Logomarca do periódico: Brazilian Journal of Physics

Open-access Brazilian Journal of Physics

Publication of: Sociedade Brasileira de Física
Area: Exact And Earth Sciences ISSN printed version: 0103-9733
ISSN online version: 1678-4448
Previous title Revista brasileira de física

Table of contents

Brazilian Journal of Physics, Volume: 36, Issue: 3a, Published: 2006

Brazilian Journal of Physics, Volume: 36, Issue: 3a, Published: 2006

Document list
Foreword
IV Brazilian Meeting on Simulational Physics Plascak, João Antonio Dickman, Ronald

A Harris-Todaro agent-based model to rural-urban migration Espíndola, Aquino L. Silveira, Jaylson J. Penna, T. J. P.

Abstract in English:

The Harris-Todaro model of the rural-urban migration process is revisited under an agent-based approach. The migration of the workers is interpreted as a process of social learning by imitation, formalized by a computational model. By simulating this model, we observe a transitional dynamics with continuous growth of the urban fraction of overall population toward an equilibrium. Such an equilibrium is characterized by stabilization of rural-urban expected wages differential (generalized Harris-Todaro equilibrium condition), urban concentration and urban unemployment. These classic results obtained originally by Harris and Todaro are emergent properties of our model.

Dense packings Herrmann, H. J. Baram, R. Mahmoodi Wackenhut, M.

Abstract in English:

Dense packings of granular systems are of fundamental importance in the manufacture of hard ceramics and ultra strong concrete. We generalize the reversible parking lot model to describe polydisperse dynamic packings. The key ingredient lies in the size distribution of grains. In the extreme case of perfect filling of spherical beads (density one), one has Apollonian tilings with a powerlaw distribution of sizes. We will present the recent discovery of 3D packings which also have the freedom to rotate (bearings) in three dimensions.

Heterogeneities in aging models of granular compaction Arenzon, Jeferson J.

Abstract in English:

Kinetically constrained models (KCM) are systems with trivial thermodynamics but often complex dynamical behavior due to constraints on the accessible paths followed by the system. Exploring these properties, the Kob-Andersen (KA) model was introduced to study the slow dynamics of glass forming liquids and later extended to granular materials. In this last context, we present new results on the heterogeneous character of both in and out of equilibrium dynamics, further stretching the granular-glass analogy.

Wang-Landau sampling in three-dimensional polymers Cunha Netto, A. G. Silva, C. J. Caparica, A. A. Dickman, R.

Abstract in English:

Monte Carlo simulations using Wang-Landau sampling are performed to study three-dimensional chains of homopolymers on a lattice. We confirm the accuracy of the method by calculating the thermodynamic properties of this system. Our results are in good agreement with those obtained using Metropolis importance sampling. This algorithm enables one to accurately simulate the usually hardly accessible low-temperature regions since it determines the density of states in a single simulation.

Monte Carlo simulations of a semi-flexible polymer chain: a first glance Seaton, D. T. Mitchell, S. J. Landau, D. P.

Abstract in English:

We present preliminary results for Monte Carlo simulations of a three dimensional semi-flexible polymer chain with continuous monomer positions. In these simulations, standard Metropolis Monte Carlo methods are used to examine the basic properties of the model, such as equilibration configurations, overall size, and transition temperatures.

Polymer dynamics: long time simulations and topological constraints Kremer, Kurt

Abstract in English:

Topological constraints, entanglements, dominate the viscoelastic behavior of high molecular weight polymeric liquids. To give a microscopic foundation of the phenomenological tube, recently a method for identifying the so called primitive path mesh that characterizes the microscopic topological state of (computer generated) conformations of long-chain polymer networks, melts and solutions was introduced. Here we give a short account of this approach and compare this to long time simulations.

Comparison among HB-inspired algorithms for continuous-spin systems and gauge fields Cucchieri, A. Frigori, R. B. Mendes, T. Mihara, A.

