1,124 research outputs found
Thermostating by Deterministic Scattering: Heat and Shear Flow
We apply a recently proposed novel thermostating mechanism to an interacting
many-particle system where the bulk particles are moving according to
Hamiltonian dynamics. At the boundaries the system is thermalized by
deterministic and time-reversible scattering. We show how this scattering
mechanism can be related to stochastic boundary conditions. We subsequently
simulate nonequilibrium steady states associated to thermal conduction and
shear flow for a hard disk fluid. The bulk behavior of the model is studied by
comparing the transport coefficients obtained from computer simulations to
theoretical results. Furthermore, thermodynamic entropy production and
exponential phase-space contraction rates in the stationary nonequilibrium
states are calculated showing that in general these quantities do not agree.Comment: 16 pages (revtex) with 9 figures (postscript
Predicting the coherence resonance curve using a semi-analytical treatment
Emergence of noise induced regularity or Coherence Resonance in nonlinear
excitable systems is well known. We explain theoretically why the normalized
variance () of inter spike time intervals, which is a measure of
regularity in such systems, has a unimodal profile. Our semi-analytic treatment
of the associated spiking process produces a general yet simple formula for
, which we show is in very good agreement with numerics in two test
cases, namely the FitzHugh-Nagumo model and the Chemical Oscillator model.Comment: 5 pages, 5 figure
Stochastic Resonance in Two Dimensional Landau Ginzburg Equation
We study the mechanism of stochastic resonance in a two dimensional Landau
Ginzburg equation perturbed by a white noise. We shortly review how to
renormalize the equation in order to avoid ultraviolet divergences. Next we
show that the renormalization amplifies the effect of the small periodic
perturbation in the system. We finally argue that stochastic resonance can be
used to highlight the effect of renormalization in spatially extended system
with a bistable equilibria
Quasi Markovian behavior in mixing maps
We consider the time dependent probability distribution of a coarse grained
observable Y whose evolution is governed by a discrete time map. If the map is
mixing, the time dependent one-step transition probabilities converge in the
long time limit to yield an ergodic stochastic matrix. The stationary
distribution of this matrix is identical to the asymptotic distribution of Y
under the exact dynamics. The nth time iterate of the baker map is explicitly
computed and used to compare the time evolution of the occupation probabilities
with those of the approximating Markov chain. The convergence is found to be at
least exponentially fast for all rectangular partitions with Lebesgue measure.
In particular, uniform rectangles form a Markov partition for which we find
exact agreement.Comment: 16 pages, 1 figure, uses elsart.sty, to be published in Physica D
Special Issue on Predictability: Quantifying Uncertainty in Models of Complex
Phenomen
Diversity-induced resonance
We present conclusive evidence showing that different sources of diversity,
such as those represented by quenched disorder or noise, can induce a resonant
collective behavior in an ensemble of coupled bistable or excitable systems.
Our analytical and numerical results show that when such systems are subjected
to an external subthreshold signal, their response is optimized for an
intermediate value of the diversity. These findings show that intrinsic
diversity might have a constructive role and suggest that natural systems might
profit from their diversity in order to optimize the response to an external
stimulus.Comment: 4 pages, 3 figure
Gravitational Waves from Warped Spacetime
We argue that the RSI model can provide a strong signature in gravitational
waves. This signal is a relic stochastic background generated during the
cosmological phase transition from an AdS-Schwarschild phase to the RS1
geometry that should occur at a temperature in the TeV range. We estimate the
amplitude of the signal in terms of the parameters of the potential stabilizing
the radion and show that over much of the parameter region in which the phase
transition completes, a signal should be detectable at the planned space
interferometer, LISA.Comment: 18 pages, 15 figures; v2: discussion improved, in particular on the
justification of the thick wall approximation. 6 figures added. 4 pi factor
corrected in perturbativity bound. N-dependence displayed. Conclusions
unchanged. JHEP versio
Stochastic resonance in periodic potentials: realization in a dissipative optical lattice
We have observed the phenomenon of stochastic resonance on the Brillouin
propagation modes of a dissipative optical lattice. Such a mode has been
excited by applying a moving potential modulation with phase velocity equal to
the velocity of the mode. Its amplitude has been characterized by the
center-of-mass (CM) velocity of the atomic cloud. At Brillouin resonance, we
studied the CM-velocity as a function of the optical pumping rate at a given
depth of the potential wells. We have observed a resonant dependence of the CM
velocity on the optical pumping rate, corresponding to the noise strength. This
corresponds to the experimental observation of stochastic resonance in a
periodic potential in the low-damping regime
Null energy condition and superluminal propagation
We study whether a violation of the null energy condition necessarily implies
the presence of instabilities. We prove that this is the case in a large class
of situations, including isotropic solids and fluids relevant for cosmology. On
the other hand we present several counter-examples of consistent effective
field theories possessing a stable background where the null energy condition
is violated. Two necessary features of these counter-examples are the lack of
isotropy of the background and the presence of superluminal modes. We argue
that many of the properties of massive gravity can be understood by associating
it to a solid at the edge of violating the null energy condition. We briefly
analyze the difficulties of mimicking in scalar tensor theories of
gravity.Comment: 46 pages, 6 figure
Brans-Dicke DGP Brane Cosmology
We consider a five dimensional DGP-brane scenario endowed with a
non-minimally coupled scalar field within the context of Brans-Dicke theory.
This theory predicts that the mass appearing in the gravitational potential is
modified by the addition of the mass of the effective intrinsic curvature on
the brane. We also derive the effective four dimensional field equations on a
3+1 dimensional brane where the fifth dimension is assumed to have an orbifold
symmetry. Finally, we discuss the cosmological implications of this setup,
predicting an accelerated expanding universe with a value of the Brans-Dicke
parameter consistent with values resulting from the solar system
observations.Comment: 12 pages, 1 figure, to appear in JCA
Coherence Resonance in Chaotic Systems
We show that it is possible for chaotic systems to display the main features
of coherence resonance. In particular, we show that a Chua model, operating in
a chaotic regime and in the presence of noise, can exhibit oscillations whose
regularity is optimal for some intermediate value of the noise intensity. We
find that the power spectrum of the signal develops a peak at finite frequency
at intermediate values of the noise. These are all signatures of coherence
resonance. We also experimentally study a Chua circuit and corroborate the
above simulation results. Finally, we analyze a simple model composed of two
separate limit cycles which still exhibits coherence resonance, and show that
its behavior is qualitatively similar to that of the chaotic Chua systemComment: 4 pages (including 4 figures) LaTeX fil
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