arXiv subjects

R. N. Costa Filho

Publications and source records attributed to R. N. Costa Filho.

At least 73 records · Page 4Linked to original sources

arXiv2007

Model for erosion-deposition patterns

We investigate through computational simulations with a pore network model the formation of patterns caused by erosion-deposition mechanisms. In this model, the geometry of the pore space changes dynamically as a consequence of the coupling between the fluid flow and the movement of particles due to local drag forces. Our results for this irreversible process show that the model is capable to reproduce typical natural patterns caused by well known erosion processes. Moreover, we observe that, within a certain range of porosity values, the grains form clusters that are tilted with respect to the horizontal with a characteristic angle. We compare our results to recent experiments for granular material in flowing water and show that they present a satisfactory agreement.

physics.comp-ph↗

arXiv2007

Transport properties of a 2DEG in the presence of a tilted magnetic field

The effect of a tilted dc magnetic field on the transport properties of a two dimensional electron gas (2DEG) is studied. The influence of the component of magnetic field parallel to the surface is analyzed and the dependence of the Fermi energy on the intensity and direction of the external field is discussed. Numerical results are obtained for the Hall conductivity considering values of electronic densities and strength of the dc magnetic field that are currently obtained in many laboratories

cond-mat.mes-hall↗

arXiv2006

Damage and Healing in Fatigue Fracture

We present an experimental and theoretical study of the fatigue failure of asphalt under cyclic compression. Varying the load amplitude, experiments reveal a finite fatigue limit below which the specimen does not break, while approaching the tensile strength of the material a rapid failure occurs. In the intermediate load range, the lifetime decreases with the load as a power law. We introduce two novel theoretical approaches, namely, a fiber bundle model and a fuse model, and show that both capture the major microscopic mechanisms of the fatigue failure of asphalt, providing an excellent agreement with the experimental findings. Model calculations show that the competition of damage accumulation and healing of microcracks gives rise to novel scaling laws for fatigue failure.

cond-mat.mtrl-sci↗

arXiv2005

Critical behavior of the 3D-Ising model on a poissonian random lattice

The single-cluster Monte Carlo algorithm and the reweighting technique are used to simulate the 3D-ferromagnetic Ising model on three dimensional Voronoi-Delaunay lattices. It is assumed that the coupling factor $J$ varies with the distance $r$ between the first neighbors as $J(r) \propto e^{-ar}$, with $a \ge 0$. The critical exponents $γ/ν$, $β/ν$, and $ν$ are calculated, and according to the present estimates for the critical exponents, we argue that this random system belongs to the same universality class of the pure three-dimensional ferromegnetic Ising model.

cond-mat.dis-nn↗

arXiv2005

Dipole-exchange spin waves in Fibonacci magnetic multilayers

A microscopic model is employed to calculate the spectrum of spin waves in quasiperiodic magnetic multilayers in the dipole-exchange regime. Results are presented for structures in which thin ferromagnetic films are separated by non-magnetic spacers following a Fibonacci sequence and extend previous magnetostatic calculations. The results show the splitting of the frequency bands and the mode mixing caused by the dipolar interaction between the films as a function of spacer thickness, as well as the fractal aspect of the spectrum induced by the non-periodic aspect of the structure.

cond-mat.mes-hall↗

arXiv2005

Pattern Selection in a Phase Field Model for Directional Solidification

A symmetric phase field model is used to study wavelength selection in two dimensions. We study the problem in a finite system using a two-pronged approach. First we construct an action and, minimizing this, we obtain the most probable configuration of the system, which we identify with the selected stationary state. The minimization is constrained by the stationary solutions of stochastic evolution equations and is done numerically. Secondly, additional support for this selected state is obtained from straightforward simulations of the dynamics from a variety of initial states.

