Strategies for Optimize Off-Lattice Aggregate Simulations

We review some computer algorithms for the simulation of off-lattice clusters grown from a seed, with emphasis on the diffusion-limited aggregation, ballistic aggregation and Eden models. Only those methods which can be immediately extended to distinct off-lattice aggregation processes are discussed. The computer efficiencies of the distinct algorithms are compared.Comment: 6 pages, 7 figures and 3 tables; published at Brazilian Journal of Physics 38, march, 2008 (http://www.sbfisica.org.br/bjp/files/v38_81.pdf

Contact process on a Voronoi triangulation

We study the continuous absorbing-state phase transition in the contact process on the Voronoi-Delaunay lattice. The Voronoi construction is a natural way to introduce quenched coordination disorder in lattice models. We simulate the disordered system using the quasistationary simulation method and determine its critical exponents and moment ratios. Our results suggest that the critical behavior of the disordered system is unchanged with respect to that on a regular lattice, i.e., that of directed percolation

Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height

The formation of mounded surfaces in epitaxial growth is attributed to the presence of barriers against interlayer diffusion in the terrace edges, known as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth using a ES barrier explicitly dependent on the step height. Our model has an intrinsic topological step barrier even in the absence of an explicit ES barrier. We show that mounded morphologies can be obtained even for a small barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma equation, is observed in absence of an explicit step barrier. The mounded surfaces are described by a super-roughness dynamical scaling characterized by locally smooth (faceted) surfaces and a global roughness exponent $\alpha>1$. The thin film limit is featured by surfaces with self-assembled three-dimensional structures having an aspect ratio (height/width) that may increase or decrease with temperature depending on the strength of step barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary materia

Two-component mixture of charged particles confined in a channel: melting

The melting of a binary system of charged particles confined in a {\it quasi}-one-dimensional parabolic channel is studied through Monte Carlo simulations. At zero temperature the particles are ordered in parallel chains. The melting is anisotropic and different melting temperatures are obtained according to the spatial direction, and the different types of particles present in the system. Melting is very different for the single-, two- and four-chain configurations. A temperature induced structural phase transition is found between two different four chain ordered states which is absent in the mono-disperse system. In the mixed regime, where the two types of particles are only slightly different, melting is almost isotropic and a thermally induced homogeneous distribution of the distinct types of charges is observed.Comment: To appear in Journal of Physics: condensed matter ; (13 pages, 12 figures

Manejo sustentável de recursos florestais em áreas de colonos na região de Marabá, Estado do Pará.

bitstream/item/57635/1/CPATU-PA175.pd

Manejo de bacurizeiro (Platonia insignis Mart.) em florestas secundárias, integrando o sistema produtivo familiar no nordeste Paraense.

O objetivo deste estudo é contribuir para o aumento do conhecimento sobre manejo da vegetação que cresce espontaneamente após o cultivo agrícola, com vistas a favorecer a espécie Platonia insignis Mart. (bacurizeiro), na perspectiva do desenvolvimento sustentável local. Buscou-se estabelecer critérios para o manejo, com enfoque na abundante regeneração natural, proveniente de rebrotações de raízes e troncos, após o cultivo de roças, utilizando técnica de baixo custo e de pleno domínio dos agricultores, que é o desbaste de povoamentos. Como procedimentos metodológicos para a seleção das áreas de estudo, usou-se a relação estabelecida nas comunidades, resultado de dez anos de trabalho de pesquisa-ação no Nordeste Paraense. Foram instaladas áreas experimentais localizadas em quatro comunidades rurais no município de Bragança, em vegetação secundária em três fases de desenvolvimento após o cultivo: inicial, intermediária e adulta. Foram aplicados desbastes em dois níveis: moderado e radical. Concluiu-se que o manejo da vegetação secundária com abundância dessa espécie é viável para os sistemas de produção familiar e certamente contribuirá de forma sustentável para a manutenção destes. O manejo dessas áreas, para produção de fruto, contribuirá para a preservação do ambiente florestal que, além de proporcionar benefícios financeiros para as famílias rurais e colaborar na diversificação da dieta alimentar das famílias, também poderá promover benefícios de ordem ambiental, como manter ciclos hidrológicos e seqüestro de carbono

A computationally efficient method for calculating the maximum conductance of disordered networks: Application to 1-dimensional conductors

Random networks of carbon nanotubes and metallic nanowires have shown to be very useful in the production of transparent, conducting films. The electronic transport on the film depends considerably on the network properties, and on the inter-wire coupling. Here we present a simple, computationally efficient method for the calculation of conductance on random nanostructured networks. The method is implemented on metallic nanowire networks, which are described within a single-orbital tight binding Hamiltonian, and the conductance is calculated with the Kubo formula. We show how the network conductance depends on the average number of connections per wire, and on the number of wires connected to the electrodes. We also show the effect of the inter-/intra-wire hopping ratio on the conductance through the network. Furthermore, we argue that this type of calculation is easily extendable to account for the upper conductivity of realistic films spanned by tunneling networks. When compared to experimental measurements, this quantity provides a clear indication of how much room is available for improving the film conductivity.Comment: 7 pages, 5 figure