Biomedical subjects

L Leuzzi

Publications and source records attributed to L Leuzzi.

4 recordsLinked to original sources

PubMed2002

First-order phase transition and phase coexistence in a spin-glass model.

We study the mean-field static solution of the Blume-Emery-Griffiths-Capel model with quenched disorder, an Ising-spin lattice gas with random magnetic interaction. The thermodynamics is worked out in the full replica symmetry breaking scheme. The model exhibits a high temperature/low density paramagnetic phase. As temperature decreases or density increases, a phase transition to a full replica symmetry breaking spin-glass phase occurs. The nature of the transition can be either of the second order or, at temperature below a given critical value, of the first order in the Ehrenfest sense, with a discontinuous jump of the order parameter, a latent heat, and coexistence of phases.

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PubMed2002

Disordered backgammon model.

In this paper we consider an exactly solvable model that displays glassy behavior at zero temperature due to entropic barriers. The new ingredient of the model is the existence of different energy scales or modes associated with different relaxational time scales. Low-temperature relaxation takes place by partial equilibration of successive lower-energy modes. An adiabatic scaling solution, defined in terms of a threshold energy scale epsilon*, is proposed. For such a solution, modes with energy epsilon>>epsilon* are equilibrated at the bath temperature, modes with epsilon<<epsilon* remain out of equilibrium, and relaxation occurs in the neighborhood of the threshold epsilon approximately epsilon*. The model is presented as a toy example to investigate the conditions related to the existence of an effective temperature in glassy systems and its possible dependence on the energy sector is probed by the corresponding observable.

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PubMed2001

Inherent structures in models for fragile and strong glass.

An analysis of the dynamics is performed of exactly solvable models for fragile and strong glasses, exploiting the partitioning of the free-energy landscape in inherent structures. The results are compared with the exact solution of the dynamics, by employing the formulation of an effective temperature used in literature. Also, a statistical mechanics formulation is introduced, based upon general statistical considerations, which performs better. Though the considered models are conceptually simple, there is no limit in which the dynamics may be exactly described by a symbolic dynamics of the system moving through consistently weighted inherent structures.

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PubMed2001

Effective temperatures in an exactly solvable model for a fragile glass.

A model glass with facilitated dynamics is considered with one type of fast process (beta type) and one type of slow process (alpha type). On time scales where the fast processes are in equilibrium, the slow ones have a dynamics that resembles that of facilitated spin models. The main features are the occurrence of a Kauzmann transition, a Vogel-Fulcher-Tammann-Hesse behavior for the relaxation time, an Adam-Gibbs relation between relaxation time and configurational entropy, and an aging regime. The model is such that its statics is simple and its (Monte Carlo type) dynamics is exactly solvable. The dynamics has been studied both on the approach to the Kauzmann transition and below it. In certain parameter regimes it is so slow that a quasiequilibrium occurs at a time dependent effective temperature. Correlation and response functions are also computed, as well as the out of equilibrium fluctuation-dissipation relation, showing the uniqueness of the effective temperature, thus giving support to the rephrasing of the problem within the framework of out of equilibrium thermodynamics.

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