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In this article, the stability of a complex plasma monolayer levitating in the sheath of the poweredelectrode of an asymmetric capacitively coupled radio-frequency argon discharge is studied. Com-pared to earlier studies, a better integration of the experimental results and theory is achieved byoperating with actual experimental control parameters such as the gas pressure and the dischargepower. It is shown that for a given microparticle monolayer at a fixed discharge power there existtwo threshold pressures: (i) above a specific pressure p cryst , the monolayer always crystallises; (ii)below a specific pressure p MCI , the crystalline monolayer undergoes the mode-coupling instability andthe two-dimensional complex plasma crystal melts. In-between p MCI and p cryst , the microparticlemonolayer can be either in the fluid phase or the crystal phase: when increasing the pressure frombelow p MCI , the monolayer remains in the fluid phase until it reaches p cryst at which it recrystallises;when decreasing the pressure from above p cryst , the monolayer remains in the crystalline phase untilit reaches p MCI at which the mode-coupling instability is triggered and the crystal melts. A simpleself-consistent sheath model is used to calculate the rf sheath profile, the microparticle charges and themicroparticle resonance frequency as a function of power and background argon pressure. Combinedwith calculation of the lattice modes the main trends of p MCI as a function of power and backgroundargon pressure are recovered. The threshold of the mode-coupling instability in the crystalline phaseis dominated by the crossing of the longitudinal in-plane lattice mode and the out-of plane latticemode induced by the change of the sheath profile. Ion wakes are shown to have a significant effecttoo., Comment: Submitted to Phys. Rev. E

Heat transport in a three-dimensional complex (dusty) plasma was experimentally studied in microgravity conditions using Plasmakristall-4 (PK-4) instrument on board the International Space Station (ISS). An extended suspension of microparticles was locally heated by a shear flow created by applying the radiation pressure force of the manipulation-laser beam. Individual particle trajectories in the flow were analysed and from these, using a fluid heat transport equation that takes viscous heating and neutral gas drag into account, the complex plasma's thermal diffusivity and kinematic viscosity were calculated. Their values are compared with previous results reported in ground-based experiments with complex plasmas., Comment: This article has been submitted to Physics of Plasmas. After it is published, it will be found at https://publishing.aip.org/resources/librarians/products/journals/

Shear flow in a three-dimensional complex plasma was experimentally studied in microgravity conditions using Plasmakristall-4 (PK-4) instrument on board the International Space Station (ISS). The shear flow was created in an extended suspension of microparticles by applying the radiation pressure force of the manipulation-laser beam. Individual particle trajectories in the flow were analyzed and from these, using the Navier-Stokes equation, an upper estimate of the complex plasma's kinematic viscosity was calculated in the range of $0.2$--$6.7~{\rm mm^2/s}$. This estimate is much lower than previously reported in ground-based experiments with 3D complex plasmas. Possible reasons of this difference are discussed., Comment: 5 pages, 4 figures

Active Janus particles suspended in a plasma were studied experimentally. The Janus particles were micron-size plastic microspheres, one half of which was coated with a thin layer of platinum. They were suspended in the plasma sheath of a radio-frequency discharge in argon at low pressure. The Janus particles moved in characteristic looped trajectories suggesting a combination of spinning and circling motion; their interactions led to the emergence of rich dynamics characterized by non-Maxwellian velocity distribution. The particle propulsion mechanism is discussed, the main force driving the particle motion is identified as photophoretic force., Comment: 6 pages, 6 figures

Three-dimensional structure of complex (dusty) plasmas was investigated under long-term microgravity conditions in the International-Space-Station-based Plasmakristall-4 facility. The microparticle suspensions were confined in a polarity-switched dc discharge. The experimental results were compared to the results of the molecular dynamics simulations with the interparticle interaction potential represented as a superposition of isotropic Yukawa and anisotropic quadrupole terms. Both simulated and experimental data exhibited qualitatively similar structural features indicating the bulk liquid-like order with the inclusion of solid-like strings aligned with the axial electric field. Individual strings were identified and their size spectrum was calculated. The decay rate of the size spectrum was found to decrease with the enhancement of string-like structural features.

