Out-of-plane mode in 2D complex plasma crystals L. Couëdel , V. Nosenko, S. Zhdanov, A. Ivlev, H. Thomas, and G. Morfill Max-Planck-Institute für extraterrestrische Physik, Giessenbachstraße, 85741 Garching, Germany Introduction In 2D complex plasmas, two in-plane wave modes can be sustained. In crystals, both modes have an acoustic dispersion, one of them is longitudinal, another is transverse. Since the strength of the vertical confinement in such systems is finite, there is a third fundamental wave mode as- sociated with the out-of- plane oscillations. Theory predicts that this mode has a negative (or inverse) optical dispersion [1]. In the presence of strong flow of the ambient plasma the screen- ing cloud around each charged particle becomes highly asymmetric. These “plasma wakes” play the role of an (external) “third body” in the interparticle interaction and hence make it non-reciprocal. It affects dust-lattice (DL) modes and the resonance between DL wave modes can trigger the mode-coupling instability which, in turn, can cause the crystal melting [1, 2]. Experimental set-up electrode video camera RF laser sheet dust particles y x k θ Figure 1: Sketch of the experimental set-up. Microparticles are confined above the rf electrode and are illuminated with a horizontal laser sheet having a Gaussian profile in the vertical direc- tion. The monolayer is levitated well below the peak of the laser intensity. Inset shows elemen- tary cell of the hexagonal lattice and the frame of reference chosen in this paper, the orientation of the wave vector k is measured in respect to the x axis. We used a capacitively coupled RF glow discharge at 13.56 MHz (Fig. 1). The argon pres- sure p was between 0.5 Pa and 1 Pa and the RF peak-to-peak voltage was between 175 V and 310 V (which corresponds to a forward RF power P between 5 W and 20 W). The self-bias voltage was between −60 V and −130 V. The plasma parameters in the bulk discharge were electron temperature T e = 2.5 eV and electron density n e = 2 × 10 9 cm −3 at p =0.66 Pa and 37 th EPS Conference on Plasma Physics O3.311