cond-mat.stat-mech↗

arXiv2004

Nanopercolation

We investigate through direct molecular mechanics calculations the geometrical properties of hydrocarbon mantles subjected to percolation disorder. We show that the structures of mantles generated at the critical percolation point have a fractal dimension $d_{f} \approx 2.5$. In addition, the solvent access surface $A_{s}$ and volume $V_{s}$ of these molecules follow power-law behavior, $A_{s} \sim L^{α_A}$ and $V_{s} \sim L^{α_V}$, where $L$ is the system size, and with both critical exponents $α_A$ and $α_V$ being significantly dependent on the radius of the accessing probing molecule, $r_{p}$. Our results from extensive simulations with two distinct microscopic topologies (i.e., square and honeycomb) indicate the consistency of the statistical analysis and confirm the self-similar characteristic of the percolating hydrocarbons. Due to their highly branched topology, some of the potential applications for this new class of disordered molecules include drug delivery, catalysis, and supramolecular structures.

cond-mat.dis-nn↗

arXiv2004

Localised magnetic excitations of coupled impurities in a transverse Ising ferromagnet

A Green's function formalism is used to calculate the spectrum of excitations of two neighboring impurities implanted in a semi-infinite ferromagnetic. The equations of motion for the Green's functions are determined in the framework of the Ising model in a transverse field and results are given for the effect of the exchange coupling, position and orientation of the impurities on the spectra of localized spin wave modes.

cond-mat.mtrl-sci↗

arXiv2004

Majority-Vote Model on a Random Lattice

The stationary critical properties of the isotropic majority vote model on random lattices with quenched connectivity disorder are calculated by using Monte Carlo simulations and finite size analysis. The critical exponents $γ$ and $β$ are found to be different from those of the Ising and majority vote on the square lattice model and the critical noise parameter is found to be $q_{c}=0.117\pm0.005$.

cond-mat.stat-mech↗

arXiv2004

Memory effects on the statistics of fragmentation

We investigate through extensive molecular dynamics simulations the fragmentation process of two-dimensional Lennard-Jones systems. After thermalization, the fragmentation is initiated by a sudden increment to the radial component of the particles' velocities. We study the effect of temperature of the thermalized system as well as the influence of the impact energy of the ``explosion'' event on the statistics of mass fragments. Our results indicate that the cumulative distribution of fragments follows the scaling ansatz $F(m)\propto m^{-α}\exp{[-(m/m_0)^γ]}$, where $m$ is the mass, $m_0$ and $γ$ are cutoff parameters, and $α$ is a scaling exponent that is dependent on the temperature. More precisely, we show clear evidence that there is a characteristic scaling exponent $α$ for each macroscopic phase of the thermalized system, i.e., that the non-universal behavior of the fragmentation process is dictated by the state of the system before it breaks down.

cond-mat.stat-mech↗

arXiv2004

Green's function theory for a magnetic impurity layer in a ferromagnetic Ising film with transverse field

A Green's function formalism is used to calculate the spectrum of localized modes of an impurity layer implanted within a ferromagnetic thin film. The equations of motion for the Green's functions are determined in the framework of the Ising model in a transverse field. We show that depending on the thickness, exchange and effective field parameters, there is a ``crossover'' effect between the surface modes and impurity localized modes. For thicker films the results show that the degeneracy of the surface modes can be lifted by the presence of an impurity layer.

cond-mat.dis-nn↗

arXiv2003

Surface Plasmons on a Quasiperiodic Grating

A method is presented for calculating the frequencies of non-retarded surface plasmons propagating on a semi-inifinite medium with a surface profile described by a one-dimension quasiperiodic function. The profiles are generated, in analogy with previous work on quasiperiodic superlattices, by repeating unitary cells constructed according to an inflation rule. Dispersion relations are obtained for a semi-infinite free-electron metal as the active medium, with surface profiles obeying the Fibonacci and Thue-Morse sequences.

