B C S Grandi | PubMed Health

PubMed2004

Short-time dynamics for the spin-3/2 Blume-Capel model.

We employed Monte Carlo simulations and short-time dynamic scaling to determine the static and dynamic critical exponents for the generalized two-dimensional Blume-Capel model of spin-3/2. We showed that the critical behavior at the second-order phase-transition line between the paramagnetic and ferromagnetic phases is in the same universality class of the two-dimensional Ising model. However, at the double critical end point, which is present in the phase diagram of the model, the critical exponent beta , associated to the order parameter, is different from that of the Ising model.

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PubMed2002

Effect of the subsurface oxygen diffusion on the Ziff-Gulari-Barshad catalytic reaction model.

We study a version of the Ziff-Gulari-Barshad model where we include the diffusion of oxygen atoms between the uppermost layer and the subsurface. When a CO molecule impinges the surface, it occupies a single site, while the O2 molecule needs two neighboring sites to be adsorbed. The oxidation of the CO molecule occurs only at the top layer, and this happens whenever a CO molecule is nearest neighbor of an O atom. Through the pair mean-field approximation we determine the phase diagram of the model for different values of the diffusion rate of oxygen atoms between the subsurface and the top layer. The diagram exhibits a continuous line that separates regions displaying O-poisoned and non-O-poisoned states. We show that above a critical value of the diffusion rate of oxygen atoms from the subsurface to the top layer, there is no more oxygen poisoning for any nonzero value of the diffusion rate from the top layer to the subsurface. This behavior is also verified in Monte Carlo simulations.

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