7,437 research outputs found
Pinning dynamic systems of networks with Markovian switching couplings and controller-node set
In this paper, we study pinning control problem of coupled dynamical systems
with stochastically switching couplings and stochastically selected
controller-node set. Here, the coupling matrices and the controller-node sets
change with time, induced by a continuous-time Markovian chain. By constructing
Lyapunov functions, we establish tractable sufficient conditions for
exponentially stability of the coupled system. Two scenarios are considered
here. First, we prove that if each subsystem in the switching system, i.e. with
the fixed coupling, can be stabilized by the fixed pinning controller-node set,
and in addition, the Markovian switching is sufficiently slow, then the
time-varying dynamical system is stabilized. Second, in particular, for the
problem of spatial pinning control of network with mobile agents, we conclude
that if the system with the average coupling and pinning gains can be
stabilized and the switching is sufficiently fast, the time-varying system is
stabilized. Two numerical examples are provided to demonstrate the validity of
these theoretical results, including a switching dynamical system between
several stable sub-systems, and a dynamical system with mobile nodes and
spatial pinning control towards the nodes when these nodes are being in a
pre-designed region.Comment: 9 pages; 3 figure
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Importance of low-angle grain boundaries in YBa2Cu3O7-delta coated conductors
Over the past ten years the perception of grain boundaries in YBa2Cu3O7-δ
conductors has changed greatly. They are no longer a problem to be eliminated but an
inevitable and potentially favourable part of the material. This change has arisen as a
consequence of new manufacturing techniques which result in excellent grain alignment,
reducing the spread of grain boundary misorientation angles. At the same time there
is considerable recent evidence which indicates that the variation of properties of grain
boundaries with mismatch angle is more complex than a simple exponential decrease in critical
current. This is due to the fact that low-angle grain boundaries represent a qualitatively
different system to high angle boundaries. The time is therefore right for a targetted
review of research into low-angle YBa2Cu3O7-δ grain boundaries. This article does
not purport to be a comprehensive review of the physics of grain boundaries as found in
YBa2Cu3O7-δ in general; for a broader overview we would recommend that the reader
consult the comprehensive review of Hilgenkamp and Mannhart (Rev. Mod. Phys., 74, 485,
2002). The purpose of this article is to review the origin and properties of the low-angle grain
boundaries found in YBa2Cu3O7-δ coated conductors both individually and as a collective
system.EPSR
Dislocation networks in helium-4 crystals
The mechanical behavior of crystals is dominated by dislocation networks,
their structure and their interactions with impurities or thermal phonons.
However, in classical crystals, networks are usually random with impurities
often forming non-equilibrium clusters when their motion freezes at low
temperature. Helium provides unique advantages for the study of dislocations:
crystals are free of all but isotopic impurities, the concentration of these
can be reduced to the ppb level, and the impurities are mobile at all
temperatures and therefore remain in equilibrium with the dislocations. We have
achieved a comprehensive study of the mechanical response of 4He crystals to a
driving strain as a function of temperature, frequency and strain amplitude.
The quality of our fits to the complete set of data strongly supports our
assumption of string-like vibrating dislocations. It leads to a precise
determination of the distribution of dislocation network lengths and to
detailed information about the interaction between dislocations and both
thermal phonons and 3He impurities. The width of the dissipation peak
associated with impurity binding is larger than predicted by a simple Debye
model, and much of this broadening is due to the distribution of network
lengths.Comment: accepted by Phys. Rev.
Deblocking of interacting particle assemblies: from pinning to jamming
A wide variety of interacting particle assemblies driven by an external force
are characterized by a transition between a blocked and a moving phase. The
origin of this deblocking transition can be traced back to the presence of
either external quenched disorder, or of internal constraints. The first case
belongs to the realm of the depinning transition, which, for example, is
relevant for flux-lines in type II superconductors and other elastic systems
moving in a random medium. The second case is usually included within the
so-called jamming scenario observed, for instance, in many glassy materials as
well as in plastically deforming crystals. Here we review some aspects of the
rich phenomenology observed in interacting particle models. In particular, we
discuss front depinning, observed when particles are injected inside a random
medium from the boundary, elastic and plastic depinning in particle assemblies
driven by external forces, and the rheology of systems close to the jamming
transition. We emphasize similarities and differences in these phenomena.Comment: 20 pages, 8 figures, submitted for a special issue of the Brazilian
Journal of Physics entitled: Statistical Mechanics of Irreversible Stochastic
Models - I
Quantum Phase Transitions and Vortex Dynamics in Superconducting Networks
Josephson junction arrays are ideal model systems where a variety of
phenomena, phase transitions, frustration effects, vortex dynamics, chaos, to
mention a few of them, can be studied in a controlled way. In this review we
focus on the quantum dynamical properties of low capacitance Josephson junction
arrays. The two characteristic energy scales in these systems are the Josephson
energy, associated to the tunneling of Cooper pairs between neighboring
islands, and the charging energy, which is the energy cost to add an extra
electron charge to a neutral island. The phenomena described in this review
stem from the competition between single electron effects with the Josephson
effect. One example is the (quantum) Superconductor-Insulator phase transition
which occurs by varying the ratio between the coupling constants and/or by
means of external magnetic/electric fields. We will describe how the phase
diagram depends on the various control paramters and the transport properties
close to the quantum critical point. The relevant topological excitations on
the superconducting side of the phase diagram are vortices. In low capacitance
junction arrays vortices behave as massive underdamped particles that can
exhibit quantum behaviour. We will report on the various experiments and
theoretical treatments on quantum vortex dynamics.Comment: To be published in Physics Reports. Better quality figures can be
obtained upon reques
Magnetization reversal in spin patterns with complex geometry
We study field-driven dynamics of spins with antiferromagnetic interaction
along the links of a complex substrate geometry, which is modeled by graphs of
a controlled connectivity distribution. The magnetization reversal occurs in
avalanches of spin flips, which are pinned by the topological constraints of
the underlying graph. The hysteresis loop and avalanche sizes are analyzed and
classified in terms of graph's connectivity and clustering. The results are
relevant for magnets with a hierarchical spatial inhomogeneity and for design
of nanoscale magnetic devices.Comment: 4 pages, 3 color figures, revtex
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