Abstract in English:

We propose a new local algorithm for the thermalization of n-vector spin models, which can also be used in the numerical simulation of SU(N) lattice gauge theories. The algorithm combines heat-bath (HB) and micro-canonical updates in a single step - as opposed to the hybrid overrelaxation method, which alternates between the two kinds of update steps - while preserving ergodicity. We test our proposed algorithm in the case of the one-dimensional 4-vector spin model and compare its performance with the standard HB algorithm and with other HB-inspired algorithms.

Wang-Landau sampling of an asymmetric ising model: a study of the critical endpoint behavior Tsai, Shan-Ho Wang, Fugao Landau, D.P.

Abstract in English:

We use the Wang-Landau algorithm to calculate a density of states for an asymmetric Ising model on a triangular lattice with two- and three-body interactions in an external field. An accurate density of states allows us to determine the phase diagram and to study the critical behavior of this model at and near the critical endpoint. We observe a divergence of the curvature of the spectator phase boundary at the critical endpoint in accordance with theoretical predictions.

What do Monte Carlo simulations tell us about compressible Ising models? Landau, D. P.

Abstract in English:

Monte Carlo simulations have begun to illuminate the nature of phase transitions and universality classes for compressible Ising models. A comprehensive analysis of a Landau-Ginsburg Wilson hamiltonian for systems with elastic degrees of freedom predicts that there should be four cases with different behavior, depending upon symmetries and thermodynamic constraints. We shall describe the results of careful Monte Carlo simulations for a simple compressible Ising model that can be easily modified to correspond to each of the four cases.

Monte Carlo study of the metamagnet Ising model in a random and uniform field Weizenmann, A. Godoy, M. Arruda, A. S. de

Abstract in English:

Monte Carlo simulation has been used to determine the phase diagram of a metamagnet Ising model in the presence of a random and uniform magnetic field. The model consists of a spin-1/2 metamagnet in which the nearest neighbor and next nearest neighbor spin interactions are antiferromagnetic (J1 < 0) and ferromagnetic (J2 > 0), respectively. We used a bimodal probability distribution for the random magnetic field. We have calculated the staggered magnetization and the fourth-order Binder cumulants in order to obtain the critical points. The phase diagram in the uniform field versus temperature plane presents continuous and first-order transition lines. The phase transition lines, together with the critical and tricritical points, have been obtained for several random field values.

Numerical simulation of N-vector spin models in a magnetic field Mendes, Tereza Cucchieri, Attilio

Abstract in English:

Three-dimensional N-vector spin models may define universality classes for such diverse phenomena as i) the superfluid transition in liquid helium (currently investigated in the micro-gravity environment of the Space Shuttle) and ii) the transition from hadronic matter to a quark-gluon plasma, studied in heavy-ion collisions at the laboratories of Brookhaven and CERN. The models have been extensively studied both by field-theoretical and by statistical mechanical methods, including Monte Carlo simulations using cluster algorithms. These algorithms are applicable also in the presence of a magnetic field. Key quantities for the description of the transitions above - such as universal critical amplitude ratios and the location of the so-called pseudo-critical line - can be obtained from the models' magnetic equation of state, which relates magnetization, external magnetic field and temperature. Here we present an improved parametrization for the equation of state of the models, allowing a better fit to the numerical data. Our proposed form is inspired by perturbation theory, with coefficients determined nonperturbatively from fits to the data.

Thermodynamic properties of small magnetic particles Leite, Vanessa Souza Figueiredo, Wagner

Abstract in English:

We investigate the equilibrium magnetic properties of a simple cubic small ferromagnetic particle under an external magnetic field. Although the particle is small, it can not be considered as a single-domain unit. The magnetic moments are represented by unitary spin vectors and we consider ferromagnetic interactions between nearest-neighbor spins. The coupling between spins is given in terms of the classical Heisenberg Hamiltonian and we also include a Zeeman contribution and a single-ion uniaxial anisotropy. The size of the particle changes from one to twelve lattice spacings. We employ in our study mean-filed calculations and Monte Carlo simulations. The magnetization and susceptibility curves as a function of temperature show that an uniaxial anisotropy can mimic a ferromagnetic or an antiferromagnetic coupling, depending on the angle between the external magnetic field and the easy axis.

