases separated by two transition lines: a continuous transition line between the ferromagnetic and paramagnetic phases, and a first-order transition line between the paramag-netic and antiferromagnetic phases. However, Monte Carlo simulations predict the same topology for the phase diagram as the pair approximation, but all the transition lines are continuous for any value of the temperature.

In this work we study a model of a catalytic reaction among three monomers in order to understand the kinetics of the selective catalytic reduction of NO by ammonia (4NO + 4NH3 + O2 -> 4N2 + 6H2O). Our model takes into account the formation of the intermediate species in the global scheme of the reaction. Using the Monte Carlo method we simulated the model on a square lattice and determined its phase diagram.

In this work we studied the effects of non active sites on the substrate of a growing surface. In our model a particle only sticks at the surface if the site where it falls is an active site. However, we allow the deposited particle to diffuse along the surface in accordance with some previously defined mechanism. Using Monte Carlo simulations, and some analytical results, we have investigated the model in (1+1) and (2+1) dimensions, considering different relaxation mechanisms. We show that the inclusion of non active sites is a crucial point in the model. In fact, we have observed that the saturation regime always disappears and that the values of the growth exponent beta go to one, at large times, for any mechanism of diffusion we considered in one and two dimensions.

The probability distribution of the order parameter is analyzed in order to obtain the criticality of magnetic systems. Monte Carlo simulations have been employed by using single spin flip Metropolis algorithm aided by finite-size scaling and histogram reweighting techniques. A method is proposed to obtain this probability distribution even when the transition temperature of the model is unknown. A test is performed on the two-dimensional spin-1/2 and spin-1 Ising model and the results show that the present procedure can be quite efficient and accurate to describe the criticality of the system.

We study the phase diagram of the three-dimensional classical ferromagnetic Heisenberg model with an easy-plane crystalline anisotropy and an easy-axis exchange anisotropy through Monte Carlo simulations. We employ the Metropolis algorithm together with single-histogram techniques in order to characterize the transitions in each region of the phase diagram. Our results reveal, besides the disordered phase, the existence of Ising-like and XY-like ordered phases which are separated by a first-order transition line.

Colloidal particles move in the carrier liquid under the action of several forces and torques. When the particles carry a dipole moment, electric or magnetic, as in ferrofluids, the rotational and translational motions are coupled because the field on a particle depends on the spatial and directional distribution of the others and the force and torque on it depends on the field. Moreover, there is Brownian, as well as dissipative forces and torques on each particle. Consequently, the numerical solution of the equations of motion requires, besides the techniques of Classical Molecular Dynamics, those of Stochastic Dynamics. The algorithm is explained in some detail and applied on a typical ferrofluid. For different values of the temperature, the possibility of the formation of structures is examined.

Cellular automata (CA) are discrete, spatially-homogeneous, locally-interacting dynamical systems of very simple construction, but which exhibit a rich intrinsic behavior. Even starting from disordered initial configurations, CA can evolve into ordered states with complex structures crystallized in space-time patterns. In this paper we concentrate on deterministic one-dimensional CA defined by rules that lead to chaotic patterns. In order to find universality classes for these rules we associate a growth process with the CA dynamics and study the temporal behavior of the growth exponent, skewness and kurtosis of the height distribution of the interface. We obtain four universality classes characterized by different values of the growth exponent. These are related to the random deposition and directed percolation classes.

We performed Monte Carlo simulations considering two different models for antiferromagnetic small particles with Ising spins. The spins of the particle are disposed at the sites of the two dimensional arrays with coordination numbers z = 4 and z = 6, around a central spin. The core spins interact antiferromagnetically and the spins at the surface of the particle are disordered. In the first model, we consider an antiferromagnetic core surrounded by a disordered surface of the spin-glass type. In the second model, the core is still antiferromagnetic, but some bonds at the surface are broken. We determined the hysteresis curves, the zero-field-cooling (ZFC) and field-cooling (FC) curves. We have shown that the model with a disordered surface of the spin-glass type fits better the experimental measurements determined for the antiferromagnetic nanoparticles.