cond-mat↗

arXiv2002

Generalized Zipf's Law in proportional voting processes

Voting data from city-councillors, state and federal deputies elections are analyzed and considered as a response function of a social system with underlying dynamics leading to complex behavior. The voting results from the last two general Brazilian elections held in 1998 and 2000 are then used as representative data sets. We show that the voting distributions follow a generalized Zipf's Law which has been recently proposed within a nonextensive statistics framework. Moreover, the voting distribution for city-councillors is clearly distinct from those of state and federal deputies in the sense that the latter depicts a higher degree of nonextensivity. We relate this finding with the different degrees of complexity corresponding to local and non-local voting processes.

cond-mat.stat-mech↗

arXiv2002

Brazilian elections: voting for a scaling democracy

The proportional elections held in Brazil in 1998 and 2002 display identical statistical signatures. In particular, the distribution of votes among candidates includes a power-law regimen. We suggest that the rationale behind this robust scaling invariance is a multiplicative process in which the voter's choice for a candidate is governed by a product of probabilities.

cond-mat.stat-mech↗

arXiv2002

Persistence in the Zero-Temperature Dynamics of the Random Ising Ferromagnet on a Voronoi-Delaunay lattice

The zero-temperature Glauber dynamic is used to investigate the persistence probability $P(t)$ in the randomic two-dimensional ferromagnetic Ising model on a Voronoi-Delaunay tessellation. We consider the coupling factor $J$ varying with the distance $r$ between the first neighbors to be $J(r) \propto e^{-αr}$, with $α\ge 0$. The persistence probability of spins flip, that does not depends on time $t$, is found to decay to a non-zero value $P(\infty)$ depending on the parameter $α$. Nevertheless, the quantity $p(t)=P(t)-P(\infty)$ decays exponentially to zero over long times. Furthermore, the fraction of spins that do not change at a time $t$ is a monotonically increasing function of the parameter $α$. Our results are consistent with the ones obtained for the diluted ferromagnetic Ising model on a square lattice.

cond-mat.stat-mech↗

arXiv2002

Statistics of the critical percolation backbone with spatial long-range correlations

We study the statistics of the backbone cluster between two sites separated by distance $r$ in two-dimensional percolation networks subjected to spatial long-range correlations. We find that the distribution of backbone mass follows the scaling {\it ansatz}, $P(M_B)\sim M_B^{-(α+1)}f(M_B/M_0)$, where $f(x)=(α+ ηx^η) \exp(-x^η)$ is a cutoff function, and $M_0$ and $η$ are cutoff parameters. Our results from extensive computational simulations indicate that this scaling form is applicable to both correlated and uncorrelated cases. We show that the exponent $α$ can be directly related to the fractal dimension of the backbone $d_B$, and should therefore depend on the imposed degree of long-range correlations.

cond-mat.stat-mech↗

arXiv2001

Polaron effects in electron channels on a helium film

Using the Feynman path-integral formalism we study the polaron effects in quantum wires above a liquid helium film. The electron interacts with two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one dimension (1D) by an harmonic potential. The obtained results are valid for arbitrary temperature ($T$), electron-phonon coupling strength ($α$), and lateral confinement ($ω_{0}$). Analytical and numerical results are obtained for limiting cases of $T$, $α$, and $ω_{0}$. We found the surprising result that reducing the electron motion from 2D to quasi-1D makes the self-trapping transition more continuous.

cond-mat.str-el↗

arXiv2000

Microscopic Theory of Damon-Eshbach Modes in Ferromagnetic Films

The surface spin wave branches in ferromagnetic films are studied using a microscopic theory which considers both magnetic dipole-dipole and Heisenberg exchange interactions. The dipole terms are expressed in a Hamiltonian formalism, and the dipole sums are calculated in a rapidly convergent form. The Damon-Eshbach surface modes are analyzed for different directions of the spin-wave propagation and also for different ratios of the strength of the dipole interactions relative to the exchange interactions. Numerical results are presented using parameters for Fe and GdCl$_3$.

cond-mat.str-el↗