224 research outputs found
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 . An analysis
of the Binder parameter for and 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
as in the mean field case. Estimates of 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}.Comment: 6 pages and 4 figures, also available at
http://chimera.roma1.infn.it/index_papers_complex.htm
Coarse grained models of stripe forming systems: phase diagrams, anomalies and scaling hypothesis
Two coarse-grained models which capture some universal characteristics of
stripe forming systems are stud- ied. At high temperatures, the structure
factors of both models attain their maxima on a circle in reciprocal space, as
a consequence of generic isotropic competing interactions. Although this is
known to lead to some universal properties, we show that the phase diagrams
have important differences, which are a consequence of the particular k
dependence of the fluctuation spectrum in each model. The phase diagrams are
computed in a mean field approximation and also after inclusion of small
fluctuations, which are shown to modify drastically the mean field behavior.
Observables like the modulation length and magnetization profiles are computed
for the whole temperature range accessible to both models and some important
differences in behavior are observed. A stripe compression modulus is computed,
showing an anomalous behavior with temperature as recently reported in related
models. Also, a recently proposed scaling hypothesis for modulated systems is
tested and found to be valid for both models studied.Comment: 9 pages, 13 figure
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.Comment: Phys. Rev. E (Rapid Communication), 1999 (probably May
Index statistical properties of sparse random graphs
Using the replica method, we develop an analytical approach to compute the
characteristic function for the probability that a
large adjacency matrix of sparse random graphs has eigenvalues
below a threshold . The method allows to determine, in principle, all
moments of , from which the typical sample to sample
fluctuations can be fully characterized. For random graph models with localized
eigenvectors, we show that the index variance scales linearly with
for , with a model-dependent prefactor that can be exactly
calculated. Explicit results are discussed for Erd\"os-R\'enyi and regular
random graphs, both exhibiting a prefactor with a non-monotonic behavior as a
function of . These results contrast with rotationally invariant
random matrices, where the index variance scales only as , with an
universal prefactor that is independent of . Numerical diagonalization
results confirm the exactness of our approach and, in addition, strongly
support the Gaussian nature of the index fluctuations.Comment: 10 pages, 5 figure
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
Nature of Long-Range Order in Stripe-Forming Systems with Long-Range Repulsive Interactions
We study two dimensional stripe forming systems with competing repulsive
interactions decaying as . We derive an effective Hamiltonian with
a short range part and a generalized dipolar interaction which depends on the
exponent . An approximate map of this model to a known XY model with
dipolar interactions allows us to conclude that, for long range
orientational order of stripes can exist in two dimensions, and establish the
universality class of the models. When no long-range order is
possible, but a phase transition in the KT universality class is still present.
These two different critical scenarios should be observed in experimentally
relevant two dimensional systems like electronic liquids () and
dipolar magnetic films (). Results from Langevin simulations of
Coulomb and dipolar systems give support to the theoretical results.Comment: 5 pages, 2 figures. Supplemental Material include
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