Dynamic behavior of cluster observables for the 2d Ising model Wanzeller, Wanderson G. Mendes, Tereza Krein, Gastão

Abstract in English:

We present results of our numerical study of the critical dynamics of percolation observables for the two-dimensional Ising model. We consider the (Monte Carlo) short-time evolution of the system with small initial magnetization and heat-bath dynamics. We find qualitatively different dynamic behaviors for the magnetization M and for omega, the so-called strength of the percolating cluster, which is the order parameter of the percolation transition. More precisely, we obtain a (leading) exponential form for omega as a function of the Monte Carlo time t, to be compared with the power-law increase encountered for M at short times. Our results suggest that, although the descriptions in terms of magnetic or percolation order parameters may be equivalent in the equilibrium regime, greater care must be taken to interpret percolation observables at short times.

Critical behavior of the spin-3/2 Blume-Capel model on a random two-dimensional lattice Lima, F. W. S. Plascak, J. A.

Abstract in English:

We investigate the critical properties of the spin-3/2 Blume-Capel model in two dimensions on a random lattice with quenched connectivity disorder. The disordered system is simulated by applying the cluster hybrid Monte Carlo update algorithm and re-weighting techniques. We calculate the critical temperature as well as the critical point exponents gamma/<FONT FACE=Symbol>n, b</FONT>/<FONT FACE=Symbol>n, a</FONT>/nu, and nu. We find that, contrary of what happens to the spin-1/2 case, this random system does not belong to the same universality class as the regular two-dimensional ferromagnetic model.

Critical behavior of ising models with random long-range (small-world) interactions Zhang, X. Novotny, M. A.

Abstract in English:

The critical scaling behavior of Ising models with long range interactions is studied. These long-range interactions, when imposed in addition to interactions on a regular lattice, lead to small world graphs. Large-scale Monte Carlo simulations, together with finite-size scaling, is used to obtain the critical behavior of a number of different models. These include the z-model introduced by Scalettar, standard small-world bonds superimposed on a square lattice, and physical small-world bonds superimposed on a square lattice. These scaling results provide further evidence to support the existence of physical (quasi-) small-world nanomaterials.

Monte Carlo simulations of ultrathin magnetic dots Rapini, M. Dias, R. A. Costa, B. V. Landau, D. P.

Abstract in English:

In this work we study the thermodynamic properties of ultrathin ferromagnetic dots using Monte Carlo simulations. We investigate the vortex density as a function of the temperature and the vortex structure in monolayer dots with perpendicular anisotropy and long-range dipole interaction. The interplay between these two terms in the hamiltonian leads to an interesting behavior of the thermodynamic quantities as well as the vortex density.

Computer simulation of the stochastic dynamics of super-paramagnetic particles in ferrofluids Scherer, Claudio

Abstract in English:

Several papers have been written on the complex problem of the stochastic dynamics of the magnetic moments of super-paramagnetic particles, simultaneously with the stochastic rotation of these colloidal particles in a ferrofluid [1-3]. None of these works, however, is sufficiently general and conveniently simple and clear to be used in sumulational works to appropriately describe the experimental super-paramagntetic resonance lines. We have a new proposal for the equations of rotational motion, which is appropriate for simulations. Those equations are stochastic differential equations with multiplicative noise. Therefore, they have to be interpreted as Stratonovich-Langevin equations and the roles of stochastic calculus have to be used in the simulations. For this reason we will briefly present the essence of the numerical algorithms used in the solutions of Stratonovich equations. Finally, the simulational results for the magnetic response functions, and the corresponding dynamic susceptibilities, will be shown and their consequences will be analyzed.

