283 research outputs found
Fluctuation phenomena in crystal plasticity - a continuum model
On microscopic and mesoscopic scales, plastic flow of crystals is
characterized by large intrinsic fluctuations. Deformation by crystallographic
slip occurs in a sequence of intermittent bursts ('slip avalanches') with
power-law size distribution. In the spatial domain, these avalanches produce
characteristic deformation patterns in the form of slip lines and slip bands
which exhibit long-range spatial correlations. We propose a generic continuum
model which accounts for randomness in the local stress-strain relationships as
well as for long-range internal stresses that arise from the ensuing plastic
strain heterogeneities. The model parameters are related to the local dynamics
and interactions of lattice dislocations. The model explains experimental
observations on slip avalanches as well as the associated slip and surface
pattern morphologies
Irreversible flow of vortex matter: polycrystal and amorphous phases
We investigate the microscopic mechanisms giving rise to plastic depinning
and irreversible flow in vortex matter. The topology of the vortex array
crucially determines the flow response of this system. To illustrate this
claim, two limiting cases are considered: weak and strong pinning interactions.
In the first case disorder is strong enough to introduce plastic effects in the
vortex lattice. Diffraction patterns unveil polycrystalline lattice topology
with dislocations and grain boundaries determining the electromagnetic response
of the system. Filamentary flow is found to arise as a consequence of
dislocation dynamics. We analize the stability of vortex lattices against the
formation of grain boundaries, as well as the steady state dynamics for
currents approaching the depinning critical current from above, when vortex
motion is mainly localized at the grain boundaries. On the contrary, a
dislocation description proves no longer adequate in the second limiting case
examined. For strong pinning interactions, the vortex array appears completely
amorphous and no remnant of the Abrikosov lattice order is left. Here we obtain
the critical current as a function of impurity density, its scaling properties,
and characterize the steady state dynamics above depinning. The plastic
depinning observed in the amorphous phase is tightly connected with the
emergence of channel-like flow. Our results suggest the possibility of
establishing a clear distinction between two topologically disordered vortex
phases: the vortex polycrystal and the amorphous vortex matter.Comment: 13 pages, 16 figure
Le dimissioni del poeta. Una lettura di Baudelaire
La comunicazione analizza la posizione di Baudelaire rispetto al ruolo della poesia nella modernità attraverso la letttura di alcuni componimenti de "Les Fleurs du Mal"
Depinning and critical current characteristics of topologically defected vortex lattices
We discuss the role of dislocation assemblies such as grain boundaries in the
dynamic response of a driven vortex lattice. We simulate the depinning of a
field-cooled vortex polycrystal and observe a general enhancement of the
critical current as well as a distinct crossover in the characterisitic of this
quantity as a function of pinning density. The results agree with analytical
predictions for grain boundary depinning. The dynamics of grain boundaries thus
proves an essential mechanism underlying the flow response of defected vortex
lattices and the corresponding transport properties of the superconducting
material. We emphasize the connection between the topological rearrangements of
the lattice and its threshold dynamics. Our theory encompasses a variety of
experimental observations in vortex matter as well as in colloidal crystals.Comment: 7 Figure
Dislocation interactions mediated by grain boundaries
The dynamics of dislocation assemblies in deforming crystals indicate the
emergence of collective phenomena, intermittent fluctuations and strain
avalanches. In polycrystalline materials, the understanding of plastic
deformation mechanisms depends on grasping the role of grain boundaries on
dislocation motion. Here the interaction of dislocations and elastic, low angle
grain boundaries is studied in the framework of a discrete dislocation
representation. We allow grain boundaries to deform under the effect of
dislocation stress fields and compare the effect of such a perturbation to the
case of rigid grain boudaries. We are able to determine, both analytically and
numerically, corrections to dislocation stress fields acting on neighboring
grains, as mediated by grain boundary deformation. Finally, we discuss
