65 research outputs found
Relaxation dynamics of the Ising -spin disordered model with finite number of variables
We study the dynamic and metastable properties of the fully connected Ising
-spin model with finite number of variables. We define trapping energies,
trapping times and self correlation functions and we analyse their statistical
properties in comparison to the predictions of trap models.Comment: 7 pages, 6 figures, final versio
Aging dynamics of +-J Edwards-Anderson spin glasses
We analyze by means of extensive computer simulations the out of equilibrium
dynamics of Edwards-Anderson spin glasses in d=4 and d=6 dimensions with +-J
interactions. In particular, we focus our analysis on the scaling properties of
the two-time autocorrelation function in a range of temperatures from T=0.07
T_c to T=0.75 T_c in both systems. We observe that the aging dynamics of the
+-J models is different from that observed in the corresponding Gaussian
models. In both the 4d and 6d models at very low temperatures we study the
effects of discretization of energy levels. Strong interrupted aging behaviors
are found. We argue that this is because in the times accessible to our
simulations the systems are only able to probe activated dynamics through the
lowest discrete energy levels and remain trapped around nearly flat regions of
the energy landscape. For temperatures T >= 0.5 T_c in 4d we find logarithmic
scalings that are compatible with dynamical ultrametricity, while in 6d the
relaxation can also be described by super-aging scalings.Comment: 7 pages, 10 figure
Distribution of Eigenvalues of Ensembles of Asymmetrically Diluted Hopfield Matrices
Using Grassmann variables and an analogy with two dimensional electrostatics,
we obtain the average eigenvalue distribution of ensembles of asymmetrically diluted Hopfield matrices in the limit . We found that in the limit of strong dilution the distribution is
uniform in a circle in the complex plane.Comment: 9 pages, latex, 4 figure
Off-Equilibrium Dynamics of a 4D Spin Glass with Asymmetric Couplings
We study the off-equilibrium dynamics of the Edwards-Anderson spin glass in
four dimensions under the influence of a non-hamiltonian perturbation. We find
that for small asymmetry the model behaves as the hamiltonian one, while for
large asymmetry the behaviour of the model can be well described by an
interrupted aging scenario. The autocorrelation function C(t_w+\tau,t_w) scales
as \tau/t_w^\beta, with \beta a function of the asymmetry. For very long
waiting times the previous regime crosses over to a time translational
invariant regime (TTI) with stretched exponential relaxation. The model does
not show signs of reaching a TTI regime for weak asymmetry, but in the aging
regime the exponent \beta is always different from one, showing a non trivial
aging scenario.Comment: Latex, 12 pages, 9 figure
Microscopic approach to orientational order of domain walls
We develop a fully microscopic, statistical mechanics approach to study phase
transitions in Ising systems with competing interactions at different scales.
Our aim is to consider orientational and positional order parameters in a
unified framework. In this work we consider two dimensional stripe forming
systems, where nematic, smectic and crystal phases are possible. We introduce a
nematic order parameter in a lattice, which measures orientational order of
interfaces. We develop a mean field approach which leads to a free energy which
is a function of both the magnetization (density) and the orientational
(nematic) order parameters. Self-consistent equations for the order parameters
are obtained and the solutions are described for a particular system, the
Dipolar Frustrated Ising Ferromagnet. We show that this system has an
Ising-nematic phase at low temperatures in the square lattice, where positional
order (staggered magnetization) is zero. At lower temperatures a crystal-stripe
phase may appear. In the continuum limit the present approach connects to a
Ginsburg-Landau theory, which has an isotropic-nematic phase transition with
breaking of a continuous symmetry.Comment: 9 pages, 7 figures, revised and expanded, published versio
Two time scales and FDT violation in a Finite Dimensional Model for Structural Glasses
We study the breakdown of fluctuation-dissipation relations between time
dependent density-density correlations and associated responses following a
quench in chemical potential in the Frustrated Ising Lattice Gas. The
corresponding slow dynamics is characterized by two well separated time scales
which are characterized by a constant value of the fluctuation-dissipation
ratio. This result is particularly relevant taking into account that activated
processes dominate the long time dynamics of the system.Comment: 4 pages, 3 figs, Phys. Rev. Lett. (in press
Landscape statistics of the p-spin Ising model
The statistical properties of the local optima (metastable states) of the
infinite range Ising spin glass with p-spin interactions in the presence of an
external magnetic field h are investigated analytically. The average number of
optima as well as the typical overlap between pairs of identical optima are
calculated for general p. Similarly to the thermodynamic order parameter, for
p>2 and small h the typical overlap q_t is a discontinuous function of the
energy. The size of the jump in q_t increases with p and decreases with h,
vanishing at finite values of the magnetic field.Comment: 12 pages,te
Testing boundary conditions efficiency in simulations of long-range interacting magnetic models
Periodic boundary conditions have not a unique implementation in magnetic
systems where all spins interact with each other through a power law decaying
interaction of the form , being the distance between spins. In
this work we present a comparative study of the finite size effects oberved in
numerical simulations by using first image convention and full infinite of
periodic boundary conditions in one and two-dimensional spin systems with those
type of interactions, including the ferromagnetic, antiferromagnetic and
competitive interactions cases. Our results show no significative differences
between the finite size effects produced by both types of boundary conditions
when the low temperature phase has zero global magnetization, while it depends
on the ratio for systems with a low temperature ferromagnetic phase.
In the last case the first image convention gives much more stronger finite
size effects than the other when the system enters into the classical regime
.Comment: 9 pages, 5 figure
Reply to Comment on "Two time scales and violation of the Fluctuation-Dissipation Theorem in a finite dimensional model for structural glasses"
In a Comment (cond-mat/0103444) on our recent paper "Two time scales and
violation of FDT in a finite dimensional model for structural glasses" [1], A.
de Candia and A. Coniglio show evidence that the equilibrium overlap
distribution P(q) of the frustrated Ising lattice gas (FILG) does not coincide
with the one that could be estimated from the off-equilibrium results presented
in [1]. In this Reply we show new results on the glassy phase of the FILG that
clarify the controversy. Nevertheless, the structure of the equilibrium P(q)
remains an open problem.
[1] F. Ricci-Tersenghi, D. A. Stariolo and J. J. Arenzon, Phys. Rev. Lett.
84,
4473 (2000).Comment: 5 pages, 2 eps figures, to appear in Phys.Rev.Let
Inevitable Irreversibility Generated by the Glass Transition of the Binary Lattice Gas Model
We numerically investigate the thermodynamic properties of the glass state.
As the object of our study, we employ a binary lattice gas model. Through Monte
Carlo simulations, we find that this model actually experiences a glass
transition. We introduce a potential into the model that represents a piston
with which we compress the glass. By measuring the work performed in this
process, we find that irreversible works exist at the glass state even in the
quasistatic limit. This implies that yield stress is created by the glass
transition.Comment: 4 pages, 5 figure
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