82 research outputs found
A random walker on a ratchet potential: Effect of a non Gaussian noise
We analyze the effect of a colored non Gaussian noise on a model of a random
walker moving along a ratchet potential. Such a model was motivated by the
transport properties of motor proteins, like kinesin and myosin. Previous
studies have been realized assuming white noises. However, for real situations,
in general we could expect that those noises be correlated and non Gaussian.
Among other aspects, in addition to a maximum in the current as the noise
intensity is varied, we have also found another optimal value of the current
when departing from Gaussian behavior. We show the relevant effects that arise
when departing from Gaussian behavior, particularly related to current's
enhancement, and discuss its relevance for both biological and technological
situations.Comment: Submitted to Europ.Phys. J. B (LaTex, 16 pgs, 8 figures
Transition from anomalous to normal hysteresis in a system of coupled Brownian motors: a mean field approach
We address a recently introduced model describing a system of periodically
coupled nonlinear phase oscillators submitted to multiplicative white noises,
wherein a ratchet-like transport mechanism arises through a symmetry-breaking
noise-induced nonequilibrium phase transition. Numerical simulations of this
system reveal amazing novel features such as negative zero-bias conductance and
anomalous hysteresis, explained resorting to a strong-coupling analysis in the
thermodynamic limit. Using an explicit mean-field approximation we explore the
whole ordered phase finding a transition from anomalous to normal hysteresis
inside this phase, estimating its locus and identifying (within this scheme) a
mechanism whereby it takes place.Comment: RevTex, 21 pgs, 15 figures. Submited to Physical Review E (2000
Limit cycle induced by multiplicative noise in a system of coupled Brownian motors
We study a model consisting of nonlinear oscillators with {\em global
periodic} coupling and {\em local multiplicative} and additive noises. The
model was shown to undergo a nonequilibrium phase transition towards a
broken-symmetry phase exhibiting noise-induced "ratchet" behavior. A previous
study \cite{[7]} focused on the relationship between the character of
thehysteresis loop, the number of ``homogeneous'' mean-field solutions and the
shape of the stationary mean-field probability distribution function. Here we
show --as suggested by the absence of stable solutions when the load force is
beyond a critical value-- the existence of a limit cycle induced by
both:multiplicative noise and {\em global periodic} coupling.Comment: Submitted to Phys. Rev. E, RevTex, 18 pgs, 5 figure
Stochastic resonance between dissipative structures in a bistable noise-sustained dynamics
We study an extended system that without noise shows a monostable dynamics,
but when submitted to an adequate multiplicative noise, an effective bistable
dynamics arise. The stochastic resonance between the attractors of the
\textit{noise-sustained dynamics} is investigated theoretically in terms of a
two-state approximation. The knowledge of the exact nonequilibrium potential
allows us to obtain the output signal-to-noise ratio. Its maximum is predicted
in the symmetric case for which both attractors have the same nonequilibrium
potential value.Comment: RevTex, 13 pages, 6 figures, accepted in Physical Review
Flow cytometric analysis of DNA content in human ovarian cancers.
A total of 155 samples from 101 patients with ovarian cancer were investigated using flow cytometry to evaluate the DNA index and the percentage of cells in the various cell cycle phases. Thirty-four samples were DNA diploid tumours, while the other 121 were DNA aneuploid tumours. The DNA index was very stable in different sites and over time in the same patient. Tumour stage and ploidy were significantly associated: stages III and IV tumour stage were more likely to be DNA aneuploid. Patients with residual tumour size at first surgery greater than 2 cm had a significantly larger number of DNA aneuploid than DNA diploid tumours. The DNA index was also related to the degree of differentiation of the tumours. The percentage of cells in the S phase of the cell cycle was significantly higher in DNA aneuploid and in poorly differentiated tumours than DNA diploid and well differentiated tumours. Multivariate analysis using the Cox model showed that the DNA index and the percentage of cells in S phase were not independent prognostic variables in this study. Prospectively collected data should be accumulated before assigning the DNA index an important role as a biological prognostic factor in ovarian cancer
Nonequilibrium phase transitions induced by multiplicative noise: effects of self-correlation
A recently introduced lattice model, describing an extended system which
exhibits a reentrant (symmetry-breaking, second-order) noise-induced
nonequilibrium phase transition, is studied under the assumption that the
multiplicative noise leading to the transition is colored. Within an effective
Markovian approximation and a mean-field scheme it is found that when the
self-correlation time of the noise is different from zero, the transition is
also reentrant with respect to the spatial coupling D. In other words, at
variance with what one expects for equilibrium phase transitions, a large
enough value of D favors disorder. Moreover, except for a small region in the
parameter subspace determined by the noise intensity and D, an increase in the
self-correlation time usually preventsthe formation of an ordered state. These
effects are supported by numerical simulations.Comment: 15 pages. 9 figures. To appear in Phys.Rev.
Extending the definition of modularity to directed graphs with overlapping communities
Complex networks topologies present interesting and surprising properties,
such as community structures, which can be exploited to optimize communication,
to find new efficient and context-aware routing algorithms or simply to
understand the dynamics and meaning of relationships among nodes. Complex
networks are gaining more and more importance as a reference model and are a
powerful interpretation tool for many different kinds of natural, biological
and social networks, where directed relationships and contextual belonging of
nodes to many different communities is a matter of fact. This paper starts from
the definition of modularity function, given by M. Newman to evaluate the
goodness of network community decompositions, and extends it to the more
general case of directed graphs with overlapping community structures.
Interesting properties of the proposed extension are discussed, a method for
finding overlapping communities is proposed and results of its application to
benchmark case-studies are reported. We also propose a new dataset which could
be used as a reference benchmark for overlapping community structures
identification.Comment: 22 pages, 11 figure
Brownian motion exhibiting absolute negative mobility
We consider a single Brownian particle in a spatially symmetric, periodic
system far from thermal equilibrium. This setup can be readily realized
experimentally. Upon application of an external static force F, the average
particle velocity is negative for F>0 and positive for F<0 (absolute negative
mobility).Comment: 4 pages, 3 figures, to be published in PR
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