2,299 research outputs found
Distribution of averages in a correlated Gaussian medium as a tool for the estimation of the cluster distribution on size
Calculation of the distribution of the average value of a Gaussian random
field in a finite domain is carried out for different cases. The results of the
calculation demonstrate a strong dependence of the width of the distribution on
the spatial correlations of the field. Comparison with the simulation results
for the distribution of the size of the cluster indicates that the distribution
of an average field could serve as a useful tool for the estimation of the
asymptotic behavior of the distribution of the size of the clusters for "deep"
clusters where value of the field on each site is much greater than the rms
disorder.Comment: 15 pages, 6 figures, RevTe
Droplets in the coexistence region of the two-dimensional Ising model
The two-dimensional Ising model with fixed magnetization is studied using
Monte Carlo techniques. At the coexistence line, the macroscopic, extensive
droplet of minority spins becomes thermally unstable by breaking up into
microscopic clusters. Intriguing finite--size effects as well as singularities
of thermal and cluster properties associated with the transition are discussed.Comment: 7 pages, 3 figures included, submitted to J. Phys. A: Math. Ge
Percolation-to-hopping crossover in conductor-insulator composites
Here, we show that the conductivity of conductor-insulator composites in
which electrons can tunnel from each conducting particle to all others may
display both percolation and tunneling (i.e. hopping) regimes depending on few
characteristics of the composite. Specifically, we find that the relevant
parameters that give rise to one regime or the other are (where is
the size of the conducting particles and is the tunneling length) and the
specific composite microstructure. For large values of , percolation
arises when the composite microstructure can be modeled as a regular lattice
that is fractionally occupied by conducting particle, while the tunneling
regime is always obtained for equilibrium distributions of conducting particles
in a continuum insulating matrix. As decreases the percolating behavior
of the conductivity of lattice-like composites gradually crosses over to the
tunneling-like regime characterizing particle dispersions in the continuum. For
values lower than the conductivity has tunneling-like
behavior independent of the specific microstructure of the composite.Comment: 8 pages, 5 figure
Non-Markovian Persistence and Nonequilibrium Critical Dynamics
The persistence exponent \theta for the global order parameter, M(t), of a
system quenched from the disordered phase to its critical point describes the
probability, p(t) \sim t^{-\theta}, that M(t) does not change sign in the time
interval t following the quench. We calculate \theta to O(\epsilon^2) for model
A of critical dynamics (and to order \epsilon for model C) and show that at
this order M(t) is a non-Markov process. Consequently, \theta is a new
exponent. The calculation is performed by expanding around a Markov process,
using a simplified version of the perturbation theory recently introduced by
Majumdar and Sire [Phys. Rev. Lett. _77_, 1420 (1996); cond-mat/9604151].Comment: 4 pages, Revtex, no figures, requires multicol.st
Persistence Probabilities of the German DAX and Shanghai Index
We present a relatively detailed analysis of the persistence probability
distributions in financial dynamics. Compared with the auto-correlation
function, the persistence probability distributions describe dynamic
correlations non-local in time. Universal and non-universal behaviors of the
German DAX and Shanghai Index are analyzed, and numerical simulations of some
microscopic models are also performed. Around the fixed point , the
interacting herding model produces the scaling behavior of the real markets
Connected component identification and cluster update on GPU
Cluster identification tasks occur in a multitude of contexts in physics and
engineering such as, for instance, cluster algorithms for simulating spin
models, percolation simulations, segmentation problems in image processing, or
network analysis. While it has been shown that graphics processing units (GPUs)
can result in speedups of two to three orders of magnitude as compared to
serial codes on CPUs for the case of local and thus naturally parallelized
problems such as single-spin flip update simulations of spin models, the
situation is considerably more complicated for the non-local problem of cluster
or connected component identification. I discuss the suitability of different
approaches of parallelization of cluster labeling and cluster update algorithms
for calculations on GPU and compare to the performance of serial
implementations.Comment: 15 pages, 14 figures, one table, submitted to PR
Lattice dynamics of mixed semiconductors (Be,Zn)Se from first-principles calculations
Vibration properties of Zn(1-x)Be(x)Se, a mixed II-VI semiconductor
haracterized by a high contrast in elastic properties of its pure constituents,
ZnSe and BeSe, are simulated by first-principles calculations of electronic
structure, lattice relaxation and frozen phonons. The calculations within the
local density approximation has been done with the Siesta method, using
norm-conserving pseudopotentials and localized basis functions; the benchmark
calculations for pure endsystems were moreover done also by all-electron WIEN2k
code. An immediate motivation for the study was to analyze, at the microscopic
level, the appearance of anomalous phonon modes early detected in Raman spectra
in the intermediate region (20 to 80%) of ZnBe concentration. This was early
discussed on the basis of a percolation phenomenon, i.e., the result of the
formation of wall-to-wall --Be--Se-- chains throughout the crystal. The
presence of such chains was explicitly allowed in our simulation and indeed
brought about a softening and splitting off of particular modes, in accordance
with experimental observation, due to a relative elongation of Be--Se bonds
along the chain as compared to those involving isolated Be atoms. The variation
of force constants with interatomic distances shows common trends in relative
independence on the short-range order.Comment: 11 pages, 10 figures, to be published in Phys. Rev.
Critical exponents and phase transition in gold nuclei fragmentation at energies 10.6 and 4.0 GeV/nucleon
An attempt to extract critical exponents gamma, beta and tau from data on
gold nuclei fragmentation due to interactions with nuclear emulsion at energies
4.0 A GeV and 10.6 A GeV is presented. Based on analysis of Campi's 2nd charge
moments, two subsets of data at each energy are selected from the inclusive
data, corresponding to 'liquid' and 'gas' phases. The extracted values of
critical exponents from the selected data sets are in agreement with
predictions of 'liquid-gas' model of phase transition.Comment: 21 pages, 15 figure
High-temperature series for the bond-diluted Ising model in 3, 4 and 5 dimensions
In order to study the influence of quenched disorder on second-order phase
transitions, high-temperature series expansions of the \sus and the free energy
are obtained for the quenched bond-diluted Ising model in --5
dimensions. They are analysed using different extrapolation methods tailored to
the expected singularity behaviours. In and 5 dimensions we confirm
that the critical behaviour is governed by the pure fixed point up to dilutions
near the geometric bond percolation threshold. The existence and form of
logarithmic corrections for the pure Ising model in is confirmed and
our results for the critical behaviour of the diluted system are in agreement
with the type of singularity predicted by renormalization group considerations.
In three dimensions we find large crossover effects between the pure Ising,
percolation and random fixed point. We estimate the critical exponent of the
\sus to be at the random fixed point.Comment: 16 pages, 10 figure
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