2D Melting of Plasma Crystals: Equilibrium and Nonequilibrium Regimes

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.

Figure 1

Maps of defects in 2D complex plasmas. The left and right columns illustrate the difference between, respectively, the uniform heating experiment I (horizontal dashed lines indicate the heating region) and the recrystallization experiment IV. The dots are individual particles located inside their Voronoi cell: green or red colors represent five- or sevenfold defects, yellow or blue are for four- or eightfold defects. Different “temperature regimes” are shown: (a) is the initial equilibrium state, before the external heating was applied; (b) and (c) are the “high-temperature” regimes, where steady-state experiments (I–III) reveal higher concentration of defects; (d) and (e) demonstrate the reversed trend at lower temperatures, where the recrystallization experiment (IV) is characterized by a higher number of defects. The temperature decreases from (b) to (e).

Figure 2

Concentration of defects versus temperature. The figures show the concentration of fivefold defects $c_5$, the symbols correspond to different experimental series: ⋄ for uniform heating (I), ▿ for temperature gradient (II), ▵ for shear flow (III), and ○ for recrystallization (IV). (a) Log-log plots of $c_5(T)$ for all experiments. Vertical arrows point to different “temperature regimes” illustrated in Figs. 1b, 1c, 1d, 1e, normalized temperature is the inverse coupling parameter ${\Gamma }^{-1}=T\Delta /Q^2$. The unsteady recrystallization series IV exhibits a power-law scaling $c(T)\propto T^{\alpha }$ (fit by a straight line). (b) Log-norm plots of $c_5$ versus $T^{-1}$ for steady-state series I–III demonstrate equilibrium Arrhenius behavior $c(T)\propto e^{-W/T}$ (fits by straight lines). The inset in panel (a) shows the bond-orientational correlation function $g_6(r)$ for different temperature regimes in the experiment I, see left column of Fig. 1.