1,819 research outputs found
Irreversible phase transitions induced by an oscillatory input
A novel kind of irreversible phase transitions (IPT's) driven by an
oscillatory input parameter is studied by means of computer simulations. Second
order IPT's showing scale invariance in relevant dynamic critical properties
are found to belong to the universality class of directed percolation. In
contrast, the absence of universality is observed for first order IPT's.Comment: 18 pages (Revtex); 8 figures (.ps); submitted to Europhysics Letters,
December 9th, 199
Study of the one-dimensional off-lattice hot-monomer reaction model
Hot monomers are particles having a transient mobility (a ballistic flight)
prior to being definitely absorbed on a surface. After arriving at a surface,
the excess energy coming from the kinetic energy in the gas phase is dissipated
through degrees of freedom parallel to the surface plane. In this paper we
study the hot monomer-monomer adsorption-reaction process on a continuum
(off-lattice) one-dimensional space by means of Monte Carlo simulations. The
system exhibits second-order irreversible phase transition between a reactive
and saturated (absorbing) phases which belong to the directed percolation (DP)
universality class. This result is interpreted by means of a coarse-grained
Langevin description which allows as to extend the DP conjecture to transitions
occurring in continuous media.Comment: 13 pages, 5 figures, final version to appear in J. Phys.
Critical behavior of a non-equilibrium interacting particle system driven by an oscillatory field
First- and second-order temperature driven transitions are studied, in a
lattice gas driven by an oscillatory field. The short time dynamics study
provides upper and lower bounds for the first-order transition points obtained
using standard simulations. The difference between upper and lower bounds is a
measure for the strength of the first-order transition and becomes negligible
small for densities close to one half. In addition, we give strong evidence on
the existence of multicritical points and a critical temperature gap, the
latter induced by the anisotropy introduced by the driving field.Comment: 12 pages, 4 figures; to appear in Europhys. Let
Critical Behavior of an Ising System on the Sierpinski Carpet: A Short-Time Dynamics Study
The short-time dynamic evolution of an Ising model embedded in an infinitely
ramified fractal structure with noninteger Hausdorff dimension was studied
using Monte Carlo simulations. Completely ordered and disordered spin
configurations were used as initial states for the dynamic simulations. In both
cases, the evolution of the physical observables follows a power-law behavior.
Based on this fact, the complete set of critical exponents characteristic of a
second-order phase transition was evaluated. Also, the dynamic exponent of the critical initial increase in magnetization, as well as the critical
temperature, were computed. The exponent exhibits a weak dependence
on the initial (small) magnetization. On the other hand, the dynamic exponent
shows a systematic decrease when the segmentation step is increased, i.e.,
when the system size becomes larger. Our results suggest that the effective
noninteger dimension for the second-order phase transition is noticeably
smaller than the Hausdorff dimension. Even when the behavior of the
magnetization (in the case of the ordered initial state) and the
autocorrelation (in the case of the disordered initial state) with time are
very well fitted by power laws, the precision of our simulations allows us to
detect the presence of a soft oscillation of the same type in both magnitudes
that we attribute to the topological details of the generating cell at any
scale.Comment: 10 figures, 4 tables and 14 page
Experimental evidence on the development of scale invariance in the internal structure of self-affine aggregates
It is shown that an alternative approach for the characterization of growing
branched patterns consists of the statistical analysis of frozen structures,
which cannot be modified by further growth, that arise due to competitive
processes among neighbor growing structures. Scaling relationships applied to
these structures provide a method to evaluate relevant exponents and to
characterize growing systems into universality classes. The analysis is applied
to quasi-two-dimensional electrochemically formed silver branched patterns
showing that the size distribution of frozen structures exhibits scale
invariance. The measured exponents, within the error bars, remind us those
predicted by the Kardar-Parisi-Zhang equation.Comment: 11 pages, 4 figure
Topological Effects caused by the Fractal Substrate on the Nonequilibrium Critical Behavior of the Ising Magnet
The nonequilibrium critical dynamics of the Ising magnet on a fractal
substrate, namely the Sierpinski carpet with Hausdorff dimension =1.7925,
has been studied within the short-time regime by means of Monte Carlo
simulations. The evolution of the physical observables was followed at
criticality, after both annealing ordered spin configurations (ground state)
and quenching disordered initial configurations (high temperature state), for
three segmentation steps of the fractal. The topological effects become evident
from the emergence of a logarithmic periodic oscillation superimposed to a
power law in the decay of the magnetization and its logarithmic derivative and
also from the dependence of the critical exponents on the segmentation step.
