arXiv subjects

A. A. Moreira

Publications and source records attributed to A. A. Moreira.

32 records · Page 2Linked to original sources

arXiv2011

Reply to the comment on: "Thermostatistics of Overdamped Motion of Interacting Particles" [arXiv:1104.0697] by Y. Levin and R. Pakter

We show that the comment [arXiv:1104.0697] by Levin and Pakter on our work [arXiv:1008.1421] is conceptually unfounded, contains misleading interpretations, and is based on results of questionable applicability. We initially provide arguments to evince that, inexplicably, these authors simply choose to categorically dismiss our elaborated and solid conceptual approach, results and analysis, without employing any fundamental concepts or tools from Statistical Physics. We then demonstrate that the results of Levin and Pakter do not present any evidence against, but rather corroborates, our conclusions. In fact, the results shown in their comment correspond to a confining potential that is 1000 times stronger than the typical valued utilized in our study, therefore explaining the discrepancy between their results and ours. Furthermore, in this regime where higher vortex densities are involved, vortex cores might get so close to each other that can no longer be treated as point-like defects. As a consequence, Ginzburg-Landau equations should be employed instead, meaning that the physical conditions implied by the results of Levin and Pakter should be considered with caution in the context of the Physics of interacting superconducting vortexes.

cond-mat.stat-mech↗

arXiv2011

Thermostatistics of overdamped motion of interacting particles

We show through a nonlinear Fokker-Planck formalism, and confirm by molecular dynamics simulations, that the overdamped motion of interacting particles at T=0, where T is the temperature of a thermal bath connected to the system, can be directly associated with Tsallis thermostatistics. For sufficiently high values of T, the distribution of particles becomes Gaussian, so that the classical Boltzmann-Gibbs behavior is recovered. For intermediate temperatures of the thermal bath, the system displays a mixed behavior that follows a novel type of thermostatistics, where the entropy is given by a linear combination of Tsallis and Boltzmann-Gibbs entropies.

cond-mat.stat-mech↗

arXiv2009

A Hamiltonian approach for explosive percolation

We introduce a cluster growth process that provides a clear connection between equilibrium statistical mechanics and an explosive percolation model similar to the one recently proposed by Achlioptas et al. [Science 323, 1453 (2009)]. We show that the following two ingredients are essential for obtaining an abrupt (first-order) transition in the fraction of the system occupied by the largest cluster: (i) the size of all growing clusters should be kept approximately the same, and (ii) the inclusion of merging bonds (i.e., bonds connecting vertices in different clusters) should dominate with respect to the redundant bonds (i.e., bonds connecting vertices in the same cluster). Moreover, in the extreme limit where only merging bonds are present, a complete enumeration scheme based on tree-like graphs can be used to obtain an exact solution of our model that displays a first-order transition. Finally, the proposed mechanism can be viewed as a generalization of standard percolation that discloses an entirely new family of models with potential application in growth and fragmentation processes of real network systems.

cond-mat.stat-mech↗

arXiv2009

Fracturing the optimal paths

Optimal paths play a fundamental role in numerous physical applications ranging from random polymers to brittle fracture, from the flow through porous media to information propagation. Here for the first time we explore the path that is activated once this optimal path fails and what happens when this new path also fails and so on, until the system is completely disconnected. In fact numerous applications can be found for this novel fracture problem. In the limit of strong disorder, our results show that all the cracks are located on a single self-similar connected line of fractal dimension $D_{b} \approx 1.22$. For weak disorder, the number of cracks spreads all over the entire network before global connectivity is lost. Strikingly, the disconnecting path (backbone) is, however, completely independent on the disorder.

cond-mat.dis-nn↗

arXiv2009

Micro-bias and macro-performance

We use agent-based modeling to investigate the effect of conservatism and partisanship on the efficiency with which large populations solve the density classification task--a paradigmatic problem for information aggregation and consensus building. We find that conservative agents enhance the populations' ability to efficiently solve the density classification task despite large levels of noise in the system. In contrast, we find that the presence of even a small fraction of partisans holding the minority position will result in deadlock or a consensus on an incorrect answer. Our results provide a possible explanation for the emergence of conservatism and suggest that even low levels of partisanship can lead to significant social costs.

physics.soc-ph↗

arXiv2009

Designing optimal transport networks

We investigate the optimal design of networks for a general transport system. Our network is built from a regular two-dimensional ($d=2$) square lattice to be improved by adding long-range connections (shortcuts) with probability $P_{ij} \sim r_{ij}^{-α}$, where $r_{ij}$ is the Euclidean distance between sites $i$ and $j$, and $α$ is a variable exponent. We introduce a cost constraint on the total length of the additional links and find optimal transport in the system for $α=d+1$. Remarkably, this condition remains optimal, regardless of the strategy used for navigation, being based on local or global knowledge of the network structure, in sharp contrast with the results obtained for unconstrained navigation using global or local information, where the optimal conditions are $α=0$ and $α=d$, respectively. The validity of our theoretical results is supported by data on the US airport network, for which $α\approx 3.0$ was recently found [Bianconi {\it et al.}, arXiv:0810.4412 (2008)].

physics.soc-ph↗

arXiv2007

Different topologies for a herding model of opinion

Understanding how opinions spread through a community or how consensus emerges in noisy environments can have a significant impact on our comprehension of social relations among individuals. In this work a model for the dynamics of opinion formation is introduced. The model is based on a nonlinear interaction between opinion vectors of agents plus a stochastic variable to account for the effect of noise in the way the agents communicate. The dynamics presented is able to generate rich dynamical patterns of interacting groups or clusters of agents with the same opinion without a leader or centralized control. Our results show that by increasing the intensity of noise, the system goes from consensus to a disordered state. Depending on the number of competing opinions and the details of the network of interactions, the system displays a first- or a second-order transition. We compare the behavior of different topologies of interactions: one-dimensional chains, and annealed and complex networks.

