2,739 research outputs found
Comment on "Ising Spin Glasses in a Magnetic Field"
In ref. cond-mat/9811419 Houdayer and Martin analyze the T=0 3d EA spin glass
with a magnetic field . By using a new, powerful method, they determine an
effective critical field as a function of the lattice size . They use
their results to deduce that the model is behaving like in the droplet approach
and not like the mean-field theory. We show here, by using some unpublished
data, that this very interesting method and numerical results are completely
compatible with the behavior implied by the Replica Symmetry Breaking theory.Comment: One page comment about ref. cond-mat/9811419, including two eps
figure
Comment on ``Triviality of the Ground State Structure in Ising Spin Glasses''
We show that the evidence of cond-mat/9906323 does not discriminate among
droplet model and mean field like behavior.Comment: 1 page comment with two .ps figures included. Rewritten version, one
error correcte
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
Reply to Comment on "Ising Spin Glasses in a Magnetic Field"
The problem of the survival of a spin glass phase in the presence of a field
has been a challenging one for a long time. To date, all attempts using
equilibrium Monte Carlo methods have been unconclusive. In their comment to our
paper, Marinari, Parisi and Zuliani use out-of-equilibrium measurements to test
for an Almeida-Thouless line. In our view such a dynamic approach is not based
on very solid foundations in finite dimensional systems and so cannot be as
compelling as equilibrium approaches. Nevertheless, the results of those
authors suggests that there is a critical field near B=0.4 at zero temperature.
In view of this quite small value (compared to the mean field value), we have
reanalyzed our data. We find that if finite size scaling is to distinguish
between that small field and a zero field, we would need to go to lattice sizes
of about 20x20x20.Comment: reply to comment cond-mat/9812401 on ref. cond-mat/981141
A Quantitative Clustering Approach to Ultrametricity in Spin Glasses
We discuss the problem of ultrametricity in mean field spin glasses by means
of a hierarchical clustering algorithm. We complement the clustering approach
with quantitative testing: we discuss both in some detail. We show that the
elimination of the (in this context accidental) spin flip symmetry plays a
crucial role in the analysis, since the symmetry hides the real nature of the
data. We are able to use in the analysis disorder averaged quantities. We are
able to exhibit a number of features of the low phase of the mean field
theory, and to claim that the full hierarchical structure can be observed
without ambiguities only on very large lattice volumes, not currently
accessible by numerical simulations.Comment: 15 pages with color figure
Numerical evidences of universal trap-like aging dynamics
Trap models have been initially proposed as toy models for dynamical
relaxation in extremely simplified rough potential energy landscapes. Their
importance has considerably grown recently thanks to the discovery that the
trap like aging mechanism is directly controlling the out-of-equilibrium
relaxation processes of more sophisticated spin models, that are considered as
the solvable counterpart of real disordered systems. Establishing on a firmer
ground the connection between these spin model out-of-equilibrium behavior and
the trap like aging mechanism would shed new light on the properties, still
largely mysterious, of the activated out-of-equilibrium dynamics of disordered
systems. In this work we discuss numerical evidences of emergent trap-like
aging behavior in a variety of disordered models. Our numerical results are
backed by analytic derivations and heuristic discussions. Such exploration
reveals some of the tricks needed to analyze the trap behavior in spite of the
occurrence of secondary processes, of the existence of dynamical correlations
and of finite system's size effects.Comment: 25 pages, 15 figure
Spontaneous energy-barrier formation in entropy-driven glassy dynamics
The description of activated relaxation of glassy systems in the multidimensional configurational space is a
long-standing open problem. We develop a phenomenological description of the out-of-equilibrium dynamics of
a model with a rough potential energy landscape and we analyze it both numerically and analytically. The model
provides an example of dynamics where typical relaxation channels go over finite-potential energy barriers despite
the presence of less-energy-demanding escaping paths in configurational space; we expect this phenomenon to
be also relevant in the thermally activated regime of realistic models of glass-formers. In this case, we found that
typical dynamical paths episodically reach an high-fixed-threshold energy, unexpectedly giving rise to a simple
thermally activated aging phenomenology. In order to unveil this peculiar aging behavior, we introduce a novel
description of the dynamics in terms of spontaneously emerging dynamical basins
The String Tension in Gauge Theories: a Suggestion for a New Measurement Method
We discuss a new method for testing confinement and measuring the string
tension (in the Coulomb gauge). Our numerical simulations demonstrate that the
problems related to Gribov copies are not harmful and that the method is
effective in the case of pure gauge Q.C.D.. We discuss the relevance of the
method for studying gauge theories coupled to fermionic matter.Comment: 9 pages, Latex style, 4 postscript figures appended, ROMA 92-90
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