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W P Ferreira

Publications and source records attributed to W P Ferreira.

8 recordsLinked to original sources
PubMed2005

Melting and evaporation in classical two-dimensional clusters confined by a Coulomb potential.

The thermal properties of a two-dimensional classical cluster of negatively charged particles bound by a punctual positive charge are presented. The melting phenomenon is analyzed and the features which characterize such a solid-liquid transition are highlighted. We found that the presence of metastable states strongly modifies the melting scenario, and that the melting temperature of the system is determined by the height of the saddle point energy separating the ground state and the metastable state. Due to the particular type of confinement potential considered in this paper, we also found that, at sufficiently large temperature, the cluster can become thermally ionized.

Journal Article↗
PubMed2005

Structure, normal mode spectra, and mixing of a binary system of charged particles confined in a parabolic trap.

We study the mixing of two different kinds of particles, having different charge and/or mass, interacting through a pure Coulomb potential, and confined in a parabolic trap. The structure of the cluster and its normal mode spectrum are analyzed as a function of the ratio of the charges (mass ratio) of the two types of particles. We show that particles are not always arranged in a shell structure. Mixing of the particles goes hand in hand with a large number of metastable states. The normal modes of the system are obtained, and we find that some of the special modes can be tuned by varying the ratio between the charges (masses) of the two species. The degree of mixing of the two type of particles is summarized in a phase diagram, and an order parameter that describes quantitatively the mixing between particles is defined.

Journal Article↗
PubMed2005

Structural phase transitions and unusual melting behavior in a classical two-dimensional Coulomb bound cluster.

The melting properties of a cluster with N equally charged particles confined by a Coulomb potential are studied. The system exhibits a structural phase transition before it melts. The melting process is not dominated by the usual thermal hops between stable states. We also show that the symmetry of the ground state configuration is a dominant factor in determining the melting temperature and that more confined particles in the cluster do not necessarily have a higher melting temperature.

Journal Article↗
PubMed2004

Nonlinear screening in large two-dimensional Coulomb clusters.

The distortion due to a fixed point impurity with variable charge placed in the center of a classical harmonically confined two-dimensional (2D) large Coulomb cluster is studied. We find that the net topological charge (N(-)-N+ ) of the system is always equal to six independent of the position and charge of the impurity. In comparison with a 2D cluster without impurity charge, only the breathing mode remains unchanged. The screening length is found to be a highly nonlinear function of the impurity charge. For values of the impurity charge smaller than the charge of the other particles, the system has almost the same screening strength. When the impurity charge is larger, the screening length is strongly enhanced. This result can be explained by the competition between the different forces active in the system.

Journal Article↗
PubMed2003

Normal mode spectra of two-dimensional classical atoms confined by a Coulomb potential.

The normal mode spectra of two-dimensional finite clusters of charged particles (-e) confined by a Coulomb potential resulting from a displaced positive charge Ze are obtained. This is a classical two-dimensional model system for atoms. We obtain the frequencies of the normal modes as a function of the confinement charge Z and the number of particles N. The analysis of the lowest normal mode frequency reveals a good agreement with the experimental results obtained in a system with screened interaction between charged particles. The dependence of the normal mode spectra as a function of a perpendicular magnetic field is also discussed and we found that the shear-like character of the modes is enhanced in the presence of the magnetic field. For large values of the magnetic fields the normal modes fall into two bands, a low frequency band with frequency approximately 1/omega(c) and a high frequency band with frequency approximately omega(c).

Journal Article↗
PubMed2003

Structure and correlations in two-dimensional classical artificial atoms confined by a Coulomb potential.

The ordering of N equally charged particles (-e) moving in two dimensions and confined by a Coulomb potential, resulting from a displaced positive charge Ze is discussed. This is a classical model system for atoms. We obtain the configurations of charged particles which, depending on the value of N and Z, may result in ring structures, hexagonal-type configurations, and for N/Z approximately 1 in an inner structure of particles which is separated by an outer ring of particles. For N/Z<<1, the Hamiltonian of the parabolic confinement case is recovered. For N/Z approximately 1, the configurations are very different from those found in the case of a parabolic confinement potential. A hydrodynamic analysis is presented in order to highlight the correlations effects.

Journal Article↗
PubMed2001

New percutaneous techniques for perforating the pulmonary valve in pulmonary atresia with intact ventricular septum.

We report new percutaneous techniques for perforating the pulmonary valve in pulmonary atresia with intact ventricular septum, in 3 newborns who had this birth defect. There was mild to moderate hypoplastic right ventricle, a patent infundibulum, and no coronary-cavitary communications. We succeeded in all cases, and no complications related to the procedure occurred. The new coaxial radiofrequency system was easy to handle, which simplified the procedure. Two patients required an additional source of pulmonary flow (Blalock-Taussig shunt) in the first week after catheterization. All patients had a satisfactory short-term clinical evolution and will undergo recatheterization within 1 year to define the next therapeutic strategy. We conclude that this technique may be safely and efficiently performed, especially when the new coaxial radiofrequency system is used, and it may become the initial treatment of choice in select neonates with pulmonary atresia and intact ventricular septum.

Catheter Ablation↗