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Mario Nicodemi

Publications and source records attributed to Mario Nicodemi.

34 records · Page 2Linked to original sources
arXiv2001

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 temperatures experimental phenomenology, previously interpreted in terms of ``quantum tunnelling'' of vortices, is reproduced by Monte Carlo simulations of a purely ``classical'' vortex model. We demonstrate that a non-zero 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.

cond-mat↗
arXiv2001

A Statistical Mechanics Approach to the Inherent States of Granular Media

We consider a Statistical Mechanics approach to granular systems by following the original ideas developed by Edwards. We use the concept of ``inherent states'', defined as the stable configurations in the potential energy landscape, introduced in the context of glasses. Under simplifying assumptions, the equilibrium inherent states can be characterised by a configurational temperature, $1/β$. We link $β$ to Edwards' compactivity and address the problem of its experimental measure. We also discuss the possibility to describe the time dependent distribution probability in the inherent states with an appropriate master equation.

cond-mat↗
arXiv2001

Memory effects in response functions of driven vortex matter

Vortex flow in driven type II superconductors shows strong memory and history dependent effects. Here, we study a schematic microscopic model of driven vortices to propose a scenario for a broad set of these kind of phenomena ranging from ``rejuvenation'' and ``stiffening'' of the system response, to ``memory'' and ``irreversibility'' in I-V characteristics.

cond-mat↗
arXiv2000

Off equilibrium magnetic properties in a model for vortices in superconductors

We study the properties of a simple lattice model of repulsive particles diffusing in a pinning landscape. The behaviour of the model is very similar to the observed physics of vortices in superconductors. We compare and discuss the equilibrium phase diagram, creep dynamics, the Bean state profiles, hysteresis of magnetisation loops (including the second peak feature), and, in particular, ``off equilibrium'' relaxations. The model is analytically tractable in replica mean field theory and numerically via Monte Carlo simulations. It offers a comprehensive schematic framework of the observed phenomenology.

cond-mat↗
arXiv2000

The jamming transition of Granular Media

We briefly review the basics ideas and results of a recently proposed statistical mechanical approach to granular materials. Using lattice models from standard Statistical Mechanics and results from a mean field replica approach and Monte Carlo simulations we find a jamming transition in granular media closely related to the glass transition in super-cooled liquids. These models reproduce the logarithmic relaxation in granular compaction and reversible-irreversible lines, in agreement with experimental data. The models also exhibit aging effects and breakdown of the usual fluctuation dissipation relation. It is shown that the glass transition may be responsible for the logarithmic relaxation and may be related to the cooperative effects underlying many phenomena of granular materials such as the Reynolds transition.

cond-mat↗
arXiv1999

Domains growth and packing properties in driven granular media subject to gravity

We study the dynamical properties of recently introduced frustrated lattice gas models (IFLG and Tetris) for granular media under gentle shaking. We consider both the case where grains have inter-grain surface interactions and the case where they have not, corresponding, for instance, to the presence or absence of moisture in the packs. To characterise the grains packing structure, we discuss the properties of density distribution. In particular, we consider the phenomenon of grains domains formation under compaction. New results amenable of experimental check are discussed along with some important differences between the dynamics of the present models.

cond-mat.stat-mech↗
arXiv1999

Off-equilibrium dynamics in a singular diffusion model

We introduce a schematic non-linear diffusion model where density fluctuations induce a rich out of equilibrium dynamics. The properties of the model are studied by numerical simulations and analytically in a mean field approximation. At low temperatures and high densities we find a long off-equilibrium glassy region, where the system evolves out of an initially pinned state showing aging and a slow decay of the autocorrelation as an enhanced power law, along with strong spatial heterogeneities and violation of the fluctuation dissipation theorem.

cond-mat.stat-mech↗
arXiv1999

Logarithmic Relaxations in a Random Field Lattice Gas Subject to Gravity

A simple lattice gas model with random fields and gravity is introduced to describe a system of grains moving in a disordered environment. Off equilibrium relaxations of bulk density and its two time correlation functions are numerically found to show logarithmic time dependences and "aging" effects. Similitudes with dry granular media are stressed. The connections with off equilibrium dynamics in others kinds of "frustrated" lattice models in presence of a directional driving force (gravity) are discussed to single out the appearance of universal features in the relaxation process.

cond-mat.dis-nn↗
arXiv1998

Dynamical response functions in models of vibrated granular media

In recently introduced schematic lattice gas models for vibrated dry granular media, we study the dynamical response of the system to small perturbations of shaking amplitudes and its relations with the characteristic fluctuations. Strong off equilibrium features appear and a generalized version of the fluctuation dissipation theorem is introduced. The relations with thermal glassy systems and the role of Edwards' compactivity are discussed.

cond-mat↗
arXiv1998

Aging and multiscaling in out of equilibrium dynamical processes in granular media

In the framework of recently introduced frustrated lattice gas models, we study the out of equilibrium dynamical processes during the compaction process in granular media. We find irreversible-reversible cycles in agreement with recent experimental observations. Moreover in analogy with the phenomenology of the glass transition we find aging effects during the compaction process In particular we find that the two time density correlation function $C(t,t')$ asymptotically scales as a function of the single variable $\ln(t')/\ln(t)$. This result is interpreted in terms of multiscaling properties of the system.

cond-mat↗
arXiv1997

Percolation and Cluster Formalism in Continuous Spin Systems

The generalization of Kasteleyn and Fortuin clusters formalism is introduced in XY (or more generally O(n)) models. Clusters geometrical structure may be linked to spin physical properties as correlation functions. To investigate percolative characteristics, the new cluster definition is analytically explored in one dimension and with Monte Carlo simulations in 2D and 3D frustrated and unfrustrated n-clock models. In these models (also in unfrustrated cases for large n) the percolative transition temperature is higher than the usual thermodynamical critical one.

cond-mat↗
arXiv1997

Force correlations and arches formation in granular assemblies

In the context of a simple microscopic schematic scalar model we study the effects of spatial correlations in force transmission in granular assemblies. We show that the parameters of the normalized weights distribution function, $P(v)\sim v^α\exp(-v/ϕ)$, strongly depend on the spatial extensions, $ξ_V$, of such correlations. We show, then, the connections between measurable macroscopic quantities and microscopic mechanisms enhancing correlations. In particular we evaluate how the exponential cut-off, $ϕ(ξ_V)$, and the small forces power law exponent, $α(ξ_V)$, depend on the correlation length, $ξ_V$. If correlations go to infinity, weights are power law distributed.

cond-mat↗
arXiv1997

The Blume-Emery-Griffiths Spin Glass Model

We study the equilibrium properties of the Blume-Emery-Griffiths model with bilinear quenched disorder in the case of attractive as well as repulsive biquadratic interactions. The global phase diagram of the system is calculated in the context of the replica symmetric mean field approximation.

cond-mat.dis-nn↗
arXiv1996

Equilibrium properties of the Ising frustrated lattice gas

We study the equilibrium properties of an Ising frustrated lattice gas with a mean field replica approach. This model bridges usual {\em Spin Glasses} and a version of {\em Frustrated Percolation} model, and has proven relevant to describe the glass transition. It shows a rich phase diagram which in a definite limit reduces to the known Sherrington-Kirkpatrick spin glass model.

cond-mat↗
arXiv1996

A model for the compaction of granular media

We introduce a lattice model, in which frustration plays a crucial role, to describe relaxation properties of granular media. We show Monte Carlo results for compaction in the presence of vibrations and gravity, which compare well with experimental data.

cond-mat↗