Two-dimensional Wigner crystal on helium films: an indication of quantum melting Cunha, J. A. R. da Cândido, Ladir

Abstract in English:

Using molecular dynamics simulation (MD) we have investigated the melting of the two-dimensional Wigner crystal on 240Å-500Å liquid helium films supported by substrates of dielectric constants epsilons = 2.2-7.3. Our results show good agreement with available theoretical and experimental results for densities below 1.0×10(10)cm-2. For higher densities, we notice some disagreements suggesting that quantum effects are important in this regime of densities.

Quasi-stationary simulation: the subcritical contact process Oliveira, Marcelo Martins de Dickman, Ronald

Abstract in English:

We apply the recently devised quasi-stationary simulation method to study the lifetime and order parameter of the contact process in the subcritical phase. This phase is not accessible to other methods because virtually all realizations of the process fall into the absorbing state before the quasi-stationary regime is attained. With relatively modest simulations, the method yields an estimate of the critical exponent nu|| with a precision of 0.5%.

Determination of prediction intervals for a future number of failures: a statistical and Monte Carlo approach Menezes, Fortunato S. de Vivanco, Mário J. Ferrua Sampaio, Luiz C.

Abstract in English:

In this work, we present a new procedure, called sub-sampling, to obtain data concerning time of failure in trials without replacement, (NRT). With this data it is possible to determine the prediction interval (PI) for the future number of failures. We also present an alternative way to evaluate the coverage probability of the prediction interval (PI). The results presented show that the method proposed is reliable and can be useful for the statistical analyses of quality control of processes.

Scaling properties of the Fermi-Ulam accelerator model Silva, Jafferson Kamphorst Leal da Ladeira, Denis Gouvêa Leonel, Edson D. McClintock, P. V. E. Kamphorst, Sylvie O.

Abstract in English:

The chaotic low energy region (chaotic sea) of the Fermi-Ulam accelerator model is discussed within a scaling framework near the integrable to non-integrable transition. Scaling results for the average quantities (velocity, roughness, energy etc.) of the simplified version of the model are reviewed and it is shown that, for small oscillation amplitude of the moving wall, they can be described by scaling functions with the same characteristic exponents. New numerical results for the complete model are presented. The chaotic sea is also characterized by its Lyapunov exponents.

Two-dimensional critical Potts and its tricritical shadow Janke, Wolfhard Schakel, Adriaan M. J.

Abstract in English:

These notes give examples of how suitably defined geometrical objects encode in their fractal structure thermal critical behavior. The emphasis is on the two-dimensional Potts model for which two types of spin clusters can be defined. Whereas the Fortuin-Kasteleyn clusters describe the standard critical behavior, the geometrical clusters describe the tricritical behavior that arises when including vacant sites in the pure Potts model. Other phase transitions that allow for a geometrical description discussed in these notes include the superfluid phase transition and Bose-Einstein condensation.

Ground state energy of charged particles clusters by quantum Monte Carlo method Moreira, N. L. Rabelo, J. N. Teixeira Cândido, L.

Abstract in English:

Using variational and diffusion Monte Carlo methods, we have calculated the ground state energy of spinless charged particles (for N < 10) interacting through a repulsive Coulomb potential, moving in two-dimensions and kept together by an external parabolic potential. Using a very simple trial wave function, we obtain results comparable to those of a sophisticated model of a quantum dot.

MD study of energetics, melting and isomerization of aluminum microclusters Böyükata, Mustafa Güvenç, Ziya B.

Abstract in English:

Voter and Chen version of an Embedded Atom Model has been applied to study the locally stable structures, energies, melting, isomerization and growth patterns of small aluminum clusters, Al n, in the size range of n = 2 - 13. Using molecular dynamics and thermal quenching simulations, the global minima and the other locally stable structures have been distinguished from those stationary structures that correspond to saddle points of the potential energy surface. A large number (10000) of independent initial configurations generated at high temperatures has been used to obtain the stable isomers, and the probabilities of sampling different basins of attractions, for each size of the clusters. Their energy spectra have been determined and melting, and isomerization dynamics are investigated.

