Melting and heating of two-dimensional Coulomb clusters in dusty plasmas

Rotating particle pair produces hot complex plasma crystals

Rotating quasipaired particles (torsions) are observed within a two-dimensional monolayer crystal suspended in an argon complex plasma for discharge powers of 1-10 W and pressures of 135-155 mTorr. The inclusion of a torsion in a crystal lattice fundamentally changes the overall lattice state to a "hot crystal." A torsion increases the particle motion and kinetic energy of other particles in the crystal, with the strongest effects on neighboring particles. The apparent effective range is to the third nearest neighbor, with the kinetic energy in the first three shells of particles increasing by at least 200% over baseline values for the crystal. However, the variance of the motion of all particles in the crystal increases by more than two times over the average background kinetic fluctuations for the whole crystal. The formation of a torsion perturbs the structure and symmetry of a plasma crystal. A single torsion causes the average interparticle spacing to increase by 11% compared to the same crystal without a torsion. Particles in the first two shells surrounding a torsion also display reduced hexagonal symmetry. The combination of the perturbed lattice structure and the larger range of motion for the microparticles contribute to a higher-energy-state crystal when torsions are present.

Phase transition of three-dimensional finite-sized charged dust clusters in a plasma environment

The dynamics of a harmonically trapped three-dimensional Yukawa ball of charged dust particles immersed in plasma is investigated as function of external magnetic field and Coulomb coupling parameter using molecular dynamics simulation. It is shown that the harmonically trapped dust particles organize themselves into nested spherical shells. The particles start rotating in a coherent order as the magnetic field reaches a critical value corresponding to the coupling parameter of the system of dust particles. The magnetically controlled charged dust cluster of finite size undergoes a first-order phase transition from disordered to ordered phase. At sufficiently high coupling and strong magnetic field, the vibrational mode of this finite-sized charged dust cluster freezes, and the system retains only rotational motion.

Rotating vortices in two-dimensional inhomogeneous strongly coupled dusty plasmas: Shear and spiral density waves

Dusty plasma experiments can be performed quite easily in a strong coupling regime. In our previous work [V. S. Dharodi, S. K. Tiwari, and A. Das, Physics of Plasmas 21, 073705 (2014)]PHPAEN1070-664X10.1063/1.4888882, we numerically explored such plasmas with constant density and observed the transverse shear (TS) waves from the rotating vortex. Laboratory dusty plasmas are good examples of homogeneous plasmas; however, heterogeneity (e.g., density, temperature, and charge) may be due to the existence of voids, different domains with different orientations, presence of external forces like magnetic and/or electric, size or charge imbalance, etc. Here, we examine how the density heterogeneity in dusty plasmas responds to the circularly rotating vortex monopoles, specifically, smooth and sharp cutoff. For this purpose, we have carried out a series of two-dimensional fluid simulations in the framework of the incompressible generalized hydrodynamics fluid model. The rotating vortices are placed at the interface of two incompressible fluids with different densities. The smooth rotating vortex causes two effects: First, the regions are stretched to form the spiral density waves; second, there is a shear in flows which consequently induces the TS waves. The TS waves move slower in the denser side than in the lighter side. The difference in speeds of the waves induces the net flow of the medium towards the lower density side. We notice that the spiral density arms are distinguishable in the early time while later they get smeared out. In sharp flows, the interplay between the TS waves and the vortices of Kelvin-Helmholtz instability distorts the formation of the regular spiral density arms around the rotor.

Experimental observation of a first-order phase transition in a complex plasma monolayer crystal

The formation and melting of a monolayered charged dust particle crystal in a DC glow discharge argon plasma is studied. The nature of the melting or formation process is established as a first-order phase transition from the variations in the Coulomb coupling parameter, the dust temperature, the structural order parameter, and from the existence of a hysteresis behavior. Our experimental results are distinctly different from existing theoretical predictions for two dimensional crystals based on the Kosterlitz-Thouless-Halperin-Nelson-Young mechanism or the grain boundary induced melting and indicate a mechanism that is akin to a fluctuation induced first-order phase transition in complex plasmas.

Melting of a finite-sized two-dimensional colloidal crystal

We have studied the melting process of a finite-sized two-dimensional colloidal crystal by video microscopy. The local area fraction ϕ and the local hexatic orientational order parameter ψ(6) have been evaluated for respective Voronoi cells in the crystal. The histogram of ϕ exhibits a peak and the peak ϕ continuously decreases with the time elapsed. The histogram of |ψ(6)| shows an abrupt broadening for ϕ < 0.65. This critical value of ϕ is the transition point between the hexatic and dense liquid phases in finite crystal. We have also evaluated ϕ and |ψ(6)| as a function of the distance from the center of the crystal r. ϕ(r) is almost constant within the crystal and monotonously decreases with the time elapsed. |ψ(6)(r)| gradually decreases with r but there is the core with |ψ(6) = 1 at earlier time stage. The temporal change of the average ϕ within the crystal is qualitatively explained by the slow diffusion of the particles situated at the crystal edge. The steric repulsion between the particles within the crystal enhances the expansion rate of the crystal edge. Overall melting behavior is same in the crystals with different sizes. We have also studied the melting of a finite-sized crystal composed of soft-core particles by Brownian dynamics simulation and verified the finite-size effect on the melting process. The simulated behavior is qualitatively in good agreement with the experimental results.

