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M Nicodemi

Publications and source records attributed to M Nicodemi.

7 recordsLinked to original sources
PubMed2002

Dynamics and thermodynamics of the spherical frustrated Blume-Emery-Griffiths model.

We introduce a spherical version of the frustrated Blume-Emery-Griffiths model and solve exactly the statics and the Langevin dynamics for zero particle-particle interaction (K=0). In this case the model exhibits an equilibrium transition from a disordered to a spin glass phase, which is always continuous for nonzero temperature. The same phase diagram results from the study of the dynamics. Furthermore, we note the existence of a nonequilibrium time regime in a region of the disordered phase, characterized by aging, as occurs in the glassy phase. Due to a finite equilibration time, the system displays in this region the pattern of interrupted aging.

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PubMed2001

Creep of superconducting vortices in the limit of vanishing temperature: a fingerprint of off-equilibrium dynamics.

We theoretically study the creep of vortex matter in superconductors. The low temperature experimental phenomenology, previously interpreted in terms of "quantum tunneling" of vortices, is reproduced by Monte Carlo simulations of a purely "classical" vortex model. We demonstrate that a nonzero creep rate in the limit of vanishing temperature is to be expected in systems with slow relaxations as a consequence of their off-equilibrium evolution in a complex free energy landscape.

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PubMed2001

Slow dynamics and aging in a constrained diffusion model.

We carry out a complete analysis of the schematic diffusive model recently introduced for the description of supercooled liquids and glassy systems above the glass temperature. The model is described by a trivial equilibrium measure and the presence of kinetics constraints is mimicked through a rapidly decreasing mobility at high particle density. The governing equation describing a sudden quench process is investigated analytically in a mean field approach and by means of numerical simulations. For deep quenches a long lasting off-equilibrium dynamics is observed in dense systems before equilibration is achieved, where time translational invariance lacks and the system ages. The kinetics is slow in this time domain since the average particle diffusivity D decreases in time, as opposed to the standard diffusion case of a constant D, that is recovered only in equilibrium. The autocorrelation function decays slower than an exponential, falling in mean field as an enhanced power law. The linear response function is computed and the modalities of the break-down of the fluctuation dissipation theorem are analytically investigated, showing that an effective temperature can be defined which slowly approaches the bath temperature from above.

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PubMed2000

Universal fluctuations in correlated systems

The probability density function (PDF) of a global measure in a large class of highly correlated systems has been suggested to be of the same functional form. Here, we identify the analytical form of the PDF of one such measure, the order parameter in the low temperature phase of the 2D XY model. We demonstrate that this function describes the fluctuations of global quantities in other correlated equilibrium and nonequilibrium systems. These include a coupled rotor model, Ising and percolation models, models of forest fires, sandpiles, avalanches, and granular media in a self-organized critical state. We discuss the relationship with both Gaussian and extremal statistics.

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PubMed1999

Density fluctuations in a model for vibrated granular media.

This paper presents the study of density fluctuations in a model for vibrated granular media. Their microscopic origin is shown to be linked to the microscopic disorder in grains packing. Varying vibrations amplitude and duration, several regimes are found for density relaxation. Its power spectrum is well described by power laws.

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