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.Comment: 12 pages, 2 postscript figure

Glass transition in models with controlled frustration

A class of models with self-generated disorder and controlled frustration is studied. Between the trivial case, where frustration is not present at all, and the limit case, where frustration is present over every length scale, a region with local frustration is found where glassy dynamics appears. We suggest that in this region, the mean field model might undergo a p-spin like transition, and increasing the range of frustration, a crossover from a 1-step replica symmetry breaking to a continuous one might be observed.Comment: 4 pages, 6 figure

Glass glass transition and new dynamical singularity points in an analytically solvable p-spin glass like model

We introduce and analytically study a generalized p-spin glass like model that captures some of the main features of attractive glasses, recently found by Mode Coupling investigations, such as a glass/glass transition line and dynamical singularity points characterized by a logarithmic time dependence of the relaxation. The model also displays features not predicted by the Mode Coupling scenario that could further describe the attractive glasses behavior, such as aging effects with new dynamical singularity points ruled by logarithmic laws or the presence of a glass spinodal line

A stochastic model dissects cell states in biological transition processes

Many biological processes, including differentiation, reprogramming, and disease transformations, involve transitions of cells through distinct states. Direct, unbiased investigation of cell states and their transitions is challenging due to several factors, including limitations of single-cell assays. Here we present a stochastic model of cellular transitions that allows underlying single-cell information, including cell-state-specific parameters and rates governing transitions between states, to be estimated from genome-wide, population-averaged time-course data. The key novelty of our approach lies in specifying latent stochastic models at the single-cell level, and then aggregating these models to give a likelihood that links parameters at the single-cell level to observables at the population level. We apply our approach in the context of reprogramming to pluripotency. This yields new insights, including profiles of two intermediate cell states, that are supported by independent single-cell studies. Our model provides a general conceptual framework for the study of cell transitions, including epigenetic transformations

Thermodynamics and statistical mechanics of frozen systems in inherent states

We discuss a Statistical Mechanics approach in the manner of Edwards to the ``inherent states'' (defined as the stable configurations in the potential energy landscape) of glassy systems and granular materials. We show that at stationarity the inherent states are distributed according a generalized Gibbs measure obtained assuming the validity of the principle of maximum entropy, under suitable constraints. In particular we consider three lattice models (a diluted Spin Glass, a monodisperse hard-sphere system under gravity and a hard-sphere binary mixture under gravity) undergoing a schematic ``tap dynamics'', showing via Monte Carlo calculations that the time average of macroscopic quantities over the tap dynamics and over such a generalized distribution coincide. We also discuss about the general validity of this approach to non thermal systems.Comment: 10 pages, 16 figure

Metastable states in the Blume-Emery-Griffiths spin glass model

We study the Blume-Emery-Griffiths spin glass model in presence of an attractive coupling between real replicas, and evaluate the effective potential as a function of the density overlap. We find that there is a region, above the first order transition of the model, where metastable states with a large density overlap exist. The line where these metastable states appear should correspond to a purely dynamical transition, with a breaking of ergodicity. Differently from what happens in p-spin glasses, in this model the dynamical transition would not be the precursor of a 1-step RSB transition, but (probably) of a full RSB transition.Comment: RevTeX, 4 pages, 2 fig

Two time scales and FDT violation in a Finite Dimensional Model for Structural Glasses

We study the breakdown of fluctuation-dissipation relations between time dependent density-density correlations and associated responses following a quench in chemical potential in the Frustrated Ising Lattice Gas. The corresponding slow dynamics is characterized by two well separated time scales which are characterized by a constant value of the fluctuation-dissipation ratio. This result is particularly relevant taking into account that activated processes dominate the long time dynamics of the system.Comment: 4 pages, 3 figs, Phys. Rev. Lett. (in press

A LEAP INTO THE BEGINNING OF THE METAL AGE: RECRYSTALLIZATION AND CARBURIZING

Although the great importance covered by the heat treatment and the thermo-mechanical process in the evolution of the history of metallurgy, the role of these processes has not been correctly considered even by some famous and recognized archeo-metallurgists. Moreover, it is difficult to agree with the prevalent opinion that the beginning of the metallurgical activity corresponds with the birth of the extractive processes which permit to obtain the metals starting from their ores. The most reliable hypothesis supposes that the first metallurgical activity is to be found the plastic working and heat treatment of the metals found in nature under their reduced form and this statement seems to be strongly confirmed by the fact that the first objects are constituted by gold, silver and copper, which are the metals which can be frequently found in the reduced form

Jamming transition in granular media: A mean field approximation and numerical simulations

In order to study analytically the nature of the jamming transition in granular material, we have considered a cavity method mean field theory, in the framework of a statistical mechanics approach, based on Edwards' original idea. For simplicity we have applied the theory to a lattice model and a transition with exactly the same nature of the glass transition in mean field models for usual glass formers is found. The model is also simulated in three dimensions under tap dynamics and a jamming transition with glassy features is observed. In particular two step decays appear in the relaxation functions and dynamic heterogeneities resembling ones usually observed in glassy systems. These results confirm early speculations about the connection between the jamming transition in granular media and the glass transition in usual glass formers, giving moreover a precise interpretation of its nature.Comment: 11 pages, 12 figure

A ``Tetris''-like model for the Compaction of Dry Granular Media

We propose a two-dimensional geometrical model, based on the concept of geometrical frustration, conceived for the study of compaction in granular media. The dynamics exhibits an interesting inverse logarithmic law that is well known from real experiments. Moreover we present a simple dynamical model of $N$ planes exchanging particles with excluded volume problems, which allows to clarify the origin of the logarithmic relaxations and the stationary density distribution. A simple mapping allows us to cast this Tetris-like model in the form of an Ising-like spin systems with vacancies.Comment: 4 pages, Latex including 2 PS figures (reference corrected). Subm. to Phys. Rev. Lett. (1997