Repositório RCAAP

We discuss finite-size effects on homogeneous nucleation in first-order phase transitions. We study their implications for cosmological phase transitions and to the hadronization of a quark-gluon plasma generated in high-energy heavy ion collisions.

Using the hydrodynamical code NeXSPheRIO, we compare predictions as usually done in hydrodynamics, using centrality windows defined through the impact parameter, and as obtainable experimentally, using windows in participant number.

We study, in a hydrodynamical approach, the energy dependence of the kaon mT spectra in central Pb+Pb (Au+Au) collisions. We show that the experimental data of the inverse slope parameter can be reproduced with a reasonable choice of both energy-dependent freeze-out temperature and initial conditions.

In this work, we study the behavior of event-by-event distribution of particle ratios in Gran Canonical and Isobaric ensemble.We show that the experimental value of the width in the K/p ratio distribution is much wider than that given by a unique grand canonical ensemble or Isobaric ensembe and maybe a mixture is required.

We report the present status on the construction of an equation of state (EoS) for the strongly interacting matter which is to be used in the hydrodynamical calculations for the ultra-relativistic heavy ion collisions. In the present version, the conservation of isospin, baryon number and strangeness are taken into account. A preliminary hydrodynamical result for our EoS, using the hydro code SPHERIO, is also shown.

Localized rain events have been found to follow power-law distributions over several decades, suggesting parallels between precipitation and seismic activity [O. Peters et al., PRL 88, 018701 (2002)]. Similar power laws can be generated by treating raindrops as passive tracers advected by the velocity field of a two-dimensional system of point vortices [R. Dickman, PRL 90, 108701 (2003)]. Here I review observational and theoretical aspects of fractal rain distributions and chaotic advection, and present new results on tracer distributions in the vortex model.

Molecular dynamics simulation was used to study structural and dynamical properties of InSb. The effective potential takes into account two and three-body interactions, considering atomic-size effects and charge-charge, charge-dipole, and dipole-dipole interactions between 1000 particles, 500 In and 500 Sb, initially within a cubic box of side L=32.397 Å . The effect of hydrostatic pressure and temperature on the structural properties like pair distribution function, coordination number, volume change and bond angle distribution and on dynamical properties like vibrational density of states, phonon anharmonicity, dynamic Debye-Waller factor, thermal expansion coefficient and structural phase transformations are correctly described, in excellent agreement with the experimental results.

We describe a new algorithm that approaches Monte Carlo simulation in statistical physics in a different way. Instead of sampling the probability distribution at a fixed temperature, a random walk is performed in energy space to directly extract an estimate for the density of states. The canonical probability can then be found at any temperature by weighting by the appropriate Boltzmann factor, and thermodynamic properties can be determined from suitable derivatives of the partition function.

We review the generalized-ensemble approach to protein studies. Focusing on the problem of secondary structure formation, we show that these sophisticated techniques allow efficient simulations of all-atom protein models and may lead to a deeper understanding of the folding mechanism in proteins.

Half a century ago a reaction-diffusion system of two chemicals was introduced by Alan Turing to account for morphogenesis, i.e., the development of patterns, shapes and structures found in nature. Here we will discuss the formation of patterns and structures obtained through numerical simulation of the Turing mechanism in two and three dimensions. The forming patterns are found to depend strongly on the initial and boundary conditions as well as system parameters, showing a rich variety of patterns, e.g. stripes and spots (2D), and lamellae and spherical droplets (3D) arranged in structures of high symmetry, with or without defects or distortions.

We have simulated a continuous time version of the Nagel-Schreckenberger model of vehicular traffic on highways and calculated the flux as a function of vehicle density. In the low density regime the flux increases linearly with density but becomes a power law when velocities are allowed to increase without bounds. We have simulated also a modified version in which the state of a vehicle depends on the velocity of the vehicle moving ahead. This model displays a phase transition from a state with nonzero flux to a jammed state at a critical density which is strictly less than the closed-packed density. We also study the relationship of this model with self-organized criticality.

We discuss how reweighting and histogram methods for classical systems can be generalized to quantum models for discrete and continuous time world line simulations, and the stochastic series expansion (SSE) method. Our approach allows to apply all classical reweighting and histogram techniques for classical systems, as well as multicanonical or Wang-Landau sampling to the quantum case.

