503 research outputs found
Erosion waves: transverse instabilities and fingering
Two laboratory scale experiments of dry and under-water avalanches of
non-cohesive granular materials are investigated. We trigger solitary waves and
study the conditions under which the front is transversally stable. We show the
existence of a linear instability followed by a coarsening dynamics and finally
the onset of a fingering pattern. Due to the different operating conditions,
both experiments strongly differ by the spatial and time scales involved.
Nevertheless, the quantitative agreement between the stability diagram, the
wavelengths selected and the avalanche morphology reveals a common scenario for
an erosion/deposition process.Comment: 4 pages, 6 figures, submitted to PR
Stabilization of unstable steady states by variable delay feedback control
We report on a dramatic improvement of the performance of the classical
time-delayed autosynchronization method (TDAS) to control unstable steady
states, by applying a time-varying delay in the TDAS control scheme in a form
of a deterministic or stochastic delay-modulation in a fixed interval around a
nominal value . The successfulness of this variable delay feedback control
(VDFC) is illustrated by a numerical control simulation of the Lorenz and
R\"{o}ssler systems using three different types of time-delay modulations: a
sawtooth wave, a sine wave, and a uniform random distribution. We perform a
comparative analysis between the VDFC method and the standard TDAS method for a
sawtooth-wave modulation by analytically determining the domains of control for
the generic case of an unstable fixed point of focus type.Comment: 7 pages, 4 figures, RevTe
Phononics: Manipulating heat flow with electronic analogs and beyond
The form of energy termed heat that typically derives from lattice
vibrations, i.e. the phonons, is usually considered as waste energy and,
moreover, deleterious to information processing. However, with this colloquium,
we attempt to rebut this common view: By use of tailored models we demonstrate
that phonons can be manipulated like electrons and photons can, thus enabling
controlled heat transport. Moreover, we explain that phonons can be put to
beneficial use to carry and process information. In a first part we present
ways to control heat transport and how to process information for physical
systems which are driven by a temperature bias. Particularly, we put forward
the toolkit of familiar electronic analogs for exercising phononics; i.e.
phononic devices which act as thermal diodes, thermal transistors, thermal
logic gates and thermal memories, etc.. These concepts are then put to work to
transport, control and rectify heat in physical realistic nanosystems by
devising practical designs of hybrid nanostructures that permit the operation
of functional phononic devices and, as well, report first experimental
realizations. Next, we discuss yet richer possibilities to manipulate heat flow
by use of time varying thermal bath temperatures or various other external
fields. These give rise to a plenty of intriguing phononic nonequilibrium
phenomena as for example the directed shuttling of heat, a geometrical phase
induced heat pumping, or the phonon Hall effect, that all may find its way into
operation with electronic analogs.Comment: 24 pages, 16 figures, modified title and revised, accepted for
publication in Rev. Mod. Phy
Particle Motion in Rapidly Oscillating Potentials: The Role of the Potential's Initial Phase
Rapidly oscillating potentials with a vanishing time average have been used
for a long time to trap charged particles in source-free regions. It has been
argued that the motion of a particle in such a potential can be approximately
described by a time independent effective potential, which does not depend upon
the initial phase of the oscillating potential. However, here we show that the
motion of a particle and its trapping condition significantly depend upon this
initial phase for arbitrarily high frequencies of the potential's oscillation.
We explain this novel phenomenon by showing that the motion of a particle is
determined by the effective potential stated in the literature only if its
initial conditions are transformed according to a transformation which we show
to significantly depend on the potential's initial phase for arbitrarily high
frequencies. We confirm our theoretical findings by numerical simulations.
Further, we demonstrate that the found phenomenon offers new ways to manipulate
the dynamics of particles which are trapped by rapidly oscillating potentials.
Finally, we propose a simple experiment to verify the theoretical findings of
this work.Comment: 9 pages, 8 figures, published in PR
Out-of-equilibrium states as statistical equilibria of an effective dynamics
We study the formation of coherent structures in a system with long-range
interactions where particles moving on a circle interact through a repulsive
cosine potential. Non equilibrium structures are shown to correspond to
statistical equilibria of an effective dynamics, which is derived using
averaging techniques. This simple behavior might be a prototype of others
observed in more complicated systems with long-range interactions, like
two-dimensional incompressible fluids or self-gravitating systems.Comment: 4 figure
Time independent description of rapidly oscillating potentials
The classical and quantum dynamics in a high frequency field are found to be
described by an effective time independent Hamiltonian. It is calculated in a
systematic expansion in the inverse of the frequency () to order
. The work is an extension of the classical result for the Kapitza
pendulum, which was calculated in the past to order . The analysis
makes use of an implementation of the method of separation of time scales and
of a quantum gauge transformation in the framework of Floquet theory. The
effective time independent Hamiltonian enables one to explore the dynamics in
presence of rapidly oscillating fields, in the framework of theories that were
developed for systems with time independent Hamiltonians. The results are
relevant, in particular, for exploration of the dynamics of cold atoms.Comment: 4 pages, 1 figure. Revised versio
Multi-threshold second-order phase transition
We present a theory of the multi-threshold second-order phase transition, and
experimentally demonstrate the multi-threshold second-order phase transition
phenomenon. With carefully selected parameters, in an external cavity diode
laser system, we observe second-order phase transition with multiple (three or
four) thresholds in the measured power-current-temperature three dimensional
phase diagram. Such controlled death and revival of second-order phase
transition sheds new insight into the nature of ubiquitous second-order phase
transition. Our theory and experiment show that the single threshold
second-order phase transition is only a special case of the more general
multi-threshold second-order phase transition, which is an even richer
phenomenon.Comment: 5 pages, 3 figure
Quantum Brownian motion under rapid periodic forcing
We study the steady state behaviour of a confined quantum Brownian particle
subjected to a space-dependent, rapidly oscillating time-periodic force. To
leading order in the period of driving, the result of the oscillating force is
an effective static potential which has a quantum dissipative contribution,
, which adds on to the classical result. This is shown using a coherent
state representation of bath oscillators. is evaluated exactly in the
case of an Ohmic dissipation bath. It is strongest for intermediate values of
the damping, where it can have pronounced effects.Comment: 11 Pages and 3 figures, Content change
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