We propose a new analytic approximation for the luminosity function of galaxies. The suggested expression behaves like the Schechter function at the faint end (f ~ La) but departs considerably at the bright end (L >> L*). We argue here that such a behavior may provide a better fit for the current observational data than does the Schechter function. Its practical interest is stressed by considering roughly the data set provided by the Stromlo-APM redshift survey. Implications on the estimates of the matter density parameter from mass-to-light ratio are also briefly discussed.

Systems Biology (system-level understanding in biological science), from the physical-chemical point of view, is involved with irreversible thermodynamics and nonlinear kinetic theory of open systems which are founded on nonequilibrium statistical mechanics. We describe a modern thermo-statistical approach for dealing with complex systems, in particular biological systems. We consider the case of a very peculiar complex behavior in open boson systems sufficiently away, from equilibrium, which appear to have large relevance in the functioning of biological systems. This is, on the one hand, the so-called Frohlich-Bose-Einstein-like condensation leading in steady-state conditions to the emergence of a particular case of quantum-large-scale coherent ordering, of the type of a selforganizing-synergetic dissipative structure. Moreover, additional complexity emerges in the form of propagation, in this condensate, of signals (information) consisting of nearly undamped and undistorted, long-distance propagating, solitary waves (the pseudoparticle soliton). It can be accompanied by a so-called Frohlich-Cherenkov cone of emission of polar vibrations, and it is also possible the formation of ¨ metastable states of the form of the so-called bioelectrets. These are phenomena apparently working in biological processes, which are presently gaining relevant status on the basis of eventually providing a large-scale quantum-coherent behavior in cytoskeletons of neurons and the conscious (non-computational) activity in the brain. Emphasis is centered on the quantum-mechanical-statistical irreversible thermodynamics of these open systems, and the informational characteristics of the phenomena. Ways for their experimental evidencing are pointed out and discussed.

Research in ultrasonography has evidenced the propagation of a peculiar kind of excitation in fluids. Such excitation, dubbed a X-wave, has characteristics resembling that of a solitary-wave type. We reconsider the problem in a medium consisting of a biological material of the like of alpha-helix proteins. It can be shown that in this case is expected an excitation of the Davydov's solitary wave type, however strongly damped in normal conditions. The case of acetanilide, an organic polymer which resembles biopolymers, is considered, and the infrared spectrum analyzed. Davydov's soliton is evidenced as a coherent state of polar vibrations. The case of acoustic (sound) vibrations is also considered, where, also, a damped Davydov-like solitary wave may be excited. However, it is shown that when traveling in conditions sufficiently away from equilibrium, more precisely, when the soliton is embeded in the resulting Frohlich-Bose-Einstein condensate, the lifetime of the solitary wave is largely enhanced. Moreover, a soliton moving in bulk with a velocity larger than that of the group velocity of the normal vibrational waves would produce a Cherenkov-like emission of phonons giving rise to the observed X-wave-like pattern. This paper is a modified and extended version of an earlier publication on the subject.

The classical technical analysis methods of financial time series based on the moving average and momentum is recalled. Illustrations use the IBM share price and Latin American (Argentinian MerVal, Brazilian Bovespa and Mexican IPC) market indices. We have also searched for scaling ranges and exponents in exchange rates between Latin American currencies (ARS, CLP, MXP) and other major currencies DEM, GBP, JPY , USD, and SDRs. We have sorted out correlations and anticorrelations of such exchange rates with respect to DEM, GBP, JPY and USD. They indicate a very complex or speculative behavior.

The recent resumption of the synthesis of new superheavy nuclei has provided relevant results for nuclear physics and chemistry. These nuclei provide experimental confirmation of theoretical calculations that predict the existence of two islands of stability, one with maximum stability in Z=114 and N=184 and the other in Z=120 or Z=126 and N=184 or N=228. Through cold fusion, recently synthesized nuclei with Z=114, 116 and 118 have confirmed the theoretical predictions, providing a possibility of reaching the maximum of these islands of stability. Because of the low production rate and the extremely short half-life of these nuclei, it has not yet been possible to determine their energy spectra. However, it has been theoretically predicted that the nuclei should acquire a spherical form in these islands of stability, thus supplying characteristic vibrational energy spectra. The predictions also indicate the existence of deformed superheavy nuclei in the region between the islands, which can possess rotational energy spectra. In this work, the hydrodynamic equation of energy was corrected by inserting terms referring to coulombian effects, shell closure and nuclear deformation, enabling us to predict the energies of the first excited state for superheavy nuclei of quadrupole deformation, the quadrupole phonon state and an octupole phonon for spherical superheavy nuclei. Four anharmonic vibrational models were then used to describe higher energy states of the nuclei in question. The energy spectra of the (266)104, (268)106, (270)108, (274)110, (278)112, (298)114, (304)120, (348)120, (354)126 nuclei and their closest neighbors were determined.