Effects of molecular rovibrational states and surface topologies for molecule-surface interaction: chemisorption dynamics of D2 collision with rigid Ni surfaces Böyükata, Mustafa Güvenç, Ziya B.

Abstract in English:

A quasiclassical and micro-canonical molecular dynamic simulation techniques have been applied for D2(v, j) + Ni-surface collision systems. Dissociative adsorptions of a D2 molecule on the rigid low index (100), (110) and (111), surfaces of the nickel are investigated to understand the effects of the different surfaces, impact sites and the initial rovibrational states of the molecule on molecule-surface collisions. Interactions between the molecule and the Ni surfaces are mimicked by a LEPS potential. Dissociative chemisorption probabilities of the D2(v, j) Molecule ( for the vibrational (v) = 0 and rotational (j) = 0,1, 3, 10, and for the v = 1, j = 0 states on different impact sites of the surfaces) are presented for the translation energies between 0.001 and 1.0 eV. The probabilities obtained at each collision site have unique behavior for the colliding molecule which is moving along the surface normal direction. It has been observed that at the low collision energies the indirect processes (steering effects) enhance the reactivity on the surfaces. The results are compared to the related studies in the literature.

Estimation of cross sections for molecule-cluster interactions by using artificial neural networks Böyükata, Mustafa Koçyigit, Yücel Güvenç, Ziya B.

Abstract in English:

The cross sections of D2(v,j)+Ni n(T), n = 19 and 20, collision systems have been estimated by using Artificial Neural Networks (ANNs). For training, previously determined cross section values via molecular dynamics simulation have been used. The performance of the ANNs for predicting any quantities in molecule-cluster interaction has been investigated. Effects of the temperature of the clusters and the rovibrational states of the molecule are analyzed. The results are in good agreement with previous studies.

Lattice gas with nearest-neighbor exclusion in a shear-like field Potiguar, Fabricio Q. Dickman, Ronald

Abstract in English:

We present Monte Carlo simulations of the lattice gas with nearest-neighbor exclusion and Kawasaki (hopping) dynamics (hard square lattice gas), under the influence of a nonuniform drive, on the square lattice. The drive, which favors motion along the +x axis and inhibits motion in the opposite direction, varies linearly in the y direction. Our lattice has rigid walls at the end points in the y direction and periodic boundaries along the drive. We find that this model has transition to a sublattice-ordered phase at a density of about 0.298, lower than in equilibrium (rhoc <FONT FACE=Symbol>@</FONT> 0.37), but somewhat higher than in the uniformly driven case at maximal bias (rhoc <FONT FACE=Symbol>@</FONT> 0.272). For smaller global densities (r < 0.33), the ordering occurs with particle accumulation in the low-drive region. Above this density we observe a surprising reversal in the density profile, with particles migrating to the high-drive region.

Temperature dependent molecular dynamic simulation of friction Dias, R. A. Rapini, M. Costa, B. V. Coura, P. Z.

Abstract in English:

In this work we present a molecular dynamics simulation of a FFM experiment. The tip-sample interaction is studied by varying the normal force in the tip and the temperature of the surface. The friction force, cA, at zero load and the friction coefficient, m, were obtained. Our results strongly support the idea that the effective contact area, A, decreases with increasing temperature and the friction coefficient presents a clear signature of the premelting process of the surface.

A new method to study stochastic growth equations: application to the Edwards-Wilkinson equation Mattos, T. G. Moreira, J. G. Atman, A. P. F.

Abstract in English:

In this work we introduce a method to study stochastic growth equations, which follows a dynamics based on cellular automata modeling. The method defines an interface growth process that depends on height differences between neighbors. The growth rules assign a probability p i(t) for site i to receive a particle at time t, where p i(t) = rho exp[<FONT FACE=Symbol>kG</FONT>i(t)]. Here r and k are two parameters and gammai(t) is a kernel that depends on height h i(t) of site i and on heights of its neighbors, at time t. We specify the functional form of this kernel by the discretization of the deterministic part of the equation that describes some growth process. In particular, we study the Edwards-Wilkinson (EW) equation which describes growth processes where surface relaxation plays a major role. In this case we have a Laplacian term dominating in the growth equation and gammai(t) = h i+1(t)+h i-1(t)-2h i(t), which follows from the discretization of <FONT FACE=Symbol>Ñ</FONT>2h. By means of simulations and statistical analysis of the height distributions of the profiles, we obtain the roughening exponents, beta, alpha and z, whose values confirm that the processes defined by the method are indeed in the universality class of the original growth equation.