physics.soc-ph↗

arXiv2005

A Fractal Space-filling Complex Network

We study in this work the properties of the $Q_{mf}$ network which is constructed from an anisotropic partition of the square, the multifractal tiling. This tiling is build using a single parameter $ρ$, in the limit of $ρ\to 1$ the tiling degenerates into the square lattice that is associated with a regular network. The $Q_{mf}$ network is a space-filling network with the following characteristics: it shows a power-law distribution of connectivity for $k>7$ and it has an high clustering coefficient when compared with a random network associated. In addition the $Q_{mf}$ network satisfy the relation $N \propto \ell^{d_f}$ where $\ell$ is a typical length of the network (the average minimal distance) and $N$ the network size. We call $d_f$ the fractal dimension of the network. In tne limit case $ρ\to 1$ we have $d_{f} \to 2$.

cond-mat.stat-mech↗

arXiv2005

Invasion Percolation Between two Sites

We investigate the process of invasion percolation between two sites (injection and extraction sites) separated by a distance r in two-dimensional lattices of size L. Our results for the non-trapping invasion percolation model indicate that the statistics of the mass of invaded clusters is significantly dependent on the local occupation probability (pressure) Pe at the extraction site. For Pe=0, we show that the mass distribution of invaded clusters P(M) follows a power-law P(M) ~ M^{-α} for intermediate values of the mass M, with an exponent α=1.39. When the local pressure is set to Pe=Pc, where Pc corresponds to the site percolation threshold of the lattice topology, the distribution P(M) still displays a scaling region, but with an exponent α=1.02. This last behavior is consistent with previous results for the cluster statistics in standard percolation. In spite of these discrepancies, the results of our simulations indicate that the fractal dimension of the invaded cluster does not depends significantly on the local pressure Pe and it is consistent with the fractal dimension values reported for standard invasion percolation. Finally, we perform extensive numerical simulations to determine the effect of the lattice borders on the statistics of the invaded clusters and also to characterize the self-organized critical behavior of the invasion percolation process.

cond-mat.dis-nn↗

arXiv2005

The Complex Topology of Chemical Plants

We show that flowsheets of oil refineries can be associated to complex network topologies that are scale-free, display small-world effect and possess hierarchical organization. The emergence of these properties from such man-made networks is explained as a consequence of the currently used principles for process design, which include heuristics as well as algorithmic techniques. We expect these results to be valid for chemical plants of different types and capacities.

cond-mat.dis-nn↗

arXiv2005

Mesoscopic modeling for nucleic acid chain dynamics

To gain a deeper insight into cellular processes such as transcription and translation, one needs to uncover the mechanisms controlling the configurational changes of nucleic acids. As a step toward this aim, we present here a novel mesoscopic-level computational model that provides a new window into nucleic acid dynamics. We model a single-stranded nucleic as a polymer chain whose monomers are the nucleosides. Each monomer comprises a bead representing the sugar molecule and a pin representing the base. The bead-pin complex can rotate about the backbone of the chain. We consider pairwise stacking and hydrogen-bonding interactions. We use a modified Monte Carlo dynamics that splits the dynamics into translational bead motion and rotational pin motion. By performing a number of tests we first show that our model is physically sound. We then focus on the study of a the kinetics of a DNA hairpin--a single-stranded molecule comprising two complementary segments joined by a non-complementary loop--studied experimentally. We find that results from our simulations agree with experimental observations, demonstrating that our model is a suitable tool for the investigation of the hybridization of single strands.

q-bio.BM↗

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↗

arXiv2002

Traveling length and minimal traveling time for flow through percolation networks with long-range spatial correlations

We study the distributions of traveling length l and minimal traveling time t through two-dimensional percolation porous media characterized by long-range spatial correlations. We model the dynamics of fluid displacement by the convective movement of tracer particles driven by a pressure difference between two fixed sites (''wells'') separated by Euclidean distance r. For strongly correlated pore networks at criticality, we find that the probability distribution functions P(l) and P(t) follow the same scaling Ansatz originally proposed for the uncorrelated case, but with quite different scaling exponents. We relate these changes in dynamical behavior to the main morphological difference between correlated and uncorrelated clusters, namely, the compactness of their backbones. Our simulations reveal that the dynamical scaling exponents for correlated geometries take values intermediate between the uncorrelated and homogeneous limiting cases.

cond-mat.dis-nn↗

arXiv2001

An extended-phase-space dynamics for the generalized nonextensive thermostatistics

We apply a variant of the Nose-Hoover thermostat to derive the Hamiltonian of a nonextensive system that is compatible with the canonical ensemble of the generalized thermostatistics of Tsallis. This microdynamical approach provides a deterministic connection between the generalized nonextensive entropy and power law behavior. For the case of a simple one-dimensional harmonic oscillator, we confirm by numerical simulation of the dynamics that the distribution of energy H follows precisely the canonical q-statistics for different values of the parameter q. The approach is further tested for classical many-particle systems by means of molecular dynamics simulations. The results indicate that the intrinsic nonlinear features of the nonextensive formalism are capable to generate energy fluctuations that obey anomalous probability laws. For q<1 a broad distribution of energy is observed, while for q>1 the resulting distribution is confined to a compact support.

cond-mat.stat-mech↗