Photoluminescence and photoreflectance measurements have been used to determine excitonic transitions in the ternary Al xGa1-xAs alloy in the temperature range from 2 to 300 K. The effect of the thermal expansion contribution on the temperature dependence of excitonic transitions for different aluminum concentrations in the Al xGa1-xAs alloy is presented. Results from this study have shown that the negative thermal expansion (NTE) in the Al xGa1-xAs alloy, in the low temperature interval, induces a small blueshift in the optical transition energy. In the temperature range from ~23 to ~95 K there is a competition between the NTE effect and the electron-phonon interaction. Using the thermal expansion coefficient in the 2 - 300 K temperature range, the thermal expansion contribution to GaAs, at room temperature, represents 21% of the total shift of the excitonic transition energy. After subtracting the thermal expansion contribution from the experimental temperature dependence of the excitonic transitions, in the Al xGa1-xAs alloy, the contribution to the electron-phonon interaction of the longitudinal optical phonon increases, relatively to the longitudinal acoustical phonon, with increasing Al concentration.

We present a class of exact vacuum solutions corresponding to de Sitter and warm inflation models in the framework of scalar-tensor cosmologies. We show that in both cases the field equations reduce to planar dynamical systems with constraints. Then, we carry out a qualitative analysis of the models by examining the phase diagrams of the solutions near the equilibrium points.

We study the conformal symmetry and the energy-momentum conservation of scalar field interacting with a curved background at D = 2. We avoid to incorporate the metric determinant into the measure of the scalar field to explain the conformal anomaly and the consequent energy-momentum conservation. Contrarily, we split the scalar field in two other fields, in such a way that just one of them can be quantized. We show that the same usual geometric quantities of the anomaly are obtained, which are accompanied by terms containing the new field of the theory.

Recent developments in string theory suggest that there might exist extra spatial dimensions, which are not small nor compact. The framework of a great number of brane cosmological models is that in which the matter fields are confined on a brane-world embedded in five dimensions (the bulk). Motivated by this we review the main results on the algebraic classification of second order symmetric tensors in 5-dimensional space-times. All possible Segre types for a symmetric two-tensor are found, and a set of canonical forms for each Segre type is obtained. A limiting diagram for the Segre types of these symmetric tensors in 5D is built. Two theorems which collect together some basic results on the algebraic structure of second order symmetric tensors in 5-D are presented. We also show how one can obtain, by induction, the classification and the canonical forms of a symmetric two-tensor on n-dimensional (n > 5) spaces from its classification in 5-D spaces, present the Segre types in n-D and the corresponding canonical forms. This classification of symmetric two-tensors in any n-D spaces and their canonical forms are important in the context of n-dimensional brane-worlds context and also in the framework of 11-D supergravity and 10-D superstrings.

By spectroscopically analyzing the light emitted from the tip-sample gap of the scanning tunneling microscope (STM), we have investigated the carrier transport as well as the luminescence properties of AlGaAs/GaAs quantum wells (QW's). The emission intensity form a target well was measured as a function of the tip position on a cleaved (110) surface of the QW structures. The thermalization length and the diffusion length of the injected electrons were determined in real space.

This paper discusses the growth and the properties of semiconductor nanostructures based on self-assembled quantum dots (QDs). These QDs confine electrons or excitons in zero-dimension (0D), similar to an artificial atom or to an artificial molecule in the case of coupled QDs with vertical alignment. They are obtained in a simple step during the epitaxy of strained III-V semiconductors such as InAs on GaAs, or InAs on InP. We will elaborate on the unique optical properties and the physics of self-assembled QDs and their applications, including QD lasers.