A monolayer plasma crystal consisting of micron-sized particles levitated in the sheath of a rf discharge was melted by applying a short electric pulse to two parallel wires located at the height of the particles. Structural properties and the particle temperature were examined during the stage of recrystallization. A liquidlike phase was followed by a transient state characterized by energy release and the restoring of long range translational order while the defect fraction was low. No long range orientational order was found, though highly ordered domains formed locally. Numerical simulations revealed the same regimes of recrystallization as those observed in the experiment.
Comprehensive experimental investigations of melting in two-dimensional complex plasmas were carried out. Different experiments were performed in steady and unsteady heating regimes. We demonstrate an Arrhenius dependence of the defect concentration on the kinetic temperature in steady-state experiments, and show the evidence of metastable quenching in unsteady experiments, where the defect concentration follows a power-law temperature scaling. In all experiments, independent indicators suggest a grain-boundary-induced melting scenario.
Viscoelastic vortical fluid motion in a strongly coupled particle system has been observed experimentally. Optical tracking of particle motion in a complex plasma monolayer reveals high grain mobility and large scale vortex flows coexistent with partial preservation of the global hexagonal lattice structure. The transport of particles is superdiffusive and ascribed to Lévy statistics on short time scales and to memory effects on the longer scales influenced by cooperative motion. At these longer time scales, the transport is governed by vortex flows covering a wide spectrum of temporal and spatial scales.
We propose a simple method to determine the local coupling strength Gamma experimentally, by linking the individual particle dynamics with the local density and crystal structure of a 2D plasma crystal. By measuring particle trajectories with high spatial and temporal resolution we obtain the first maps of Gamma and temperature at individual particle resolution. We employ numerical simulations to test this new method, and discuss the implications to characterize strongly coupled systems.
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