67 research outputs found
Static and Dynamic Correlations in Many-Particle Lyapunov Vectors
We introduce static and dynamic correlation functions for the spatial
densities of Lyapunov vector fluctuations. They enable us to show, for the
first time, the existence of hydrodynamic Lyapunov modes in chaotic
many-particle systems with soft core interactions, which indicates universality
of this phenomenon. Our investigations for Lennard-Jones fluids yield, in
addition to the Lyapunov exponent - wave vector dispersion, the collective
dynamic excitations of a given Lyapunov vector. In the limit of purely
translational modes the static and dynamic structure factor are recovered.Comment: 5 pages, 5 figure
Preisach models of hysteresis driven by Markovian input processes
We study the response of Preisach models of hysteresis to stochastically
fluctuating external fields. We perform numerical simulations which indicate
that analytical expressions derived previously for the autocorrelation
functions and power spectral densities of the Preisach model with uncorrelated
input, hold asymptotically also if the external field shows exponentially
decaying correlations. As a consequence, the mechanisms causing long-term
memory and 1/f-noise in Preisach models with uncorrelated inputs still apply in
the presence of fast decaying input correlations. We collect additional
evidence for the importance of the effective Preisach density previously
introduced even for Preisach models with correlated inputs. Additionally, we
present some new results for the output of the Preisach model with uncorrelated
input using analytical methods. It is found, for instance, that in order to
produce the same long-time tails in the output, the elementary hysteresis loops
of large width need to have a higher weight for the generic Preisach model than
for the symmetric Preisach model. Further, we find autocorrelation functions
and power spectral densities to be monotonically decreasing independently of
the choice of input and Preisach density
Lyapunov instabilities of Lennard-Jones fluids
Recent work on many particle system reveals the existence of regular
collective perturbations corresponding to the smallest positive Lyapunov
exponents (LEs), called hydrodynamic Lyapunov modes. Until now, however, these
modes are only found for hard core systems. Here we report new results on
Lyapunov spectra and Lyapunov vectors (LVs) for Lennard-Jones fluids. By
considering the Fourier transform of the coordinate fluctuation density
, it is found that the LVs with are
highly dominated by a few components with low wave-numbers.
These numerical results provide strong evidence that hydrodynamic Lyapunov
modes do exist in soft-potential systems, although the collective Lyapunov
modes are more vague than in hard-core systems. In studying the density and
temperature dependence of these modes, it is found that, when the value of
Lyapunov exponent is plotted as function of the dominant
wave number of the corresponding LV, all data from simulations with
different densities and temperatures collapse onto a single curve. This shows
that the dispersion relation vs. for
hydrodynamical Lyapunov modes appears to be universal irrespective of the
particle density and temperature of the system.
Despite the wave-like character of the LVs, no step-like structure exists in
the Lyapunov spectrum of the systems studied here, in contrast to the hard-core
case. Further numerical simulations show that the finite-time LEs fluctuate
strongly. We have also investigated localization features of LVs and propose a
new length scale to characterize the Hamiltonian spatio-temporal chaotic
states.Comment: 12 pages, 20 figure
Comparison of covariant and orthogonal Lyapunov vectors
Two sets of vectors, covariant and orthogonal Lyapunov vectors (CLVs/OLVs),
are currently used to characterize the linear stability of chaotic systems. A
comparison is made to show their similarity and difference, especially with
respect to the influence on hydrodynamic Lyapunov modes (HLMs). Our numerical
simulations show that in both Hamiltonian and dissipative systems HLMs formerly
detected via OLVs survive if CLVs are used instead. Moreover the previous
classification of two universality classes works for CLVs as well, i.e. the
dispersion relation is linear for Hamiltonian systems and quadratic for
dissipative systems respectively. The significance of HLMs changes in different
ways for Hamiltonian and dissipative systems with the replacement of OLVs by
CLVs. For general dissipative systems with nonhyperbolic dynamics the long wave
length structure in Lyapunov vectors corresponding to near-zero Lyapunov
exponents is strongly reduced if CLVs are used instead, whereas for highly
hyperbolic dissipative systems the significance of HLMs is nearly identical for
CLVs and OLVs. In contrast the HLM significance of Hamiltonian systems is
always comparable for CLVs and OLVs irrespective of hyperbolicity. We also find
that in Hamiltonian systems different symmetry relations between conjugate
pairs are observed for CLVs and OLVs. Especially, CLVs in a conjugate pair are
statistically indistinguishable in consequence of the micro- reversibility of
Hamiltonian systems. Transformation properties of Lyapunov exponents, CLVs and
hyperbolicity under changes of coordinate are discussed in appendices
Infinite invariant density in a semi-Markov process with continuous state variables
We report on a fundamental role of a non-normalized formal steady state,
i.e., an infinite invariant density, in a semi-Markov process where the state
is determined by the inter-event time of successive renewals. The state
describes certain observables found in models of anomalous diffusion, e.g., the
velocity in the generalized L\'evy walk model and the energy of a particle in
