951 research outputs found
Superdiffusion in the Dissipative Standard Map
We consider transport properties of the chaotic (strange) attractor along
unfolded trajectories of the dissipative standard map. It is shown that the
diffusion process is normal except of the cases when a control parameter is
close to some special values that correspond to the ballistic mode dynamics.
Diffusion near the related crisises is anomalous and non-uniform in time: there
are large time intervals during which the transport is normal or ballistic, or
even superballistic. The anomalous superdiffusion seems to be caused by
stickiness of trajectories to a non-chaotic and nowhere dense invariant Cantor
set that plays a similar role as cantori in Hamiltonian chaos. We provide a
numerical example of such a sticky set. Distribution function on the sticky set
almost coincides with the distribution function (SRB measure) of the chaotic
attractor.Comment: 10 Figure
Dynamics of the Chain of Oscillators with Long-Range Interaction: From Synchronization to Chaos
We consider a chain of nonlinear oscillators with long-range interaction of
the type 1/l^{1+alpha}, where l is a distance between oscillators and 0< alpha
<2. In the continues limit the system's dynamics is described by the
Ginzburg-Landau equation with complex coefficients. Such a system has a new
parameter alpha that is responsible for the complexity of the medium and that
strongly influences possible regimes of the dynamics. We study different
spatial-temporal patterns of the dynamics depending on alpha and show
transitions from synchronization of the motion to broad-spectrum oscillations
and to chaos.Comment: 22 pages, 10 figure
Maximal width of the separatrix chaotic layer
The main goal of the paper is to find the {\it absolute maximum} of the width
of the separatrix chaotic layer as function of the frequency of the
time-periodic perturbation of a one-dimensional Hamiltonian system possessing a
separatrix, which is one of the major unsolved problems in the theory of
separatrix chaos. For a given small amplitude of the perturbation, the width is
shown to possess sharp peaks in the range from logarithmically small to
moderate frequencies. These peaks are universal, being the consequence of the
involvement of the nonlinear resonance dynamics into the separatrix chaotic
motion. Developing further the approach introduced in the recent paper by
Soskin et al. ({\it PRE} {\bf 77}, 036221 (2008)), we derive leading-order
asymptotic expressions for the shape of the low-frequency peaks. The maxima of
the peaks, including in particular the {\it absolute maximum} of the width, are
proportional to the perturbation amplitude times either a logarithmically large
factor or a numerical, still typically large, factor, depending on the type of
system. Thus, our theory predicts that the maximal width of the chaotic layer
may be much larger than that predicted by former theories. The theory is
verified in simulations. An application to the facilitation of global chaos
onset is discussed.Comment: 18 pages, 16 figures, submitted to PR
Finite Element Analysis of Strain Effects on Electronic and Transport Properties in Quantum Dots and Wires
Lattice mismatch in layered semiconductor structures with submicron length
scales leads to extremely high nonuniform strains. This paper presents a finite
element technique for incorporating the effects of the nonuniform strain into
an analysis of the electronic properties of SiGe quantum structures. Strain
fields are calculated using a standard structural mechanics finite element
package and the effects are included as a nonuniform potential directly in the
time independent Schrodinger equation; a k-p Hamiltonian is used to model the
effects of multiple valence subband coupling. A variational statement of the
equation is formulated and solved using the finite element method. This
technique is applied to resonant tunneling diode quantum dots and wires; the
resulting densities of states confined to the quantum well layers of the
devices are compared to experimental current-voltage I(V) curves.Comment: 17 pages (LaTex), 18 figures (JPEG), submitted to Journal of Applied
Physic
Dynamic instabilities in resonant tunneling induced by a magnetic field
We show that the addition of a magnetic field parallel to the current induces
self sustained intrinsic current oscillations in an asymmetric double barrier
structure. The oscillations are attributed to the nonlinear dynamic coupling of
the current to the charge trapped in the well, and the effect of the external
field over the local density of states across the system. Our results show that
the system bifurcates as the field is increased, and may transit to chaos at
large enough fields.Comment: 4 pages, 3 figures, accepted in Phys. Rev. Letter
Stochastic heating of a molecular nanomagnet
We study the excitation dynamics of a single molecular nanomagnet by static
and pulsed magnetic fields. Based on a stability analysis of the classical
magnetization dynamics we identify analytically the fields parameters for which
the energy is stochastically pumped into the system in which case the
magnetization undergoes diffusively and irreversibly a large angle deflection.
An approximate analytical expression for the diffusion constant in terms of the
fields parameters is given and assessed by full numerical calculations.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Web-assisted tunneling in the kicked harmonic oscillator
We show that heating of harmonically trapped ions by periodic delta kicks is
dramatically enhanced at isolated values of the Lamb-Dicke parameter. At these
values, quasienergy eigenstates localized on island structures undergo avoided
crossings with extended web-states.Comment: 4 pages, 4 figures. Accepted for publication in Phys. Rev. Let
On the unconstrained expansion of a spherical plasma cloud turning collisionless : case of a cloud generated by a nanometer dust grain impact on an uncharged target in space
Nano and micro meter sized dust particles travelling through the heliosphere
at several hundreds of km/s have been repeatedly detected by interplanetary
spacecraft. When such fast moving dust particles hit a solid target in space,
an expanding plasma cloud is formed through the vaporisation and ionisation of
the dust particles itself and part of the target material at and near the
impact point. Immediately after the impact the small and dense cloud is
dominated by collisions and the expansion can be described by fluid equations.
However, once the cloud has reached micro-m dimensions, the plasma may turn
collisionless and a kinetic description is required to describe the subsequent
expansion. In this paper we explore the late and possibly collisionless
spherically symmetric unconstrained expansion of a single ionized ion-electron
plasma using N-body simulations. Given the strong uncertainties concerning the
early hydrodynamic expansion, we assume that at the time of the transition to
the collisionless regime the cloud density and temperature are spatially
uniform. We do also neglect the role of the ambient plasma. This is a
reasonable assumption as long as the cloud density is substantially higher than
the ambient plasma density. In the case of clouds generated by fast
interplanetary dust grains hitting a solid target some 10^7 electrons and ions
are liberated and the in vacuum approximation is acceptable up to meter order
cloud dimensions. ..
Decoherence and the quantum-classical limit in the presence of chaos
We investigate how decoherence affects the short-time separation between
quantum and classical dynamics for classically chaotic systems, within the
framework of a specific model. For a wide range of parameters, the distance
between the corresponding phase-space distributions depends on a single
parameter that relates an effective Planck constant ,
the Lyapunov coeffficient, and the diffusion constant. This distance peaks at a
time that depends logarithmically on , in agreement with
previous estimations of the separation time for Hamiltonian systems. However,
for , the separation remains small, going down with , so the concept of separation time loses its meaning.Comment: 5 pages, 4 figures (in 6 postscript files) two of them are color
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