The full melting of a two-dimensional plasma crystal was induced in a principally stable monolayer by localized laser stimulation. Two distinct behaviors of the crystal after laser stimulation were observed depending on the amount of injected energy: (i) below a well-defined threshold, the laser melted area recrystallized; (ii) above the threshold, it expanded outwards in a similar fashion to mode-coupling instability induced melting, rapidly destroying the crystalline order of the whole complex plasma monolayer. The reported experimental observations are due to the fluid mode-coupling instability which can pump energy into the particle monolayer at a rate surpassing the heat transport and damping rates in the energetic localized melted spot, resulting in its further growth. This behavior exhibits remarkable similarities with impulsive spot heating and thermal runaway (explosion) in ordinary reactive matter.

We report an experimental observation of coupling of the transverse vertical and longitudinal in-plane dust-lattice wave modes in a two-dimensional complex plasma crystal in the absence of mode crossing. A new large diameter rf plasma chamber was used to suspend the plasma crystal. The observations are confirmed with molecular-dynamics simulations. The coupling manifests itself in traces of the transverse vertical mode appearing in the measured longitudinal spectra and vice versa. We calculate the expected ratio of the trace to the principal mode with a theoretical analysis of the modes in a crystal with finite temperature and find good agreement with the experiment and simulations., Comment: 4 figures, 5 pages, accepted for publication in PRL Nov 2017

An interaction of upstream extra particles with a monolayer highly-ordered complex plasma is studied. A principally new abnormal turbulent wake formed behind the supersonic upstream particle is discovered. An anomalous type of the turbulence wake clearly manifests in anomalously low thermal diffusivity and two orders of magnitude larger particle kinetic temperature compared to that of the 'normal' wake (Mach cone) observed by Du et al [Europhys. Lett. 99, 55001 (2012)]., Comment: 6 pages, 4 figures

An interferometric imaging technique has been proposed to instantly measure the diameter of individual spherical dust particles suspended in a gas discharge plasma. The technique is based on the defocused image analysis of both spherical particles and their binary agglomerates. Above a critical diameter, the defocused images of spherical particles contain stationary interference fringe patterns and the fringe number increases with particle diameters. Below this critical diameter, the particle size has been measured using the rotational interference fringe patterns which appear only on the defocused images of binary agglomerates. In this case, a lower cut-off limit of particle diameter has been predicted, below which no such rotational fringe patterns are observed for the binary agglomerates. The method can be useful as a diagnostics for complex plasma experiments on earth as well as under microgravity condition.

A new type of quasi-two-dimensional complex plasma system was observed which consisted of monodisperse microspheres and their binary agglomerations (dimers). The particles and their dimers levitated in a plasma sheath at slightly different heights and formed two distinct sublayers. The sys- tem did not crystallize and may be characterized as disordered solid. The dimers were identified based on their characteristic appearance in defocused images, i.e., rotating interference fringe pat- terns. The in-plane and inter-plane particle separations exhibit nonmonotonic dependence on the discharge pressure which agrees well with theoretical predictions.

The collective motion of dust particles during the mode-coupling induced melting of a two-dimensional plasma crystal is explored in molecular dynamics simulations. The crystal is compressed horizontally by an anisotropic confinement. This compression leads to an asymmetric triggering of the mode-coupling instability which is accompanied by alternating chains of in-phase and anti-phase oscillating particles. A new order parameter is proposed to quantify the synchronization with respect to different directions of the crystal. Depending on the orientation of the confinement anisotropy, mode-coupling instability and synchronized motion are observed in one or two directions. Notably, the synchronization is found to be direction-dependent. The good agreement with experiments suggests that the confinement anisotropy can be used to explain the observed synchronization process., Comment: 6 pages, 4 figures

The transition between linear and nonlinear regimes of the mode-coupling instability (MCI) operating in a monolayer plasma crystal is studied. The mode coupling is triggered at the centre of the crystal and a melting front is formed, which travels through the crystal. At the nonlinear stage, the mode coupling results in synchronisation of the particle motion and the kinetic temperature of the particles grows exponentially. After melting of the crystalline structure, the mean kinetic energy of the particles continued to grow further, preventing recrystallisation of the melted phase. The effect could not be reproduced in simulations employing a simple point-like wake model. This shows that at the nonlinear stage of the MCI a heating mechanism is working which was not considered so far., Comment: 6 pages, 4 figures

