221 research outputs found
Control of unstable steady states by extended time-delayed feedback
Time-delayed feedback methods can be used to control unstable periodic orbits
as well as unstable steady states. We present an application of extended time
delay autosynchronization introduced by Socolar et al. to an unstable focus.
This system represents a generic model of an unstable steady state which can be
found for instance in a Hopf bifurcation. In addition to the original
controller design, we investigate effects of control loop latency and a
bandpass filter on the domain of control. Furthermore, we consider coupling of
the control force to the system via a rotational coupling matrix parametrized
by a variable phase. We present an analysis of the domain of control and
support our results by numerical calculations.Comment: 11 pages, 16 figure
Temperature and frequency dependent optical properties of ultra-thin Au films
While the optical properties of thin metal films are well understood in the
visible and near-infrared range, little has been done in the mid- and
far-infrared region. Here we investigate ultra-thin gold films prepared on
Si(111)(7 x 7) in UHV by measuring in the frequency range between 500 cm-1 and
7000 cm-1 and for temperatures between 300 K and 5 K. The nominal thickness of
the gold layers ranges between one monolayer and 9 nm. The frequency and
temperature dependences of the thicker films can be well described by the Drude
model of a metal, when taking into account classical size effects due to
surface scattering. The films below the percolation threshold exhibit a
non-metallic behavior: the reflection increases with frequency and decreases
with temperature. The frequency dependence can partly be described by a
generalized Drude model. The temperature dependence does not follow a simple
activation process. For monolayers we observe a transition between surface
states around 1100 cm-1.Comment: 7 pages, 10 figure
Control of unstable steady states by time-delayed feedback methods
We show that time-delayed feedback methods, which have successfully been used
to control unstable periodic ortbits, provide a tool to stabilize unstable
steady states. We present an analytical investigation of the feedback scheme
using the Lambert function and discuss effects of both a low-pass filter
included in the control loop and non-zero latency times associated with the
generation and injection of the feedback signal.Comment: 8 pages, 11 figure
Tip-induced distortions in STM imaging of carbon nanotubes
By means of STM measurements and fully self-consistent
transport calculations we analyze how STM trajectories for the
mapping of nanostructures on surfaces are affected by the
atomic structure of the tip.
For the particular case of carbon nanotubes we show that
considerable distortions of the STM trajectory with respect to
the actual structure, position and diameter of the nanotube
can occur for certain tip geometries. Comparison between
theory and experiment can allow to characterize and correct
these distortions
Adaptive Tuning of Feedback Gain in Time-Delayed Feedback Control
We demonstrate that time-delayed feedback control can be improved by
adaptively tuning the feedback gain. This adaptive controller is applied to the
stabilization of an unstable fixed point and an unstable periodic orbit
embedded in a chaotic attractor. The adaptation algorithm is constructed using
the speed-gradient method of control theory. Our computer simulations show that
the adaptation algorithm can find an appropriate value of the feedback gain for
single and multiple delays. Furthermore, we show that our method is robust to
noise and different initial conditions.Comment: 7 pages, 6 figure
Применение программного продукта Eclipse для оптимизации разработки нефтяных месторождений Западно-Сибирского региона
Применение программного продукта Eclipse для оптимизации разработки нефтяных месторождений Западно-Сибирского регионаApplication software Eclipse to optimize the development of oil fields of West Siberi
Coherence resonance in a network of FitzHugh-Nagumo systems: interplay of noise, time-delay and topology
We systematically investigate the phenomena of coherence resonance in
time-delay coupled networks of FitzHugh-Nagumo elements in the excitable
regime. Using numerical simulations, we examine the interplay of noise,
time-delayed coupling and network topology in the generation of coherence
resonance. In the deterministic case, we show that the delay-induced dynamics
is independent of the number of nearest neighbors and the system size. In the
presence of noise, we demonstrate the possibility of controlling coherence
resonance by varying the time-delay and the number of nearest neighbors. For a
locally coupled ring, we show that the time-delay weakens coherence resonance.
For nonlocal coupling with appropriate time-delays, both enhancement and
weakening of coherence resonance are possible
Controlling cluster synchronization by adapting the topology
We suggest an adaptive control scheme for the control of zero-lag and cluster
synchronization in delay-coupled networks. Based on the speed-gradient method,
our scheme adapts the topology of a network such that the target state is
realized. It is robust towards different initial condition as well as changes
in the coupling parameters. The emerging topology is characterized by a
delicate interplay of excitatory and inhibitory links leading to the
stabilization of the desired cluster state. As a crucial parameter determining
this interplay we identify the delay time. Furthermore, we show how to
construct networks such that they exhibit not only a given cluster state but
also with a given oscillation frequency. We apply our method to coupled
Stuart-Landau oscillators, a paradigmatic normal form that naturally arises in
an expansion of systems close to a Hopf bifurcation. The successful and robust
control of this generic model opens up possible applications in a wide range of
systems in physics, chemistry, technology, and life science
Excitability in autonomous Boolean networks
We demonstrate theoretically and experimentally that excitable systems can be
built with autonomous Boolean networks. Their experimental implementation is
realized with asynchronous logic gates on a reconfigurabe chip. When these
excitable systems are assembled into time-delay networks, their dynamics
display nanosecond time-scale spike synchronization patterns that are
controllable in period and phase.Comment: 6 pages, 5 figures, accepted in Europhysics Letters
(epljournal.edpsciences.org
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