511 research outputs found
Breathers in inhomogeneous nonlinear lattices: an analysis via centre manifold reduction
We consider an infinite chain of particles linearly coupled to their nearest
neighbours and subject to an anharmonic local potential. The chain is assumed
weakly inhomogeneous. We look for small amplitude discrete breathers. The
problem is reformulated as a nonautonomous recurrence in a space of
time-periodic functions, where the dynamics is considered along the discrete
spatial coordinate. We show that small amplitude oscillations are determined by
finite-dimensional nonautonomous mappings, whose dimension depends on the
solutions frequency. We consider the case of two-dimensional reduced mappings,
which occurs for frequencies close to the edges of the phonon band. For an
homogeneous chain, the reduced map is autonomous and reversible, and
bifurcations of reversible homoclinics or heteroclinic solutions are found for
appropriate parameter values. These orbits correspond respectively to discrete
breathers, or dark breathers superposed on a spatially extended standing wave.
Breather existence is shown in some cases for any value of the coupling
constant, which generalizes an existence result obtained by MacKay and Aubry at
small coupling. For an inhomogeneous chain the study of the nonautonomous
reduced map is in general far more involved. For the principal part of the
reduced recurrence, using the assumption of weak inhomogeneity, we show that
homoclinics to 0 exist when the image of the unstable manifold under a linear
transformation intersects the stable manifold. This provides a geometrical
understanding of tangent bifurcations of discrete breathers. The case of a mass
impurity is studied in detail, and our geometrical analysis is successfully
compared with direct numerical simulations
Bright and dark breathers in Fermi-Pasta-Ulam lattices
In this paper we study the existence and linear stability of bright and dark
breathers in one-dimensional FPU lattices. On the one hand, we test the range
of validity of a recent breathers existence proof [G. James, {\em C. R. Acad.
Sci. Paris}, 332, Ser. 1, pp. 581 (2001)] using numerical computations.
Approximate analytical expressions for small amplitude bright and dark
breathers are found to fit very well exact numerical solutions even far from
the top of the phonon band. On the other hand, we study numerically large
amplitude breathers non predicted in the above cited reference. In particular,
for a class of asymmetric FPU potentials we find an energy threshold for the
existence of exact discrete breathers, which is a relatively unexplored
phenomenon in one-dimensional lattices. Bright and dark breathers superposed on
a uniformly stressed static configuration are also investigated.Comment: 11 pages, 16 figure
Linear response in the uniformly heated granular gas
We analyse the linear response properties of the uniformly heated granular
gas. The intensity of the stochastic driving fixes the value of the granular
temperature in the non-equilibrium steady state reached by the system. Here, we
investigate two specific situations. First, we look into the ``direct''
relaxation of the system after a single (small) jump of the driving intensity.
This study is carried out by two different methods. Not only do we linearise
the evolution equations around the steady state, but also derive generalised
out-of-equilibrium fluctuation-dissipation relations for the relevant response
functions. Second, we investigate the behaviour of the system in a more complex
experiment, specifically a Kovacs-like protocol with two jumps in the driving.
