344 research outputs found
Real space application of the mean-field description of spin glass dynamics
The out of equilibrium dynamics of finite dimensional spin glasses is
considered from a point of view going beyond the standard `mean-field theory'
versus `droplet picture' debate of the last decades. The main predictions of
both theories concerning the spin glass dynamics are discussed. It is shown, in
particular, that predictions originating from mean-field ideas concerning the
violations of the fluctuation-dissipation theorem apply quantitatively,
provided one properly takes into account the role of the spin glass coherence
length which plays a central role in the droplet picture. Dynamics in a uniform
magnetic field is also briefly discussed.Comment: 4 pages, 4 eps figures. v2: published versio
Critical phenomena in a highly constrained classical spin system: Neel ordering from the Coulomb phase
Many classical, geometrically frustrated antiferromagnets have
macroscopically degenerate ground states. In a class of three-dimensional
systems, the set of degenerate ground states has power-law correlations and is
an example of a Coulomb phase. We investigate Neel ordering from such a Coulomb
phase, induced by weak additional interactions that lift the degeneracy. We
show that the critical point belongs to a universality class that is different
from the one for the equivalent transition out of the paramagnetic phase, and
that it is characterised by effective long-range interactions; alternatively,
ordering may be discontinuous. We suggest that a transition of this type may be
realised by applying uniaxial stress to a pyrochlore antiferromagnet.Comment: 4 pages, 3 figure
Universal Finite Size Scaling Functions in the 3D Ising Spin Glass
We study the three-dimensional Edwards-Anderson model with binary
interactions by Monte Carlo simulations. Direct evidence of finite-size scaling
is provided, and the universal finite-size scaling functions are determined.
Monte Carlo data are extrapolated to infinite volume with an iterative
procedure up to correlation lengths xi \approx 140. The infinite volume data
are consistent with a conventional power law singularity at finite temperature
Tc. Taking into account corrections to scaling, we find Tc = 1.156 +/- 0.015,
nu = 1.8 +/- 0.2 and eta = -0.26 +/- 0.04. The data are also consistent with an
exponential singularity at finite Tc, but not with an exponential singularity
at zero temperature.Comment: 4 pages, Revtex, 4 postscript figures include
The Glassy Potts Model
We introduce a Potts model with quenched, frustrated disorder, that enjoys of
a gauge symmetry that forbids spontaneous magnetization, and allows the glassy
phase to extend from down to T=0. We study numerical the 4 dimensional
model with states. We show the existence of a glassy phase, and we
characterize it by studying the probability distributions of an order
parameter, the binder cumulant and the divergence of the overlap
susceptibility. We show that the dynamical behavior of the system is
characterized by aging.Comment: 4 pages including 4 (color) ps figures (all on page 4
Nature of the Spin-glass State in the Three-dimensional Gauge Glass
We present results from simulations of the gauge glass model in three
dimensions using the parallel tempering Monte Carlo technique. Critical
fluctuations should not affect the data since we equilibrate down to low
temperatures, for moderate sizes. Our results are qualitatively consistent with
earlier work on the three and four dimensional Edwards-Anderson Ising spin
glass. We find that large scale excitations cost only a finite amount of energy
in the thermodynamic limit, and that those excitations have a surface whose
fractal dimension is less than the space dimension, consistent with a scenario
proposed by Krzakala and Martin, and Palassini and Young.Comment: 5 pages, 7 figure
How the Replica-Symmetry-Breaking Transition Looks Like in Finite-Size Simulations
Finite-size effects in the mean-field Ising spin glass and the mean-field
three-state Potts glass are investigated by Monte Carlo simulations. In the
thermodynamic limit, each model is known to exhibit a continuous phase
transition into the ordered state with a full and a one-step replica-symmetry
breaking (RSB), respectively. In the Ising case, Binder parameter g calculated
for various finite sizes remains positive at any temperature and crosses at the
transition point, while in the Potts case g develops a negative dip without
showing a crossing in the g>0 region. By contrast, non-self averaging
parameters always remain positive and show a clear crossing at the transition
temperature in both cases. Our finding suggests that care should be taken in
interpreting the numerical data of the Binder parameter, particularly when the
system exhibits a one-step-like RSB.Comment: 7 pages, 8 figure
Continuous phase transition in a spin-glass model without time-reversal symmetry
We investigate the phase transition in a strongly disordered short-range
three-spin interaction model characterized by the absence of time reversal
symmetry in the Hamiltonian. In the mean-field limit the model is well
described by the Adam-Gibbs-DiMarzio scenario for the glass transition; however
in the short-range case this picture turns out to be modified. The model
presents a finite temperature continuous phase transition characterized by a
divergent spin-glass susceptibility and a negative specific heat exponent. We
expect the nature of the transition in this 3-spin model to be the same as the
transition in the Edwards-Anderson model in a magnetic field, with the
advantage that the strong crossover effects present in the latter case are
absent.Comment: 19 pages, Latex, 16 ps figure
Localization of Denaturation Bubbles in Random DNA Sequences
We study the thermodynamic and dynamic behaviors of twist-induced
denaturation bubbles in a long, stretched random sequence of DNA. The small
bubbles associated with weak twist are delocalized. Above a threshold torque,
the bubbles of several tens of bases or larger become preferentially localized
to \AT-rich segments. In the localized regime, the bubbles exhibit ``aging''
and move around sub-diffusively with continuously varying dynamic exponents.
These properties are derived using results of large-deviation theory together
with scaling arguments, and are verified by Monte-Carlo simulations.Comment: TeX file with postscript figure
Classical and Quantum Behavior in Mean-Field Glassy Systems
In this talk I review some recent developments which shed light on the main
connections between structural glasses and mean-field spin glass models with a
discontinuous transition. I also discuss the role of quantum fluctuations on
the dynamical instability found in mean-field spin glasses with a discontinuous
transition. In mean-field models with pairwise interactions in a transverse
field it is shown, in the framework of the static approximation, that such
instability is suppressed at zero temperature.Comment: 9 Pages (including 5 Figures), Revtex, Proceedings of the XIV Sitges
Conference, June 1996 (Barcelona) Spai
Evidence for the double degeneracy of the ground-state in the 3D spin glass
A bivariate version of the multicanonical Monte Carlo method and its
application to the simulation of the three-dimensional Ising spin glass
are described. We found the autocorrelation time associated with this
particular multicanonical method was approximately proportional to the system
volume, which is a great improvement over previous methods applied to
spin-glass simulations. The principal advantage of this version of the
multicanonical method, however, was its ability to access information
predictive of low-temperature behavior. At low temperatures we found results on
the three-dimensional Ising spin glass consistent with a double
degeneracy of the ground-state: the order-parameter distribution function
converged to two delta-function peaks and the Binder parameter
approached unity as the system size was increased. With the same density of
states used to compute these properties at low temperature, we found their
behavior changing as the temperature is increased towards the spin glass
transition temperature. Just below this temperature, the behavior is consistent
with the standard mean-field picture that has an infinitely degenerate ground
state. Using the concept of zero-energy droplets, we also discuss the structure
of the ground-state degeneracy. The size distribution of the zero-energy
droplets was found to produce the two delta-function peaks of .Comment: 33 pages with 31 eps figures include
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