Generalized Manna sandpile model with height restrictions Dantas, Wellington Gomes Stilck, Jürgen F.

Abstract in English:

Sandpile models with conserved number of particles (also called fixed energy sandpiles) may undergo phase transitions between active and absorbing states. We generalize the Manna sandpile model with fixed number of particles, introducing a parameter -1 < lambda < 1 related to the toppling of particles from active sites to its first neighbors. In particular, we discuss a model with height restrictions, allowing for at most two particles on a site. Sites with double occupancy are active, and their particles may be transfered to first neighbor sites, if the height restriction do allow the change. For lambda = 0 each one of the two particles is independently assigned to one of the two first neighbors and the original stochastic sandpile model is recovered. For lambda = 1 exactly one particle will be placed on each first neighbor and thus a deterministic (BTW) sandpile model is obtained. When lambda = -1 two particles are moved to one of the first neighbors, and this implies that the density of active sites is conserved in the evolution of the system, and no phase transition is observed. Through simulations of the stationary state, we estimate the critical density of particles and the critical exponents as functions of lambda.
Regular Articles
A scale-free network of evoked words Ferreira, A. A. A. Corso, G. Piuvezam, G. Alves, M. S. C. F.

Abstract in English:

We use a set of evoked words to define the vertices of a network. The connections between vertices are established by individuals in a population that evoke these words. The resulting graph is called an Evoked Words Network, EWN. The data of evoked words comes from an epidemiological research in odontological public health. In this research we consider three concept themes or evocative words: mouth, disease, and health. We investigate these words in two populations: an upper middle class and a poor district of the city of Natal. We compare and analyze six EWNs. The distribution of connectivities of all of these EWNs indicates a scale-free structure, with the data fitting a power law. The analyzed quantities of the EWNs depend more on the concept theme than on the income of population. This conclusion is discussed in the context of language-based networks.
Regular Articles
Electrostatic waves in a Maxwellian dusty plasma with variable charge on dust particles Schneider, R. S. Ziebell, L. F. Juli, M. C. de Jatenco-Pereira, V.

Abstract in English:

A kinetic formulation developed to analyze wave propagation in dusty plasmas, which takes into account the variation of the charge of the dust particles due to inelastic collisions with electrons and ions, is utilized to study the propagation and damping of electrostatic waves with wave number exactly parallel to the external magnetic field and Maxwellian distributions for the electrons and ions in the equilibrium. It is shown that, due to the presence of the dust, the damping of Langmuir waves in the region of large wavelengths is increased as compared to conventional Landau damping. Langmuir waves in the occurrence of collisional charging of dust particles also feature weak damping for small wavelengths, which vanishes if the effect of collisional charging of the dust particles is neglected in the dispersion relation. It is also shown that the damping of ion-acoustic waves is modified by the presence of the dust, and that some damping effect due to the dust particles remains even if the effect of collisional charging of dust particles is neglected in the dispersion relation.
Regular Articles
Stability in a class of quartessence models Velásquez-Toribio, A. M.