The emergence of anomalous Kovacs response is explained in terms of the
properties of the direct relaxation function: it is the second mode changing
sign at the critical value of the inelasticity that demarcates anomalous from
normal behaviour. The analytical results are compared with numerical
simulations of the kinetic equation, and a good agreement is found.Comment: 14 pages, 10 figures; major revision; completely new section on
non-equilibrium FDR; accepted for publication in PR
Collective Coordinates Theory for Discrete Soliton Ratchets in the sine-Gordon Model
A collective coordinate theory is develop for soliton ratchets in the damped
discrete sine-Gordon model driven by a biharmonic force. An ansatz with two
collective coordinates, namely the center and the width of the soliton, is
assumed as an approximated solution of the discrete non-linear equation. The
evolution of these two collective coordinates, obtained by means of the
Generalized Travelling Wave Method, explains the mechanism underlying the
soliton ratchet and captures qualitatively all the main features of this
phenomenon. The theory accounts for the existence of a non-zero depinning
threshold, the non-sinusoidal behaviour of the average velocity as a function
of the difference phase between the harmonics of the driver, the non-monotonic
dependence of the average velocity on the damping and the existence of
non-transporting regimes beyond the depinning threshold. In particular it
provides a good description of the intriguing and complex pattern of subspaces
corresponding to different dynamical regimes in parameter space
Non-equilibrium memory effects: granular fluids and beyond
In this perspective paper, we look into memory effects in out-of-equilibrium
systems. To be concrete, we exemplify memory effects with the paradigmatic case
of granular fluids, although extensions to other contexts such as molecular
fluids with non-linear drag are also considered. The focus is put on two
archetypal memory effects: the Kovacs and Mpemba effects. In brief, the first
is related to imperfectly reaching a steady state -- either equilibrium or
non-equilibrium, whereas the second is related to reaching a steady state
faster despite starting further. Connections to optimal control theory thus
naturally emerge and are briefly discussed.Comment: Perspective paper for EPL, 7 pages, 6 figure
Breathers in a system with helicity and dipole interaction
Recent papers that have studied variants of the Peyrard-Bishop model for DNA,
have taken into account the long range interaction due to the dipole moments of
the hydrogen bonds between base pairs. In these models the helicity of the
double strand is not considered. In this particular paper we have performed an
analysis of the influence of the helicity on the properties of static and
moving breathers in a Klein--Gordon chain with dipole-dipole interaction. It
has been found that the helicity enlarges the range of existence and stability
of static breathers, although this effect is small for a typical helical
structure of DNA. However the effect of the orientation of the dipole moments
is considerably higher with transcendental consequences for the existence of
mobile breathers.Comment: 4pages, 5 eps figure
Simple model with facilitated dynamics for granular compaction
A simple lattice model is used to study compaction in granular media. As in
real experiments, we consider a series of taps separated by large enough
waiting times. The relaxation of the density exhibits the characteristic
inverse logarithmic law. Moreover, we have been able to identify analytically
the relevant time scale, leading to a relaxation law independent of the
specific values of the parameters. Also, an expression for the asymptotic
density reached in the compaction process has been derived. The theoretical
predictions agree fairly well with the results from the Monte Carlo simulation.Comment: 15 pages, 4 figures, REVTeX file; no changes except for
single-spacing to save paper (previous version 22 pages
Recommended from our members
Investigating the impact of poverty on colonization and infection with drug-resistant organisms in humans: a systematic review
Background
Poverty increases the risk of contracting infectious diseases and therefore exposure to antibiotics. Yet there is lacking evidence on the relationship between income and non-income dimensions of poverty and antimicrobial resistance. Investigating such relationship would strengthen antimicrobial stewardship interventions.
Methods
A systematic review was conducted following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. PubMed, Ovid, MEDLINE, EMBASE, Scopus, CINAHL, PsychINFO, EBSCO, HMIC, and Web of Science databases were searched in October 2016. Prospective and retrospective studies reporting on income or non-income dimensions of poverty and their influence on colonisation or infection with antimicrobial-resistant organisms were retrieved. Study quality was assessed with the Integrated quality criteria for review of multiple study designs (ICROMS) tool.
Results
Nineteen articles were reviewed. Crowding and homelessness were associated with antimicrobial resistance in community and hospital patients. In high-income countries, low income was associated with Streptococcus pneumoniae and Acinetobacter baumannii resistance and a seven-fold higher infection rate. In low-income countries the findings on this relation were contradictory. Lack of education was linked to resistant S. pneumoniae and Escherichia coli. Two papers explored the relation between water and sanitation and antimicrobial resistance in low-income settings.
Conclusions
Despite methodological limitations, the results suggest that addressing social determinants of poverty worldwide remains a crucial yet neglected step towards preventing antimicrobial resistance
Energy funneling in a bent chain of Morse oscillators with long-range coupling
A bent chain of coupled Morse oscillators with long-range dispersive
interaction is considered. Moving localized excitations may be trapped in the
bending region. Thus chain geometry acts like an impurity. An energy funneling
effect is observed in the case of random initial conditions.Comment: 6 pages, 12 figures. Submitted to Physical Review E, Oct. 13, 200
- …