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 figures

A two-dimensional plasma crystal was melted by suddenly applying localized shear stress. A stripe of particles in the crystal was pushed by the radiation pressure force of a laser beam. We found that the response of the plasma crystal to stress and the eventual shear melting depended strongly on the crystal's angular orientation relative to the laser beam. Shear stress and strain rate were measured, from which the spatially resolved shear viscosity was calculated. The latter was shown to have minima in the regions with high velocity shear, thus demonstrating shear thinning. Shear-induced reordering was observed in the steady-state flow, where particles formed strings aligned in the flow direction., Comment: 7 pages, 8 figures, submitted to Physical Review E

The structure of driven three-dimensional complex plasma clusters was studied experimentally. The clusters consisted of around 60 hollow glass spheres with a diameter of 22 microns that were suspended in a plasma of rf discharge in argon. The particles were confined in a glass box with conductive yet transparent coating on its four side walls, this allowed to manipulate the particle cluster by biasing the confining walls in a certain sequence. In this work, a rotating electric field was used to drive the clusters. Depending on the excitation frequency, the clusters rotated (10^4 - 10^7 times slower than the rotating field) or remained stationary. The cluster structure was neither that of nested spherical shells nor simple chain structure. Strings of various lengths were found consisting of 2 to 5 particles, their spatial and temporal correlations were studied. The results are compared to recent simulations.

Two-dimensional plasma crystals are characterized by a strong up-and-down asymmetry not only due to gravity but also due to the presence of plasma flow at the location of particles. We study for the first time the interaction of a single-layer plasma crystal with charged extra particles located above it (upstream of the flow of ions). Upstream extra particles tend to move between the rows of particles in the crystal, accelerate to supersonic speeds, and excite attraction-dominated Mach cones and wakes in the crystal.

A defocusing imaging technique has been used as a diagnostic to identify binary agglomerates (dimers) in complex plasmas. Quasi-two-dimensional plasma crystal consisting of monodisperse spheres and binary agglomerates has been created where the agglomerated particles levitate just below the spherical particles without forming vertical pairs. Unlike spherical particles, the defocused images of binary agglomerates show distinct, stationary/periodically rotating interference fringe patterns. The results can be of fundamental importance for future experiments on complex plasmas.

The dependence of initial magnetic permeability {\mu}i of based on the Fe-Si-B amorphous and nanocrystalline alloys on the {\gamma}-irradiation dose was investigated by the method of inductance factor determination. It was determined that the alloying of amorphous Fe-Si-B alloys by nickel and molybdenum increases the radiation sensitivity of the {\mu}i. Initial magnetic permeability of nanocrystalline magnetic alloys is less sensitive to the action of {\gamma}-irradiation as compared with the alloyed amorphous alloys. Has been suggested that radiation influence on the initial magnetic permeability are caused by creation of non-magnetic incorporation in structure of amorphous alloys as well as amorphous matrix of nanocrystalline alloys., Comment: (5 pages, 4 figures, 2 tables); (http://www.ujp.bitp.kiev.ua)

The microstructure of a strongly coupled liquid undergoing a shear flow was studied experimentally. The liquid was a shear melted two-dimensional plasma crystal, i.e., a single-layer suspension of micrometer-size particles in a rf discharge plasma. Trajectories of particles were measured using video microscopy. The resulting microstructure was anisotropic, with compressional and extensional axes at around $\pm 45^{\circ}$ to the flow direction. Corresponding ellipticity of the pair correlation function $g({\bf r})$ or static structure factor $S(\bf{k})$ gives the (normalized) shear rate of the flow., Comment: 5 pages, 6 figures

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.

The speed-stress relation for gliding edge dislocations was experimentally measured for the first time. The experimental system used, a two-dimensional plasma crystal, allowed observation of individual dislocations at the "atomistic" level and in real time. At low applied stress dislocations moved subsonically, at higher stress their speed abruptly increased to 1.9 times the speed of shear waves, then slowly grew with stress. There is evidence that immediately after nucleation dislocations can move faster than pressure waves., Comment: 5 pages, 4 figures

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 Letters

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 Letters

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, $C_T$. 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.

The shear viscosity of a two-dimensional liquid-state dusty plasma was measured experimentally. A monolayer of highly charged polymer microspheres, with a Yukawa interaction, was suspended in a plasma sheath. Two counter-propagating Ar laser beams pushed the particles, causing shear-induced melting of the monolayer and a shear flow in a planar Couette configuration. By fitting the particle velocity profiles in the shear flow to a Navier-Stokes model, the kinematic viscosity was calculated; it was of order 1 mm^2/s, depending on the monolayer's parameters and shear stress applied., Comment: all 3 figures are combined in a separate pdf file