These oscillations are discussed in the framework of the discrete scale
invariance of the substrate and carefully characterized in order to determine
the critical temperature of the second-order phase transition and the critical
exponents corresponding to the short-time regime. The exponent of the
initial increase in the magnetization was also obtained and the results suggest
that it would be almost independent of the fractal dimension of the susbstrate,
provided that is close enough to d=2.Comment: 9 figures, 3 tables, 10 page
Measurements of the Yield Stress in Frictionless Granular Systems
We perform extensive molecular dynamics simulations of 2D frictionless
granular materials to determine whether these systems can be characterized by a
single static yield shear stress. We consider boundary-driven planar shear at
constant volume and either constant shear force or constant shear velocity.
Under steady flow conditions, these two ensembles give similar results for the
average shear stress versus shear velocity. However, near jamming it is
possible that the shear stress required to initiate shear flow can differ
substantially from the shear stress required to maintain flow. We perform
several measurements of the shear stress near the initiation and cessation of
flow. At fixed shear velocity, we measure the average shear stress
in the limit of zero shear velocity. At fixed shear force, we
measure the minimum shear stress required to maintain steady flow
at long times. We find that in finite-size systems ,
which implies that there is a jump discontinuity in the shear velocity from
zero to a finite value when these systems begin flowing at constant shear
force. However, our simulations show that the difference , and thus the discontinuity in the shear velocity, tend to zero in
the infinite system size limit. Thus, our results indicate that in the large
system limit, frictionless granular systems are characterized by a single
static yield shear stress. We also monitor the short-time response of these
systems to applied shear and show that the packing fraction of the system and
shape of the velocity profile can strongly influence whether or not the shear
stress at short times overshoots the long-time average value.Comment: 7 pages and 6 figure
Analytic and Gevrey Hypoellipticity for Perturbed Sums of Squares Operators
We prove a couple of results concerning pseudodifferential perturbations of
differential operators being sums of squares of vector fields and satisfying
H\"ormander's condition. The first is on the minimal Gevrey regularity: if a
sum of squares with analytic coefficients is perturbed with a
pseudodifferential operator of order strictly less than its subelliptic index
it still has the Gevrey minimal regularity. We also prove a statement
concerning real analytic hypoellipticity for the same type of
pseudodifferential perturbations, provided the operator satisfies to some extra
conditions (see Theorem 1.2 below) that ensure the analytic hypoellipticity
Non-monotonous crossover between capillary condensation and interface localisation/delocalisation transition in binary polymer blends
Within self-consistent field theory we study the phase behaviour of a
symmetric binary AB polymer blend confined into a thin film. The film surfaces
interact with the monomers via short range potentials. One surface attracts the
A component and the corresponding semi-infinite system exhibits a first order
wetting transition. The surface interaction of the opposite surface is varied
as to study the crossover from capillary condensation for symmetric surface
fields to the interface localisation/delocalisation transition for
antisymmetric surface fields. In the former case the phase diagram has a single
critical point close to the bulk critical point. In the latter case the phase
diagram exhibits two critical points which correspond to the prewetting
critical points of the semi-infinite system. The crossover between these
qualitatively different limiting behaviours occurs gradually, however, the
critical temperature and the critical composition exhibit a non-monotonic
dependence on the surface field.Comment: to appear in Europhys.Let
Effect of Gravity and Confinement on Phase Equilibria: A Density Matrix Renormalization Approach
The phase diagram of the 2D Ising model confined between two infinite walls
and subject to opposing surface fields and to a bulk "gravitational" field is
calculated by means of density matrix renormalization methods. In absence of
gravity two phase coexistence is restricted to temperatures below the wetting
temperature. We find that gravity restores the two phase coexistence up to the
bulk critical temperature, in agreement with previous mean-field predictions.
We calculate the exponents governing the finite size scaling in the temperature
and in the gravitational field directions. The former is the exponent which
describes the shift of the critical temperature in capillary condensation. The
latter agrees, for large surface fields, with a scaling assumption of Van
Leeuwen and Sengers. Magnetization profiles in the two phase and in the single
phase region are calculated. The profiles in the single phase region, where an
interface is present, agree well with magnetization profiles calculated from a
simple solid-on-solid interface hamiltonian.Comment: 4 pages, RevTeX and 4 PostScript figures included. Final version as
published. To appear in Phys. Rev. Let
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