28 research outputs found
Long Range Anticorrelations and Non-Gaussian Behavior of a Leaky Faucet
We find that intervals between successive drops from a leaky faucet display
scale-invariant, long-range anticorrelations characterized by the same
exponents of heart beat-to-beat intervals of healthy subjects. This behavior is
also confirmed by numerical simulations on lattice and it is faucet-width- and
flow-rate-independent. The histogram for the drop intervals is also well
described by a L\'evy distribution with the same index for both histograms of
healthy and diseased subjects. This additional result corroborates the evidence
for similarities between leaky faucets and healthy hearts underlying dynamics.Comment: Self-extracting uuencoded postscript file. Phys.Rev.E (Rap.Comm.).
Related papers can be found at http://www.if.uff.br/~tjpp/tjppe.htm
Flat histogram simulation of lattice polymer systems
We demonstrate the use of a new algorithm called the Flat Histogram sampling
algorithm for the simulation of lattice polymer systems. Thermodynamics
properties, such as average energy or entropy and other physical quantities
such as end-to-end distance or radius of gyration can be easily calculated
using this method. Ground-state energy can also be determined. We also explore
the accuracy and limitations of this method.
Key words: Monte Carlo algorithms, flat histogram sampling, HP model, lattice
polymer systemsComment: 7 RevTeX two-column page
Bit-String Models for Parasex
We present different bit-string models of haploid asexual populations in
which individuals may exchange part of their genome with other individuals
(parasex) according to a given probability. We study the advantages of this
parasex concerning population sizes, genetic fitness and diversity. We find
that the exchange of genomes always improves these features.Comment: 12 pages including 7 figure
Fragmentation Experiment and Model for Falling Mercury Drops
The experiment consists of counting and measuring the size of the many
fragments observed after the fall of a mercury drop on the floor. The size
distribution follows a power-law for large enough fragments. We address the
question of a possible crossover to a second, different power-law for small
enough fragments. Two series of experiments were performed. The first uses a
traditional film photographic camera, and the picture is later treated on a
computer in order to count the fragments and classify them according to their
sizes. The second uses a modern digital camera. The first approach has the
advantage of a better resolution for small fragment sizes. The second, although
with a poorer size resolution, is more reliable concerning the counting of all
fragments up to its resolution limit. Both together clearly indicate the real
existence of the quoted crossover.
The model treats the system microscopically during the tiny time interval
when the initial drop collides with the floor. The drop is modelled by a
connected cluster of Ising spins pointing up (mercury) surrounded by Ising
spins pointing down (air). The Ising coupling which tends to keep the spins
segregated represents the surface tension. Initially the cluster carries an
extra energy equally shared among all its spins, corresponding to the coherent
kinetic energy due to the fall. Each spin which touches the floor loses its
extra energy transformed into a thermal, incoherent energy represented by a
temperature used then to follow the dynamics through Monte Carlo simulations.
Whenever a small piece becomes disconnected from the big cluster, it is
considered a fragment, and counted. The results also indicate the existence of
the quoted crossover in the fragment-size distribution.Comment: 6 pages, 3 figure
Critical Exponents for Nuclear Multifragmentation: dynamical lattice model
We present a dynamical and dissipative lattice model, designed to mimic
nuclear multifragmentation. Monte-Carlo simulations with this model show clear
signature of critical behaviour and reproduce experimentally observed
correlations. In particular, using techniques devised for finite systems, we
could obtain two of its critical exponents, whose values are in agreement with
those of the universality class to which nuclear multifragmentation is supposed
to belong.Comment: 10 pages, 3 figures, to be published in Nuclear Physics
Monte Carlo Simulations of Sexual Reproduction
Modifying the Redfield model of sexual reproduction and the Penna model of
biological aging, we compare reproduction with and without recombination in
age-structured populations. In contrast to Redfield and in agreement with
Bernardes we find sexual reproduction to be preferred to asexual one. In
particular, the presence of old but still reproducing males helps the survival
of younger females beyond their reproductive age.Comment: 8 pages, plain tex, 7 EPS figures, to appear in PHYSICA
Determining the density of states for classical statistical models: A random walk algorithm to produce a flat histogram
We describe an efficient Monte Carlo algorithm using a random walk in energy
space to obtain a very accurate estimate of the density of states for classical
statistical models. The density of states is modified at each step when the
energy level is visited to produce a flat histogram. By carefully controlling
the modification factor, we allow the density of states to converge to the true
value very quickly, even for large systems. This algorithm is especially useful
for complex systems with a rough landscape since all possible energy levels are
visited with the same probability. In this paper, we apply our algorithm to
both 1st and 2nd order phase transitions to demonstrate its efficiency and
accuracy. We obtained direct simulational estimates for the density of states
for two-dimensional ten-state Potts models on lattices up to
and Ising models on lattices up to . Applying this approach to
a 3D spin glass model we estimate the internal energy and entropy at
zero temperature; and, using a two-dimensional random walk in energy and
order-parameter space, we obtain the (rough) canonical distribution and energy
landscape in order-parameter space. Preliminary data suggest that the glass
transition temperature is about 1.2 and that better estimates can be obtained
with more extensive application of the method.Comment: 22 pages (figures included