We review recently developed decomposition algorithms for molecular dynamics and spin dynamics simulations of many-body systems. These methods are time reversible, symplectic, and the error in the total energy thus generated is bounded. In general, these techniques are accurate for much larger time steps than more standard integration methods. Illustrations of decomposition algorithms performance are shown for spin dynamics simulations of a Heisenberg ferromagnet.

We studied a layered mixed-spin Ising model, with spins s = 1/2 and S = 1, distributed on the sites of a hexagonal lattice. For this spin arrangement, any spin at one lattice site has two nearest-neigbor spins of the same type, and four of the other type. We assumed that the exchange interaction between spins s and S is antiferromagnetic, with the value J1. J2 is the exchange interaction between two nearest neighbor s spins, and J3 is the coupling between two nearest neighbor S spins. We also considered a single-ion crystal-field contribution D to the S sites. We performed mean-field calculations and Monte Carlo simulations to determine the compensation point of the model. We have shown that a compensation point can be present for any positive value of D. We have also found a negative lower bound for D, below which a compensation point can not appear. For each value of D, we determined the range of values of the J2 and J3 couplings for which a compensation point is realizable.

The possibility of materials that are governed by a fixed point related to small world networks is discussed. In particular, large-scale Monte Carlo simulations are performed on Ising ferromagnetic models on two different small-world networks generated from a one-dimensional spin chain. One has the small-world bond strengths independent of the length, and exhibits a finite-temperature phase transition. The other has small-world bonds built from atoms, and although there is no finite-temperature phase transition the system shows a slow power-law change of the effective critical temperature of a finite system as a function of the system size. An outline of a possible synthesis route for quasi small-world nanomaterials is presented.

In this work we use molecular dynamics simulation to study the clustering of dust particles deposited on a liquid surface. Our results are compatible with the picture that one particle attracts another due to the deformation of the liquid medium which creates an attractive potential around the deposited particle. The similarity to other physical phenomena is discussed.

Using Monte Carlo simulations we have investigated the critical behavior of the classical fully frustrated XY model in two dimensions in a square lattice. There are two phase transitions in the model, one of the Berezinskii-Kosterlitz-Thouless type and a Z2 transition at higher temperature. We show that the vortex anti-vortex density has a clear signature of the Z2 phase transition at exactly the percolation threshold.

Diblock copolymers are linear chain molecules consisting of two subchains A and B grafted covalently to each other. Below some critical temperature Tc these two blocks tend to segregate, but due to the covalent bond they can segregate at best locally to form periodic structures (microdomains). For molecules whose subchains have the same length, the equilibrium pattern is lamellar. In the bulk regime, these microdomains are ordered at random. To obtain an oriented lamellar pattern it is necessary to consider some asymmetry. In the presence of an external field, the lamellae will align to it. Directional quenching also can lead to the growth of oriented microphase separation. The effect of boundary conditions (confinement between parallel walls) also generates well-aligned lamellae, parallel to the walls. If the distance between the walls is comparable to the molecular sizes, another constraint is imposed on the system since the domains are forced to accommodate between the walls and, for certain conditions we will see a frustration phenomenon. If we allow the walls to move with a certain velocity during phase segregation, the accommodation of the lamellae can be changed. We use a cell dynamical system, which is a very efficient computational method, in order to investigate the effect of moving walls in lamellae formation.

We study structural and spectral properties of finite classical systems of N two-dimensional charged particles,confined by a parabolic potential µ, and interacting via inverse power-law potentials µ 1/r n' . Molecular dynamics simulations are performed for different cluster sizes (N = 30 to 230) and n, n' values 1, 2, 3 and 10. We also analyze the phase transition from a ring-like configuration to a Wigner structure as a function of parameter n' and anisotropy. We compare our results with Monte Carlo simulations of Bedanov and Peeters, obtaining good agreement. In addition, we determine the Voronoi structure of the cluster. Our work complements that of Cândido, Rino and Studart, who analyzed confinement in a screened parabolic potential.

The thermal behavior of the (010), (110) and (111) copper surfaces is studied by molecular dynamics simulation. We have used a many-body potential based on the tight-binding model in order to describe the Cu-Cu interaction. The calculations we have performed correspond to simulations in the temperature range between 600 and 1800 K. The observed order in the stability follows the same order as in the packing density, i. e., (110), (010) and (111). The (110) disorder results from anharmonic effects and by vacancy-adatom formation. On the other end, the (111) surface is very stable, and remains so up to temperatures of the order of the bulk melting point. The melting proceeds by a layer-by-layer mechanism.