5,302 research outputs found
Growth Kinetics in the N-Component Model. Conserved Order Parameter
We extend the discussion of the growth kinetics of the large-N time-dependent
Ginzburg-Landau model with an order parameter described by a free
energy functional, to the conserved case. Quenches from a high temperature
initial state to a zero temperature state are studied for different selections
of parameters entering the effective potential. In all cases we obtain the
asymptotic form of the structure factor. As expected in analogy with the well
studied model, we find multiscaling behavior whenever stable
equilibrium is reached in the ordering region. On the other hand the present
model also displays a novel feature, namely the occurrence of metastable
relaxation.Comment: 20 pages,Plain Late
Cooling of a lattice granular fluid as an ordering process
We present a new microscopic model of granular medium to study the role of
dynamical correlations and the onset of spatial order induced by the
inelasticity of the interactions. In spite of its simplicity, it features
several different aspects of the rich phenomenology observed in granular
materials and allows to make contact with other topics of statistical mechanics
such as diffusion processes, domain growth, persistence, aging phenomena.
Interestingly, while local observables being controlled by the largest
wavelength fluctuations seem to suggest a purely diffusive behavior, the
formation of spatially extended structures and topological defects, such as
vortices and shocks, reveals a more complex scenario.Comment: 4 pages, 4 figure
Growth in systems of vesicles and membranes
We present a theoretical study for the intermediate stages of the growth of
membranes and vesicles in supersaturated solutions of amphiphilic molecules.
The problem presents important differences with the growth of droplets in the
classical theory of Lifshitz-Slyozov-Wagner, because the aggregates are
extensive only in two dimensions, but still grow in a three dimensional bath.
The balance between curvature and edge energy favours the nucleation of small
planar membranes, but as they grow beyond a critical size they close themselves
to form vesicles. We obtain a system of coupled equations describing the growth
of planar membranes and vesicles, which is solved numerically for different
initial conditions. Finally, the range of parameters relevant in experimental
situations is discussed.Comment: 13 pages and 5 postscript figures. To appear in Phys. Rev
Comment on "Entropy Production and Fluctuation Theorems for Active Matter"
This is a comment to a letter by D. Mandal, K. Klymko and M. R. DeWeese
published as Phys. Rev. Lett. 119, 258001 (2017).Comment: 2 pages without figures, in press as a Comment on Physical Review
Letter
Critical properties of Ising model on Sierpinski fractals. A finite size scaling analysis approach
The present paper focuses on the order-disorder transition of an Ising model
on a self-similar lattice. We present a detailed numerical study, based on the
Monte Carlo method in conjunction with the finite size scaling method, of the
critical properties of the Ising model on some two dimensional deterministic
fractal lattices with different Hausdorff dimensions. Those with finite
ramification order do not display ordered phases at any finite temperature,
whereas the lattices with infinite connectivity show genuine critical behavior.
In particular we considered two Sierpinski carpets constructed using different
generators and characterized by Hausdorff dimensions d_H=log 8/log 3 = 1.8927..
and d_H=log 12/log 4 = 1.7924.., respectively.
The data show in a clear way the existence of an order-disorder transition at
finite temperature in both Sierpinski carpets.
By performing several Monte Carlo simulations at different temperatures and
on lattices of increasing size in conjunction with a finite size scaling
analysis, we were able to determine numerically the critical exponents in each
case and to provide an estimate of their errors.
Finally we considered the hyperscaling relation and found indications that it
holds, if one assumes that the relevant dimension in this case is the Hausdorff
dimension of the lattice.Comment: 21 pages, 7 figures; a new section has been added with results for a
second fractal; there are other minor change
Self-propulsion against a moving membrane: enhanced accumulation and drag force
Self-propulsion (SP) is a main feature of active particles (AP), such as
bacteria or biological micromotors, distinguishing them from passive colloids.
