4,979 research outputs found
Bifurcations and Chaos in Time Delayed Piecewise Linear Dynamical Systems
We reinvestigate the dynamical behavior of a first order scalar nonlinear
delay differential equation with piecewise linearity and identify several
interesting features in the nature of bifurcations and chaos associated with it
as a function of the delay time and external forcing parameters. In particular,
we point out that the fixed point solution exhibits a stability island in the
two parameter space of time delay and strength of nonlinearity. Significant
role played by transients in attaining steady state solutions is pointed out.
Various routes to chaos and existence of hyperchaos even for low values of time
delay which is evidenced by multiple positive Lyapunov exponents are brought
out. The study is extended to the case of two coupled systems, one with delay
and the other one without delay.Comment: 34 Pages, 14 Figure
Multistability in nonlinear left-handed transmission lines
Employing a nonlinear left-handed transmission line as a model system, we
demonstrate experimentally the multi-stability phenomena predicted
theoretically for microstructured left-handed metamaterials with a nonlinear
response. We show that the bistability is associated with the period doubling
which at higher power may result in chaotic dynamics of the transmission line
Triggering up states in all-to-all coupled neurons
Slow-wave sleep in mammalians is characterized by a change of large-scale
cortical activity currently paraphrased as cortical Up/Down states. A recent
experiment demonstrated a bistable collective behaviour in ferret slices, with
the remarkable property that the Up states can be switched on and off with
pulses, or excitations, of same polarity; whereby the effect of the second
pulse significantly depends on the time interval between the pulses. Here we
present a simple time discrete model of a neural network that exhibits this
type of behaviour, as well as quantitatively reproduces the time-dependence
found in the experiments.Comment: epl Europhysics Letters, accepted (2010
Do columnar defects produce bulk pinning?
From magneto-optical imaging performed on heavy-ion irradiated YBaCuO single
crystals, it is found that at fields and temperatures where strong single
vortex pinning by individual irradiation-induced amorphous columnar defects is
to be expected, vortex motion is limited by the nucleation of vortex kinks at
the specimen surface rather than by half-loop nucleation in the bulk. In the
material bulk, vortex motion occurs through (easy) kink sliding. Depinning in
the bulk determines the screening current only at fields comparable to or
larger than the matching field, at which the majority of moving vortices is not
trapped by an ion track.Comment: 5 pages, 5 figures, submitted to Physical Review Letter
Stellar Populations in the Phoenix Dwarf (dIrr/dSph) Galaxy as Observed by HST/WFPC2
We present HST/WFPC2 photometry of the central regions of the Phoenix dwarf.
Accurate photometry allows us to: 1) confirm the existence of the horizontal
branch previously detected by ground-based observations, and use it to
determine a distance to Phoenix, 2) clearly detect the existence of multiple
ages in the stellar population of Phoenix, 3) determine a mean metallicity of
the old red giant branch stars in Phoenix, and suggest that Phoenix has evolved
chemically over its lifetime, 4) extract a rough star formation history for the
central regions which suggests that Phoenix has been forming stars roughly
continuously over its entire lifetime.Comment: Accepted by AJ, 22 pages including 6 figures + 1 figure in JPEG
forma
Separation of suspended particles in microfluidic systems by directional-locking in periodic fields
We investigate the transport and separation of overdamped particles under the
action of a uniform external force in a two-dimensional periodic energy
landscape. Exact results are obtained for the deterministic transport in a
square lattice of parabolic, repulsive centers that correspond to a
piecewise-continuous linear-force model. The trajectories are periodic and
commensurate with the obstacle lattice and exhibit phase-locking behavior in
that the particle moves at the same average migration angle for a range of
orientation of the external force. The migration angle as a function of the
orientation of the external force has a Devil's staircase structure. The first
transition in the migration angle was analyzed in terms of a Poincare map,
showing that it corresponds to a tangent bifurcation. Numerical results show
that the limiting behavior for impenetrable obstacles is equivalent to the high
Peclet number limit in the case of transport of particles in a periodic pattern
of solid obstacles. Finally, we show how separation occurs in these systems
depending on the properties of the particles
Non-monotonic pseudo-gap in high-Tc cuprates
The mechanism of high temperature superconductivity is not resolved for so
long because the normal state of cuprates is not yet understood. Here we show
that the normal state pseudo-gap exhibits an unexpected non-monotonic
temperature dependence, which rules out the possibility to describe it by a
single mechanism such as superconducting phase fluctuations. Moreover, this
behaviour, being remarkably similar to the behaviour of the charge ordering gap
in the transition-metal dichalcogenides, completes the correspondence between
these two classes of compounds: the cuprates in the PG state and the
dichalcogenides in the incommensurate charge ordering state reveal virtually
identical spectra of one-particle excitations as function of energy, momentum
and temperature. These results suggest that the normal state pseudo-gap, which
was considered to be very peculiar to cuprates, seems to be a general complex
phenomenon for 2D metals. This may not only help to clarify the normal state
electronic structure of 2D metals but also provide new insight into electronic
properties of 2D solids where the metal-insulator and metal-superconductor
transitions are considered on similar basis as instabilities of particle-hole
and particle-particle interaction, respectively
Renormalization group theory for finite-size scaling in extreme statistics
We present a renormalization group (RG) approach to explain universal
features of extreme statistics, applied here to independent, identically
distributed variables. The outlines of the theory have been described in a
previous Letter, the main result being that finite-size shape corrections to
the limit distribution can be obtained from a linearization of the RG
transformation near a fixed point, leading to the computation of stable
perturbations as eigenfunctions. Here we show details of the RG theory which
exhibit remarkable similarities to the RG known in statistical physics. Besides
the fixed points explaining universality, and the least stable eigendirections
accounting for convergence rates and shape corrections, the similarities
include marginally stable perturbations which turn out to be generic for the
Fisher-Tippett-Gumbel class. Distribution functions containing unstable
perturbations are also considered. We find that, after a transitory divergence,
they return to the universal fixed line at the same or at a different point
depending on the type of perturbation.Comment: 15 pages, 8 figures, to appear in Phys. Rev.
Stochastic gain in population dynamics
We introduce an extension of the usual replicator dynamics to adaptive
learning rates. We show that a population with a dynamic learning rate can gain
an increased average payoff in transient phases and can also exploit external
noise, leading the system away from the Nash equilibrium, in a reasonance-like
fashion. The payoff versus noise curve resembles the signal to noise ratio
curve in stochastic resonance. Seen in this broad context, we introduce another
mechanism that exploits fluctuations in order to improve properties of the
system. Such a mechanism could be of particular interest in economic systems.Comment: accepted for publication in Phys. Rev. Let
Superconductor strip with transport current: Magneto-optical study of current distribution and its relaxation
The dynamics of magnetic flux distributions across a YBaCuO strip carrying
transport current is measured using magneto-optical imaging at 20 K. The
current is applied in pulses of 40-5000 ms duration and magnitude close to the
critical one, 5.5 A. During the pulse some extra flux usually penetrates the
strip, so the local field increases in magnitude. When the strip is initially
penetrated by flux, the local field either increases or decreases depending
both on the spatial coordinate and the current magnitude. Meanwhile, the
current density always tends to redistribute more uniformly. Despite the
relaxation, all distributions remain qualitatively similar to the Bean model
predictions.Comment: RevTeX, 9 pages, 9 figures, submitted to Supercond. Sci. Technol.
Revision: MO image and more refs are adde
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