539 research outputs found
Local and Global relations between the number of contacts and density in monodisperse sphere packs
The topological structure resulting from the network of contacts between
grains (\emph{contact network}) is studied for large samples of monosized
spheres with densities (fraction of volume occupied by the spheres) ranging
from 0.59 to 0.64. We retrieve the coordinates of each bead in the pack and we
calculate the average coordination number by using three different methods. We
show that, in the range of density investigated, the coordination number is
larger than 4 and it increases with the packing fraction. At local level we
also observe a positive correlation between local packing fraction and number
of neighbors. We discover a dependence between the local densities of
configurations with few neighbors in contact and the global sample-denities.
This might indicate that local configurations with small number of neighbors
are able to deform plastically when the sample is compactifying.
PACS: 45.70.-n, Granular Systems; 45.70.Cc, Static sandpiles; Granular
Compaction.Comment: 10 pages, 6 figure
Surface instabilities in granular matter and ion-sputtered surfaces
We apply a theoretical approach, originally introduced to describe aeolian
ripples formation in sandy deserts, to the study of surface instability in ion
sputtered surfaces. The two phenomena are distinct by several orders of
magnitudes and by several physical mechanisms, but they obey to similar
geometrical constraints and therefore they can be described by means of the
same approach. This opens a novel conceptual framework for the study of the
dynamical surface roughening and ripple formation on crystal and amorphous
surfaces during ion sputtering.Comment: 14 pages, 3 figure
Volume fluctuations and geometrical constraints in granular packs
Structural organization and correlations are studied in very large packings
of equally sized acrylic spheres, reconstructed in three-dimensions by means of
X-ray computed tomography. A novel technique, devised to analyze correlations
among more than two spheres, shows that the structural organization can be
conveniently studied in terms of a space-filling packing of irregular
tetrahedra. The study of the volume distribution of such tetrahedra reveals an
exponential decay in the region of large volumes; a behavior that is in very
good quantitative agreement with theoretical prediction. I argue that the
system's structure can be described as constituted of two phases: 1) an
`unconstrained' phase which freely shares the volume; 2) a `constrained' phase
which assumes configurations accordingly with the geometrical constraints
imposed by the condition of non-overlapping between spheres and mechanical
stability. The granular system exploits heterogeneity maximizing freedom and
entropy while constraining mechanical stability.Comment: 5 pages, 4 figure
The Geometrical Structure of Disordered Sphere Packings
The three dimensional structure of large packings of monosized spheres with
volume fractions ranging between 0.58 and 0.64 has been studied with X-ray
Computed Tomography. We search for signatures of organization, we classify
local arrangements and we explore the effects of local geometrical constrains
on the global packing. This study is the largest and the most accurate
empirical analysis of disordered packings at the grain-scale to date with over
140,000 sphere coordinates mapped. We discuss topological and geometrical ways
to characterize and classify these systems, and discuss implications that local
geometry can have on the mechanisms of formation of these amorphous structures.Comment: 15 pages; 16 figure
Exchanges in complex networks: income and wealth distributions
We investigate the wealth evolution in a system of agents that exchange
wealth through a disordered network in presence of an additive stochastic
Gaussian noise. We show that the resulting wealth distribution is shaped by the
degree distribution of the underlying network and in particular we verify that
scale free networks generate distributions with power-law tails in the
high-income region. Numerical simulations of wealth exchanges performed on two
different kind of networks show the inner relation between the wealth
distribution and the network properties and confirm the agreement with a
self-consistent solution. We show that empirical data for the income
distribution in Australia are qualitatively well described by our theoretical
predictions.Comment: 8 pages, 11 figure
Causal construction of the massless vertex diagram
The massless one-loop vertex diagram is constructed by exploiting the causal
structure of the diagram in configuration space, which can be translated
directly into dispersive relations in momentum space.Comment: 14 pages, LATEX with style file, corresponds to published versio
Long term memories of developed and emerging markets: using the scaling analysis to characterize their stage of development
The scaling properties encompass in a simple analysis many of the volatility
characteristics of financial markets. That is why we use them to probe the
different degree of markets development. We empirically study the scaling
properties of daily Foreign Exchange rates, Stock Market indices and fixed
income instruments by using the generalized Hurst approach. We show that the
scaling exponents are associated with characteristics of the specific markets
and can be used to differentiate markets in their stage of development. The
robustness of the results is tested by both Monte-Carlo studies and a
computation of the scaling in the frequency-domain.Comment: 46 pages, 7 figures, accepted for publication in Journal of Banking &
Financ
Perturbative quantum gauge invariance: Where the ghosts come from
A condensed introduction to quantum gauge theories is given in the
perturbative S-matrix framework; path integral methods are used nowhere. This
approach emphasizes the fact that it is not necessary to start from classical
gauge theories which are then subject to quantization, but it is also possible
to recover the classical group structure and coupling properties from purely
quantum mechanical principles. As a main tool we use a free field version of
the Becchi-Rouet-Stora-Tyutin gauge transformation, which contains no
interaction terms related to a coupling constant. This free gauge
transformation can be formulated in an analogous way for quantum
electrodynamics, Yang-Mills theories with massless or massive gauge bosons and
quantum gravity.Comment: 28 pages, LATEX. Some typos corrected, version to be publishe
- …