210 research outputs found
Critical fluctuations in the breakdown of disordered systems
In this paper some critical aspects of the behaviour of breaking lattices
subject to slow driving forces are briefly reviewed. In particular fluctuations
in the response to the variation of external parameters are discussed.Comment: Presented at the workshop honoring Antonio Coniglio's 60th birthda
Lattice models of disorder with order
This paper describes the use of simple lattice models for studying the
properties of structurally disordered systems like glasses and granulates. The
models considered have crystalline states as ground states, finite
connectivity, and are not subject to constrained evolution rules. After a short
review of some of these models, the paper discusses how two particularly simple
kinds of models, the Potts model and the exclusion models, evolve after a
quench at low temperature to glassy states rather than to crystalline states
How log-normal is your country? An analysis of the statistical distribution of the exported volumes of products
We have considered the statistical distributions of the volumes of the
different products exported by 148 countries. We have found that the form of
these distributions is not unique but heavily depends on the level of
development of the nation, as expressed by macroeconomic indicators like GDP,
GDP per capita, total export and a recently introduced measure for countries'
economic complexity called fitness. We have identified three major classes: a)
an incomplete log-normal shape, truncated on the left side, for the less
developed countries, b) a complete log-normal, with a wider range of volumes,
for nations characterized by intermediate economy, and c) a strongly asymmetric
shape for countries with a high degree of development. The ranking curves of
the exported volumes from each country seldom cross each other, showing a clear
hierarchy of export volumes. Finally, the log-normality hypothesis has been
checked for the distributions of all the 148 countries through different tests,
Kolmogorov-Smirnov and Cramer-Von Mises, confirming that it cannot be rejected
only for the countries of intermediate economy.Comment: 10 pages, 5 figures, submitted to IWcee15 conferenc
Random neighbour model for yielding
We introduce a model for yielding, inspired by fracture models and the
failure of a sheared granular medium in which the applied shear is resisted by
self-organized force chains. The force chains in the granular medium (GM) are
considered as a bundle of fibres of finite strength amongst which stress is
randomly redistributed after any other fibre breaks under excessive load. The
model provides an exponential distribution of the internal stress and a
log-normal shaped distribution of failure stress, in agreement with
experimental observations. The model displays critical behaviour which
approaches mean field as the number of random neighbours becomes large and
also displays a failure strength which remains finite in the limit of infinite
size. From comparison with different models it is argued that this is an effect
of uncorrelation. All these macroscopic properties appear statistically stable
with respect to the choice of the chains' initial strength distribution. The
investigated model is relevant for all systems in which some generic external
load or pressure is borne by a number of units, independent of one another
except when failure of a unit causes load transfer to some random choice of
neighbouring units.Comment: latex, 17 pages, 8 figure
Anomalous metastability in a temperature-driven transition
Langer theory of metastability provides a description of the lifetime and
properties of the metastable phase of the Ising model field-driven transition,
describing the magnetic field-driven transition in ferromagnets and the
chemical potential-driven transition of fluids. An immediate further step is to
apply it to the study of a transition driven by the temperature, as the one
underwent by the two-dimensional Potts model. For this model a study based on
the analytical continuation of the free energy (Meunier, Morel 2000) predicts
the anomalous vanishing of the metastable temperature range in the limit of
large system size, an issue that has been controversial since the eighties.
With a parallel-GPU algorithm we compare the Monte Carlo dynamics with the
theory, obtaining agreement and characterizing the anomalous system size
dependence. We discuss the microscopic origin of these metastable phenomena,
essentially different with respect to the Ising case.Comment: 5 pages, 3 figure
Phase ordering and symmetries of the Potts model
We have studied the ordering of the q-colours Potts model in two dimensions
on a square lattice. On the basis of our observations we propose that if q is
large enough the system is not able to break global and local null
magnetisation symmetries at zero temperature: when q<4 the system forms domains
with a size proportional to the system size while for q>4 it relaxes towards a
non-equilibrium phase with energy larger than the ground state energy, in
agreement with the previous findings of De Oliveira et al. (M. J. de Oliveira,
A. Petri, T. Tome, Europhys. Lett., 65, 20 (2004)).Comment: 6 pages, 3 figures; minor text rewordings and changes in figures
styl
Brownian motor in a granular medium
In this work we experimentally study the behavior of a freely-rotating
asymmetric probe immersed in a vibrated granular medium. For a wide variety of
vibration conditions the probe exhibits a steady rotation whose direction is
constant with respect to the asymmetry. By changing the vibration amplitude and
by filtering the noise in different frequency bands we show that the velocity
of rotation does not depend only on the RMS acceleration , but also on
the amount of energy provided to two separate frequency bands which are
revealed to be important for the dynamics of the granular medium: the first
band governs the transfer of energy from the grains to the probe, and the
second affects the dynamics by altering the viscosity of the vibro-fluidized
material.Comment: Latex, 10 pages, 4 figure
- …