the trap model. In our model, the inter-event-time distribution follows a
fat-tailed distribution, which makes the state value more likely to be zero
because long inter-event times imply small state values. We find two scaling
laws describing the density for the state value, which accumulates in the
vicinity of zero in the long-time limit. These laws provide universal behaviors
in the accumulation process and give the exact expression of the infinite
invariant density. Moreover, we provide two distributional limit theorems for
time-averaged observables in these non-stationary processes. We show that the
infinite invariant density plays an important role in determining the
distribution of time averages.Comment: 16 pages, 7 figure
Characterizing N-dimensional anisotropic Brownian motion by the distribution of diffusivities
Anisotropic diffusion processes emerge in various fields such as transport in
biological tissue and diffusion in liquid crystals. In such systems, the motion
is described by a diffusion tensor. For a proper characterization of processes
with more than one diffusion coefficient an average description by the mean
squared displacement is often not sufficient. Hence, in this paper, we use the
distribution of diffusivities to study diffusion in a homogeneous anisotropic
environment. We derive analytical expressions of the distribution and relate
its properties to an anisotropy measure based on the mean diffusivity and the
asymptotic decay of the distribution. Both quantities are easy to determine
from experimental data and reveal the existence of more than one diffusion
coefficient, which allows the distinction between isotropic and anisotropic
processes. We further discuss the influence on the analysis of projected
trajectories, which are typically accessible in experiments. For the
experimentally relevant cases of two- and three-dimensional anisotropic
diffusion we derive specific expressions, determine the diffusion tensor,
characterize the anisotropy, and demonstrate the applicability for simulated
trajectories.Comment: v2: 14 pages, 4 figures, added section about curvature, added
references, several clarifications and enhancements; v1: 13 pages, 4 figure
Dimensional collapse and fractal attractors of a system with fluctuating delay times
A frequently encountered situation in the study of delay systems is that the
length of the delay time changes with time, which is of relevance in many
fields such as optics, mechanical machining, biology or physiology. A
characteristic feature of such systems is that the dimension of the system
dynamics collapses due to the fluctuations of delay times. In consequence, the
support of the long-trajectory attractors of this kind of systems is found
being fractal in contrast to the fuzzy attractors in most random systems
Normal and anomalous random walks of 2-d solitons
Solitons, which describe the propagation of concentrated beams of light
through nonlinear media, can exhibit a variety of behaviors as a result of the
intrinsic dissipation, diffraction, and the nonlinear effects. One of these
phenomena, modeled by the complex Ginzburg-Landau equation, are chaotic
explosions, transient enlargements of the soliton that may induce random
transversal displacements, which in the long run lead to a random walk of the
soliton center. As we show in this work, the transition from non-moving to
moving solitons is not a simple bifurcation but includes a sequence of normal
and anomalous random walks. We analyze their statistics with the distribution
of generalized diffusivities, a novel approach that has been used successfully
for characterizing anomalous diffusion.Comment: 10 pages, 5 figure
Anomalous diffusion of dissipative solitons in the cubic-quintic complex Ginzburg-Landau equation in two spatial dimensions
We demonstrate the occurrence of anomalous diffusion of dissipative solitons
in a `simple' and deterministic prototype model: the cubic-quintic complex
Ginzburg-Landau equation in two spatial dimensions. The main features of their
dynamics, induced by symmetric-asymmetric explosions, can be modeled by a
subdiffusive continuous-time random walk, while in the case dominated by only
asymmetric explosions it becomes characterized by normal diffusion.Comment: 6 pages, 6 figure
How to compare diffusion processes assessed by single-particle tracking and pulsed field gradient nuclear magnetic resonance
Heterogeneous diffusion processes occur in many different fields such as
transport in living cells or diffusion in porous media. A characterization of
the transport parameters of such processes can be achieved by ensemble-based
methods, such as pulsed field gradient nuclear magnetic resonance (PFG NMR), or
by trajectory-based methods obtained from single-particle tracking (SPT)
experiments. In this paper, we study the general relationship between both
methods and its application to heterogeneous systems. We derive analytical
expressions for the distribution of diffusivities from SPT and further relate
it to NMR spin-echo diffusion attenuation functions. To exemplify the
applicability of this approach, we employ a well-established two-region
exchange model, which has widely been used in the context of PFG NMR studies of
multiphase systems subjected to interphase molecular exchange processes. This
type of systems, which can also describe a layered liquid with layer-dependent
self-diffusion coefficients, has also recently gained attention in SPT
experiments. We reformulate the results of the two-region exchange model in
terms of SPT-observables and compare its predictions to that obtained using the
exact transformation which we derived.Comment: v2: 14 pages, 6 figures, several enhancements, added references; v1:
7 pages, 3 figure
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