Nonequilibrium finite dust clusters: connecting normal modes and wakefields

The dynamic properties of finite three-dimensional dust clusters in a dusty plasma under the influence of an ion focus are studied by normal modes. The mode analysis has been extended to account for the ion focus using the point-charge model for the horizontal interaction of the dust particles. From that, an analytical model for a few-particle system is derived accounting for three distinct dynamical regimes at different focus strengths, namely, absolutely unstable and fully stable configurations as well as an unstable oscillatory regime. The techniques of normal mode analysis (NMA) and instantaneous normal modes (INM) extended by the ion focus have been applied to dust systems in the experiment and compared to the model. From this, the ion focus strength has been derived. The specific sensitivity of NMA and INM allows one to identify equilibrium configurations in this nonequilibrium environment for these finite clusters.

Polygon construction to investigate melting in two-dimensional strongly coupled dusty plasma

The polygon construction method of Glaser and Clark is used to characterize melting and crystallization in a two-dimensional (2D) strongly coupled dusty plasma. Using particle positions measured by video microscopy, bonds are identified by triangulation, and unusually long bonds are deleted. The resulting polygons have three or more sides. Geometrical defects, which are polygons with more than three sides, are found to proliferate during melting. Pentagons are found in liquids, where they tend to cluster with other pentagons. Quadrilaterals are a less severe defect, so that disorder can be characterized by the ratio of quadrilaterals to pentagons. This ratio is found to be less in a liquid than in a solid or a superheated solid. Another measure of disorder is the abundance of different kinds of vertices, according to the type of polygons that adjoin there. Unexpectedly, spikes are observed in the abundance of certain vertex types during rapid temperature changes. Hysteresis, revealed by a plot of a disorder parameter vs temperature, is examined to study sudden heating. The hysteresis diagram also reveals features suggesting a possibility of latent heat in the melting and rapid cooling processes.

Normal modes of a small bilayer system of binary classical charged particles trapped in a parabolic confinement potential

The normal modes and the melting character of a bilayer system consisting of binary charged particles with different charge and/or different mass, interacting through a Coulomb potential and confined in a parabolic trap are investigated. The normal mode spectrum is discussed as a function of the charge ratio (CR) and mass ratio (MR) of the two kinds of charged particles as well as the interlayer separation. We show that the dependence of the normal modes on the excited states can be tuned by varying the CR, the MR, and the interlayer distance. Once the interlayer distance is larger than a critical value, the first excited state corresponds only to the intershell rotation mode. In addition, the intershell rotation melting temperature is discussed as a function of the CR and MR as well as the interlayer separation.

Image force on a charged projectile moving over a two-dimensional strongly coupled Yukawa system

We use both analytical theory and numerical simulations to study the image force on a charged particle moving parallel to a two-dimensional strongly coupled Yukawa system. Special attention is paid to the effects of strong correlation and nonlinear response in the Yukawa system on the dependences of the image force on the particle velocity and its distance from the Yukawa system. Those effects are elucidated by comparing the results obtained from a Brownian dynamics simulation with those from linear-dielectric-response theories based on both the quasilocalized charge approximation and the standard Vlasov random phase approximation.

Energy loss of a charged particle moving over a 2D strongly coupled dusty plasma

We use molecular dynamics (MD) simulation to evaluate the energy loss of a charged projectile moving parallel to a two-dimensional strongly coupled dusty plasma and compare the results with those obtained from the quasilocalized charge approximation (QLCA) and the Vlasov-random phase approximation. Good agreement is found between the QLCA and MD results when the projectile-dust coupling is weak. In the opposite regime, nonlinear effects in the dust-layer response render the QLCA model increasingly inadequate for calculating the energy losses at low projectile speeds.

Finite-size effects in the dynamics and thermodynamics of two-dimensional Coulomb clusters

The dynamics and thermodynamics of melting in two-dimensional Coulomb clusters is revisited using molecular dynamics and Monte Carlo simulations. Several parameters are considered, including the Lindemann index, the largest Lyapunov exponent, and the diffusion constant. In addition to the orientational and radial melting processes, isomerizations and complex size effects are seen to occur in a very similar way to atomic and molecular clusters. The results are discussed in terms of the energy landscape represented through disconnectivity graphs, with proper attention paid to the broken ergodicity problems in simulations. Clusters bound by 1/r3 and e(-kappar)/r forces, and heterogeneous clusters made of singly and doubly charged species, are also studied, as well as the evolution toward larger systems.

Laser heating of particles in dusty plasmas

Experiments on the heating and melting of two-dimensional finite dust crystals are performed using random laser excitation of the dust particles by a rapidly moving laser beam. The achievable dust temperatures scale with the square of the laser power. The heating process is described for different dust clusters under various plasma and cluster conditions. A single-particle model is developed to explain the observed behavior of the cluster under the random laser excitation. Good quantitative agreement is found when the radiation pressure is made responsible for the particle excitation by the laser. The dynamical properties of the system during heating are analyzed and the dominant modes are identified. From this, it is demonstrated that the heating process is of a nearly equilibrium nature in contrast to previous melting experiments. Finally, the melting of the dust cluster by laser heating is studied. From these experiments, a precise determination of the critical coupling parameter for the solid-fluid transition was possible. It is measured as Gamma = 270-480 for an N = 18 cluster.