A renowned consequence of SP is accumulation at static interfaces, even in the
absence of hydrodynamic interactions. Here we address the role of SP in the
interaction between AP and a moving semipermeable membrane. In particular, we
implement a model of noninteracting AP in a channel crossed by a partially
penetrable wall, moving at a constant velocity . With respect to both the
cases of passive colloids with and AP with , the AP with finite
show enhancement of accumulation in front of the obstacle and experience a
largely increased drag force. This effect is understood in terms of an
effective potential localised at the interface between particles and membrane,
of height proportional to , where is the AP's re-orientation
time and the width characterising the surface's smoothness (
for hard core obstacles). An approximate analytical scheme is able to reproduce
the observed density profiles and the measured drag force, in very good
agreement with numerical simulations. The effects discussed here can be
exploited for automatic selection and filtering of AP with desired parameters.Comment: 13 pages, 3 figure
Driven granular gases with gravity
We study fluidized granular gases in a stationary state determined by the
balance between an external driving and the bulk dissipation. The two
considered situations are inspired by recent experiments, where the gravity
plays a major role as a driving mechanism: in the first case gravity acts only
in one direction and the bottom wall is vibrated, in the second case gravity
acts in both directions and no vibrating walls are present. Simulations
performed under the molecular chaos assumption show averaged profiles of
density, velocity and granular temperature which are in good agreement with the
experiments. Moreover we measure the velocity distributions which show strong
non-Gaussian behavior, as experiments pointed out, but also density
correlations accounting for clustering, at odds with the experimental results.
The hydrodynamics of the first model is discussed and an exact solution is
found for the density and granular temperature as functions of the distance
from the vibrating wall. The limitations of such a solution, in particular in a
broad layer near the wall injecting energy, are discussed.Comment: Revised version accepted for publication. New results added and
discussions considering tangential forces. 27 pages (19 figures included), to
appear in Phys.Rev.
An exact Coulomb cutoff technique for supercell calculations
We present a new reciprocal space analytical method to cutoff the long range
interactions in supercell calculations for systems that are infinite and
periodic in 1 or 2 dimensions, extending previous works for finite systems. The
proposed cutoffs are functions in Fourier space, that are used as a
multiplicative factor to screen the bare Coulomb interaction. The functions are
analytic everywhere but in a sub-domain of the Fourier space that depends on
the periodic dimensionality. We show that the divergences that lead to the
non-analytical behaviour can be exactly cancelled when both the ionic and the
Hartree potential are properly screened. This technique is exact, fast, and
very easy to implement in already existing supercell codes. To illustrate the
performance of the new scheme, we apply it to the case of the Coulomb
interaction in systems with reduced periodicity (as one-dimensional chains and
layers). For those test cases we address the impact of the cutoff in different
relevant quantities for ground and excited state properties, namely: the
convergence of the ground state properties, the static polarisability of the
system, the quasiparticle corrections in the GW scheme and in the binding
energy of the excitonic states in the Bethe-Salpeter equation. The results are
very promising.Comment: Submitted to Physical Review B on Dec 23rd 200
Noise Rectification and Fluctuations of an Asymmetric Inelastic Piston
We consider a massive inelastic piston, whose opposite faces have different
coefficients of restitution, moving under the action of an infinitely dilute
gas of hard disks maintained at a fixed temperature. The dynamics of the piston
is Markovian and obeys a continuous Master Equation: however, the asymmetry of
restitution coefficients induces a violation of detailed balance and a net
drift of the piston, as in a Brownian ratchet. Numerical investigations of such
non-equilibrium stationary state show that the velocity fluctuations of the
piston are symmetric around the mean value only in the limit of large piston
mass, while they are strongly asymmetric in the opposite limit. Only taking
into account such an asymmetry, i.e. including a third parameter in addition to
the mean and the variance of the velocity distribution, it is possible to
obtain a satisfactory analytical prediction for the ratchet drift velocity.Comment: 6 pages, 5 figures, to be published on Europhysics Letters; some
references have been adde
A Soluble Phase Field Model
The kinetics of an initially undercooled solid-liquid melt is studied by
means of a generalized Phase Field model, which describes the dynamics of an
ordering non-conserved field phi (e.g. solid-liquid order parameter) coupled to
a conserved field (e.g. thermal field). After obtaining the rules governing the
evolution process, by means of analytical arguments, we present a discussion of
the asymptotic time-dependent solutions. The full solutions of the exact
self-consistent equations for the model are also obtained and compared with
computer simulation results. In addition, in order to check the validity of the
present model we confronted its predictions against those of the standard Phase
field model and found reasonable agreement. Interestingly, we find that the
system relaxes towards a mixed phase, depending on the average value of the
conserved field, i.e. on the initial condition. Such a phase is characterized
by large fluctuations of the phi field.Comment: 13 pages, 8 figures, RevTeX 3.1, submitted to Physical Review
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