arXiv subjects

Daniel A. Stariolo

Publications and source records attributed to Daniel A. Stariolo.

49 records · Page 3Linked to original sources

arXiv2003

Time correlation functions between Inherent Structures: a connection between landscape topology and the dynamics of glassy systems

We introduce time correlation functions between Inherent Structures (IS) of a supercooled liquid. We show that these functions are useful to relate the slowing down of the dynamics to the structure of the energy landscape near the glass transition temperature. They show a short time regime during which the system remains in the basin of a particular IS and a long time regime where it explores the neighborhood of an IS. We compare the behavior of these functions in a binary Lennard-Jones supercooled liquid and in a model of traps and show that they behave qualitatively different. This comparison reflects the presence/absence of structure in the landscape of the Lennard-Jones/traps models. Possible scenarios for the structure of the landscape which are compatible with these results are discussed.

cond-mat.dis-nn↗

arXiv2003

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 $1/r^α$, $r$ 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 $α/d$ 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 $α/d \leq 3/2$.

cond-mat.stat-mech↗

arXiv2003

On the Aging Dynamics in an Immune Network Model

Recently we have used a cellular automata model which describes the dynamics of a multi-connected network to reproduce the refractory behavior and aging effects obtained in immunization experiments performed with mice when subjected to multiple perturbations. In this paper we investigate the similarities between the aging dynamics observed in this multi-connected network and the one observed in glassy systems, by using the usual tools applied to analyze the latter. An interesting feature we show here is that the model reproduces the biological aspects observed in the experiments during the long transient time it takes to reach the stationary state. Depending on the initial conditions, and without any perturbation, the system may reach one of a family of long-period attractors. The pertrubations may drive the system from its natural attractor to other attractors of the same family. We discuss the different roles played by the small random perturbations (noise) and by the large periodic perturbations (immunizations).

cond-mat.stat-mech↗

arXiv2003

Heterogeneities in systems with quenched disorder

We study the strong role played by structural (quenched) heterogeneities on static and dynamic properties of the Frustrated Ising Lattice Gas in two dimensions, already in the liquid phase. Differently from the dynamical heterogeneities observed in other glass models in this case they may have infinite lifetime and be spatially pinned by the quenched disorder. We consider a measure of local frustration, show how it induces the appearance of spatial heterogeneities and how this reflects in the observed behavior of equilibrium density distributions and dynamic correlation functions.

cond-mat.dis-nn↗

arXiv2002

The distance between Inherent Structures and the influence of saddles on approaching the mode coupling transition in a simple glass former

We analyze through molecular dynamics simulations of a Lennard-Jones binary mixture the statistics of the distances between inherent structures (IS) sampled at temperatures above the mode coupling transition temperature T_MCT. We take equilibrated configurations and randomly perturb the coordinates of a given number of particles. After that we take the nearest IS of both the original configuration and the perturbed one and evaluate the distance between them. This distance presents an inflection point near T~1 with a strong decrease below this temperature and goes to a small but nonzero value on approaching T_MCT. In the low temperature region we study the statistics of events which give zero distance, i.e. dominated by minima, and find evidence that the number of saddles decreases exponentially near T_MCT. This implies that saddles continue to exist even for T<=T_MCT. As at T_MCT the extrapolated diffusivity goes to zero our results imply that there are saddles associated with nondiffusional events at T<T_MCT.

cond-mat.dis-nn↗

arXiv2001

Dynamic ultrametricity in finite dimensional spin glasses

We show results of simulations of a weakly driven four dimensional Edwards- Anderson spin glass, which presents clear signatures of dynamical ultrametricity at low temperatures. The presence of a hierarchical organization of time scales is evident from the appearance of a triangular relation between correlations at three characteristic long times in the asymptotic limit of small drive, and also in the scaling form of stationary correlations in this regime. The differences between this scenario and known results of the aging dynamics of the model are also analyzed.

cond-mat.dis-nn↗

arXiv2000

Dynamics of ferromagnetic spherical spin models with power law interactions: exact solution