conclusions and consequences for the avalanche statistics, as observed in
polycrystalline samples.Comment: 13 pages, 5 figure
Electroweak Evolution Equations
Enlarging a previous analysis, where only fermions and transverse gauge
bosons were taken into account, we write down infrared-collinear evolution
equations for the Standard Model of electroweak interactions computing the full
set of splitting functions. Due to the presence of double logs which are
characteristic of electroweak interactions (Bloch-Nordsieck violation), new
infrared singular splitting functions have to be introduced. We also include
corrections related to the third generation Yukawa couplings.Comment: 15 pages, 3 figure
Growth of a vortex polycrystal in type II superconductors
We discuss the formation of a vortex polycrystal in type II superconductors
from the competition between pinning and elastic forces. We compute the elastic
energy of a deformed grain boundary, that is strongly non-local, and obtain the
depinning stress for weak and strong pinning. Our estimates for the grain size
dependence on the magnetic field strength are in good agreement with previous
experiments on NbMo. Finally, we discuss the effect of thermal noise on grain
growth.Comment: 4 pages, 2 figure
Yielding and irreversible deformation below the microscale: Surface effects and non-mean-field plastic avalanches
Nanoindentation techniques recently developed to measure the mechanical
response of crystals under external loading conditions reveal new phenomena
upon decreasing sample size below the microscale. At small length scales,
material resistance to irreversible deformation depends on sample morphology.
Here we study the mechanisms of yield and plastic flow in inherently small
crystals under uniaxial compression. Discrete structural rearrangements emerge
as series of abrupt discontinuities in stress-strain curves. We obtain the
theoretical dependence of the yield stress on system size and geometry and
elucidate the statistical properties of plastic deformation at such scales. Our
results show that the absence of dislocation storage leads to crucial effects
on the statistics of plastic events, ultimately affecting the universal scaling
behavior observed at larger scales.Comment: Supporting Videos available at
http://dx.plos.org/10.1371/journal.pone.002041
Mean-field analysis of the q-voter model on networks
We present a detailed investigation of the behavior of the nonlinear q-voter
model for opinion dynamics. At the mean-field level we derive analytically, for
any value of the number q of agents involved in the elementary update, the
phase diagram, the exit probability and the consensus time at the transition
point. The mean-field formalism is extended to the case that the interaction
pattern is given by generic heterogeneous networks. We finally discuss the case
of random regular networks and compare analytical results with simulations.Comment: 20 pages, 10 figure
The evolution of the cold gas fraction in nearby clusters ram-pressure stripped galaxies
Cluster galaxies are affected by the surrounding environment, which
influences, in particular, their gas, stellar content and morphology. In
particular, the ram-pressure exerted by the intracluster medium promotes the
formation of multi-phase tails of stripped gas detectable both at optical
wavelengths and in the sub-mm and radio regimes, tracing the cold molecular and
atomic gas components, respectively. In this work we analyze a sample of
sixteen galaxies belonging to clusters at redshift showing evidence
of an asymmetric HI morphology (based on MeerKAT observations) with and without
a star forming tail. To this sample we add three galaxies with evidence of a
star forming tail and no HI detection. Here we present the galaxies
gas content from APEX observations of the CO(2-1) emission. We find that in
most galaxies with a star forming tail the global content is
enhanced with respect to undisturbed field galaxies with similar stellar
masses, suggesting an evolutionary path driven by the ram-pressure stripping.
As galaxies enter into the clusters their HI is displaced but also partially
converted into , so that they are enriched when they
pass close to the pericenter, i. e. when they develop the star forming tails
that are visible in UV/B broad bands and in H emission. An inspection
of the phase-space diagram for our sample suggests an anticorrelation between
the HI and gas phases as galaxies fall into the cluster potential.
This peculiar behaviour is a key signature of the ram-pressure stripping in
action.Comment: 19 pages, 8 figures, accepted for publication in Ap
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