Recrystallization of a 2D Plasma Crystal

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.

Figure 1

Sketch of the experimental setup. (a) Oblique view. A monolayer of particles is levitated above the biased electrode and illuminated with a laser sheet. Two parallel wires are mounted 1.1 mm above the particle layer. (b) Schematic top view of the setup. The dashed square outlines the field of view of the camera. The horizontal dotted line denoting the position where the waves from the two wires meet divides the field of view into areas A and B, also used for the temperature analysis.

Figure 2

Mean particle temperature $T$ vs time. Solid red lines correspond to exponential fits in four consecutive regimes with different decays, numbered I to IV (marked by vertical dashed lines). The gap in the time series marks the phase of extremely fast particle motion shortly after the disturbance. Inset: Temperature from simulation. Solid red lines are the same exponentials as those found for the experimental data.

Figure 3

Squared pair correlation peak width ${\sigma }_0^2$ vs particle temperature. Solid lines are linear fits to three temperature intervals. The arrows indicate the regime borders of Fig. 2. The inset shows a close-up of the temperature range $\le 1\mathrm{eV}$.

Figure 4

(a) Defect fractions $N_5/N$ (red) and $N_7/N$ (blue) vs time, and (b) correlation lengths $\xi /\Delta$ (red) and ${\xi }_6/\Delta$ (blue) vs time. The dashed lines mark the regimes I–IV of temperature decay. (c) Defect fractions $N_5/N$ (red) and $N_7/N$ (blue) vs temperature. The solid line is $\propto T^{0.37}$. (d) Correlation lengths $\xi /\Delta$ (red) and ${\xi }_6/\Delta$ (blue) vs temperature. The solid line is $\propto T^{-0.29}$. The arrows in (c),(d) show the position of the regime borders.

Figure 5

Color-coded maps of the crystal for different regimes of recrystallization. The background color corresponds to the value of $|{\mathbf{\Psi }}_{6,k}|$ (see colorbar at the top). The arrows represent the vector field of $\arg ({\mathbf{\Psi }}_{6,k})$. Defects are marked by red (fivefold) and blue (sevenfold) dots. (a) Crystal structure before melting (regime 0); (b) liquidlike state immediately after melting (regime I); (c),(d) consecutive regimes II and III of crystallization; (e) metastable regime IV; (f) crystallite structure observed in simulations.