Figure 1

Sketch of the experimental setup. Microparticles are confined above the rf electrode and are illuminated with a horizontal laser sheet having a Gaussian profile in the vertical direction (with a standard deviation $\sigma =75\mu \mathrm{m}$). The monolayer is levitated well below the peak of the laser intensity, which results in strong intensity variations of the scattered light upon the vertical displacement of individual particles. The upward (downward) displacement corresponds to positive (negative) intensity variation. Microparticles are recorded from the top at a speed of 250 frames per second.Reuse & Permissions

Figure 2

Example of the instability onset in a 2D plasma crystal. The shown experiment is performed at a rf power of 20 W with particles of $8.77\mu \mathrm{m}$ diameter. The pressure of argon was ramped down in steps of 0.02 Pa (every $\simeq 300\mathrm{s}$), and the instability was triggered at 0.90 Pa. The particle trajectories (superposition of consecutive video frames during 0.04 s) are plotted about 50 s after the critical pressure decrease: (a) side view; (b) top view. The profile of the particle density in the monolayer (c) was measured before the instability onset [averaged across the 5 mm stripe marked in (b) by two horizontal dashed lines]. In the upper left corner the elementary cell of the hexagonal lattice is depicted with the frame of reference chosen in this Letter; due to the lattice symmetry we consider the wave vector $\mathbf{k}$ at $0°\le \theta \le 30°$.Reuse & Permissions

Figure 3

Evolution of the DL dispersion relations near the instability onset. The dispersion relations (“fluctuation spectra”) were measured in the experiment performed at argon pressure of 0.76 Pa with particles of $9.19\mu \mathrm{m}$ diameter, at a rf power of 7 W (left column, stable regime) and 5 W (right column, the instability onset). The logarithmic color coding is used for the “fluctuation intensity” $\Delta ϵ/\Delta k\Delta f$ (energy density, for unit mass, per unit wave number and frequency): Yellow ridges represent stable wave modes, and the dark-red “hot spots” are the unstable hybrid mode; (a) and (b) show the measured out-of-plane mode, and (c) and (d) depict the pair of in-plane modes. The solid lines in both panels are theoretical results [32] (see text for details). The wave number $k$ is normalized by the inverse interparticle distance $a^{-1}$ (determined from the first peak of the pair correlation function; the uncertainty of $a$ was $\simeq 4\%$ due to density variations—see, e.g., Fig. 2c). The shown range of $k$ corresponds to the first two Brillouin zones at $\theta =30°$ (branches are symmetric with respect to the border between the two zones). Below the fluctuation spectra the magnified hybrid mode in the unstable regime (marked by a rectangular box) is shown. (e) and (f) illustrate the distribution of the “fluctuation energy” $\Delta ϵ/\Delta k$ at fixed frequency: In the stable case the frequency ($\simeq 12.5\mathrm{Hz}$) is chosen in the middle between the longitudinal in-plane branch and the out-of-plane branch (measured at the border between the Brillouin zones); for the instability onset the frequency ($\simeq 11\mathrm{Hz}$) corresponds to the hybrid mode.Reuse & Permissions