We solve the Langevin dynamics of d-dimensional ferromagnetic spherical models with interactions that decay with distance as r^-(d+sigma). The long time dynamics of correlations and responses are studied in the different dynamical regimes and the validity of fluctuation-dissipation relations (or its violation) are shown. In particular, we show that the fluctuation-dissipation ratio X(t+t_w,t_w) is assymptotically a function only of the waiting time t_w in the aging regime and that X -> 0 as t_w -> oo . The results are valid in any finite dimension d and for 0 < sigma < 2, where short range behaviour is recovered. We also solve the T=0 Cahn-Hilliard dynamics of this model (conserved order parameter). An analysis of the multiscaling behaviour of the autocorrelation function is presented.

cond-mat.stat-mech↗

arXiv2000

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.

cond-mat.dis-nn↗

arXiv1999

Off equilibrium dynamics of the Frustrated Ising Lattice Gas

We study by means of Monte Carlo simulations the off equilibrium properties of a model glass, the Frustrated Ising Lattice Gas (FILG) in three dimensions. We have computed typical two times quantities, like density-density autocorrelations and the autocorrelation of internal degrees of freedom. We find an aging scenario particularly interesting in the case of the density autocorrelations in real space which is very reminiscent of spin glass phenomenology. While this model captures the essential features of structural glass dynamics, its analogy with spin glasses may bring the possibility of its complete description using the tools developed in spin glass theory.

cond-mat.dis-nn↗

arXiv1999

Violation of the Fluctuation-Dissipation Theorem in a Two-Dimensional Ising Model with Dipolar Interactions

The violation of the Fluctuation-Dissipation Theorem (FDT) in a two-dimensional Ising model with both ferromagnetic exchange and antiferromagnetic dipolar interactions is established and investigated via Monte Carlo simulations. Through the computation of the autocorrelation C(t+t_w,t_w) and the integrated response (susceptibility) functions we obtain the FDT violation factor X(t+t_w,t_w) for different values of the temperature, the waiting time t_w and the quotient delta=J_0/J_d, J_0 and J_d being the strength of exchange and dipolar interactions respectively. For positive values of delta this system develops a striped phase at low temperatures, in which the non-equilibrium dynamics presents two different regimes. Our results show that such different regimes are not reflected in the FDT violation factor, where X goes always to zero for high values of t_w in the aging regime, a result that appears in domain growth processes in non-frustrated ordered systems.

cond-mat.dis-nn↗

arXiv1996

Aging in Models of Non-linear Diffusion

We show that for a family of problems described by non-linear diffusion equations an exact calculation of the two time correlation function gives C(t,t')=f(t-t')g(t'), t>t', exhibiting normal and anomalous diffusions, as well as aging effects, depending on the degree of non-linearity. We discuss also the form in which FDT is violated in this class of systems. Finally we argue that in this type of models aging may be consequence of the non conservation of the "total mass".

cond-mat↗

arXiv1996

Heisenberg Spin Glass on a Hypercubic Cell

We present results of a Monte Carlo simulation of an Heisenberg Spin Glass model on a hipercubic cell of size 2 in {\it D} dimensions. Each spin interacts with {\it D} nearest neighbors and the lattice is expected to recover the completely connected (mean field) limit as $D\rightarrow \infty$. An analysis of the Binder parameter for $D=8, 9$ and $10$ shows clear evidence of the presence of a spin glass phase at low temperatures. We found that in the high temperature regime the inverse spin glass susceptibility grows linearly with $T^2$ as in the mean field case. Estimates of $T_c$ from the high temperature data are in very good agreement with the results of a Bethe-Peierls approximation for an Heisenberg Spin Glass with coordination number {\it D}.

cond-mat↗

arXiv1995

Generalized Simulated Annealing

We propose a new stochastic algorithm (generalized simulated annealing) for computationally finding the global minimum of a given (not necessarily convex) energy/cost function defined in a continuous D-dimensional space. This algorithm recovers, as particular cases, the so called classical ("Boltzmann machine") and fast ("Cauchy machine") simulated annealings, and can be quicker than both. Key-words: simulated annealing; nonconvex optimization; gradient descent; generalized statistical mechanics.

cond-mat↗