Abstract in English:

Quartessence cosmological models with exponential and logarithmic equation of state are investigated using dynamical systems methods. We focus our analysis on the stability of these models.
Regular Articles
Analysis of the relativistic brownian motion in momentum space Fa, Kwok Sau

Abstract in English:

We investigate the relativistic Brownian motion in the context of Fokker-Planck equation. Due to the multiplicative noise term of the corresponding relativistic Langevin equation many Fokker-Planck equations can be generated. Here, we only consider the Ito, Stratonovich and Hänggi-Klimontovich approaches. We analyze the behaviors of the second moment of momentum in terms of temperature. We show that the second moment increases with the temperature T for all three approaches. Also, we present differential equations for more complicated averages of the momentum. In a specific case, in the Ito approach, we can obtain an analytical solution of the temporal evolution of an average of the momentum. We present approximate solutions for the probability density for all three cases.
Regular Articles
Study of confinement improvement in presence of a MARFE in the HT-7 superconducting tokamak Asif, M.

Abstract in English:

A new set of actively cooled toroidal double-ring graphite limiters have been developed on the HT-7 tokamk for long pulse operation. In this paper, a Multifaceted asymmetric radiation from the edge (MARFE) phenomena and improved particle confinement induced by a MARFE, characterized by Halpha line emissions drops and the line-averaged density increase have been studied in the HT-7 ohmic discharges (where the plasma current Ip=145 kA, loop voltage Vloop =2-3 V, toroidal field B T =1.7 T, and Zeff =2-8). It is found that the improved particle confinement phase exists for about 90 ms and the particle confinement time tP increased about 1.6 times.
Regular Articles
Design and performance of a matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometer Santacruz, C. P. Ayala, E. Costa-Vera, C.

Abstract in English:

A matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometer has been built with the capabilities to perform structural analyzes. The first time-of-flight mass spectrometer includes a timing ion gate to select the ions to be analyzed. After the selection process, the ions are activated using an electrically floating collision cell. The fragments produced in the collision cell are analyzed in the second time-of-flight stage. A two-step mass calibration procedure is introduced that allows one to obtain accurate mass values. The characteristics of this calibration procedure make possible to automate it. The experimental results of the tandem mass spectrometer and calibration procedure using different standard peptides are presented.
Regular Articles
Universal curve of ionic conductivities in binary alkali tellurite glasses Nascimento, Marcio Luis Ferreira Watanabe, Shigueo

Abstract in English:

The main objective of this work is to present an analysis and brief discussion of experimental ionic conductivity s data in the binary alkali tellurite system, including on 47 glasses that extend the ionic conductivity range by more than 14 orders of magnitude in a wide compositional range. A 'universal' behavior is obtained, using log sigma or log sigmaT vs. E A/kB T, where E A is the activation enthalpy for conduction, kB is the Boltzmann constant and T is the absolute temperature. This finding further indicates the importance of a scaling factor F recently proposed, that is correlated to the free volume of glass composition. For a given value of E A/kB T, the difference between large and small values of sigma is only one order of magnitude in 87% of these glass systems. The influence of alkali content and temperature was minor on the pre-exponential terms, considering both expressions log10sigma and log10sigmaT. Indeed, the pre-exponential term sigma0 varies around an average value of 50 omega-1cm-1 considering different compositions in this system. The fact that s lies on these single 'universal' curves for so many ion-conducting binary tellurite glasses means that sigma is governed mainly by E A. The composition dependence of the activation enthalpy is explained in the context of the Anderson-Stuart theory.
Review Article
Density functional theory method for non-equilibrium charge transport calculations: TRANSAMPA Novaes, Frederico D. Silva, Antônio J. R. da Fazzio, A.

Abstract in English:

We describe a procedure to calculate charge transport properties across a nanosystem. This scheme is based on a Green's Function formalism to treat a non-equilibrium problem, coupled to the Density Functional Theory to describe the electronic structure. As an illustration, we perform calculations for the charge transport across a (5,5) carbon nanotube with a vacancy.
location_on
Sociedade Brasileira de Física Caixa Postal 66328, 05315-970 São Paulo SP - Brazil, Tel.: +55 11 3091-6922, Fax: (55 11) 3816-2063 - São Paulo - SP - Brazil
E-mail: sbfisica@sbfisica.org.br
rss_feed Stay informed of issues for this journal through your RSS reader
Acessibilidade / Reportar erro