1,205 research outputs found
Supersonic dislocations observed in a plasma crystal
Experimental results on the dislocation dynamics in a two-dimensional plasma
crystal are presented. Edge dislocations were created in pairs in lattice
locations where the internal shear stress exceeded a threshold and then moved
apart in the glide plane at a speed higher than the sound speed of shear waves,
. The experimental system, a plasma crystal, allowed observation of this
process at an atomistic (kinetic) level. The early stage of this process is
identified as a stacking fault. At a later stage, supersonically moving
dislocations generated shear-wave Mach cones
Observation of particle pairing in a two-dimensional plasma crystal
The observation is presented of naturally occurring pairing of particles and
their cooperative drift in a two-dimensional plasma crystal. A single layer of
plastic microspheres was suspended in the plasma sheath of a capacitively
coupled rf discharge in argon at a low pressure of 1 Pa. The particle dynamics
were studied by combining the top-view and side-view imaging of the suspension.
Cross analysis of the particle trajectories allowed us to identify naturally
occurring metastable pairs of particles. The lifetime of pairs was long enough
for their reliable identification.Comment: 5 pages, 4 figure
Supersonic dislocations observed in a plasma crystal
Experimental results on the dislocation dynamics in a two-dimensional plasma
crystal are presented. Edge dislocations were created in pairs in lattice
locations where the internal shear stress exceeded a threshold and then moved
apart in the glide plane at a speed higher than the sound speed of shear waves,
. The experimental system, a plasma crystal, allowed observation of this
process at an atomistic (kinetic) level. The early stage of this process is
identified as a stacking fault. At a later stage, supersonically moving
dislocations generated shear-wave Mach cones
Direct observation of mode-coupling instability in two-dimensional plasma crystals
Dedicated experiments on melting of 2D plasma crystals were carried out. The
melting was always accompanied by spontaneous growth of the particle kinetic
energy, suggesting a universal plasma-driven mechanism underlying the process.
By measuring three principal dust-lattice (DL) wave modes simultaneously, it is
unambiguously demonstrated that the melting occurs due to the resonance
coupling between two of the DL modes. The variation of the wave modes with the
experimental conditions, including the emergence of the resonant (hybrid)
branch, reveals exceptionally good agreement with the theory of mode-coupling
instability.Comment: 4 pages, submitted to Physical Review Letter
Three-dimensional structure of Mach cones in monolayer complex plasma
Structure of Mach cones in a crystalline complex plasma has been studied
experimentally using an intensity sensitive imaging, which resolved particle
motion in three dimensions. This revealed a previously unknown out-of-plane
cone structure, which appeared due to excitation of the vertical wave mode. The
complex plasma consisted of micron sized particles forming a monolayer in a
plasma sheath of a gas discharge. Fast particles, spontaneously moving under
the monolayer, created Mach cones with multiple structures. The in-plane cone
structure was due to compressional and shear lattice waves.Comment: Accepted for publication in Physical Review Letter
First direct measurement of optical phonons in 2D plasma crystals
Spectra of phonons with out-of-plane polarization were studied experimentally
in a 2D plasma crystal. The dispersion relation was directly measured for the
first time using a novel method of particle imaging. The out-of-plane mode was
proven to have negative optical dispersion, comparison with theory showed good
agreement. The effect of the plasma wakes on the dispersion relation is briefly
discussed.Comment: submitted to Physical Review Letter
Wave mode coupling due to plasma wakes in two-dimensional plasma crystals: In-depth view
Experiments with two-dimensional (2D) plasma crystals are usually carried out
in rf plasma sheaths, where the interparticle interactions are modified due to
the presence of plasma wakes. The wake-mediated interactions result in the
coupling between wave modes in 2D crystals, which can trigger the mode-coupling
instability and cause melting. The theory predicts a number of distinct
fingerprints to be observed upon the instability onset, such as the emergence
of a new hybrid mode, a critical angular dependence, a mixed polarization, and
distinct thresholds. In this paper we summarize these key features and provide
their detailed discussion, analyze the critical dependence on experimental
parameters, and highlight the outstanding issues
Group classification of heat conductivity equations with a nonlinear source
We suggest a systematic procedure for classifying partial differential
equations invariant with respect to low dimensional Lie algebras. This
procedure is a proper synthesis of the infinitesimal Lie's method, technique of
equivalence transformations and theory of classification of abstract low
dimensional Lie algebras. As an application, we consider the problem of
classifying heat conductivity equations in one variable with nonlinear
convection and source terms. We have derived a complete classification of
nonlinear equations of this type admitting nontrivial symmetry. It is shown
that there are three, seven, twenty eight and twelve inequivalent classes of
partial differential equations of the considered type that are invariant under
the one-, two-, three- and four-dimensional Lie algebras, correspondingly.
Furthermore, we prove that any partial differential equation belonging to the
class under study and admitting symmetry group of the dimension higher than
four is locally equivalent to a linear equation. This classification is
compared to existing group classifications of nonlinear heat conductivity
equations and one of the conclusions is that all of them can be obtained within
the framework of our approach. Furthermore, a number of new invariant equations
are constructed which have rich symmetry properties and, therefore, may be used
for mathematical modeling of, say, nonlinear heat transfer processes.Comment: LaTeX, 51 page
Autowaves in a dc complex plasma confined behind a de Laval nozzle
Experiments to explore stability conditions and topology of a dense
microparticle cloud supported against gravity by a gas flow were carried out.
By using a nozzle shaped glass insert within the glass tube of a dc discharge
plasma chamber a weakly ionized gas flow through a de Laval nozzle was
produced. The experiments were performed using neon gas at a pressure of 100 Pa
and melamine-formaldehyde particles with a diameter of 3.43 {\mu}m. The
capturing and stable global confining of the particles behind the nozzle in the
plasma were demonstrated. The particles inside the cloud behaved as a single
convection cell inhomogeneously structured along the nozzle axis in a tube-like
manner. The pulsed acceleration localized in the very head of the cloud
mediated by collective plasma-particle interactions and the resulting wave
pattern were studied in detail.Comment: 6 pages, 4 figure
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