Vertical wave packets in a two-dimensional strongly coupled complex (dusty) plasma

Summary form only given. Propagation of vertical wave packets was observed experimentally in a hexagonal monolayer complex plasma. The complex plasma was formed of 8.9 /spl mu/m charged monodisperse plastic microspheres suspended a few millimeters above a flat electrode in a capacitively coupled radio-frequency discharge. Vertical oscillations were excited by a short electrostatic pulse applied to a wire stretched below the lattice. The lattice was illuminated by a thin laser sheet and observed with a top view high speed video camera. A new method of vertical motion diagnostics based on particle visible brightness was used. Wave packets propagating away from the excitation source were observed. They kept their width L=18 mm (equal to the modulating wavelength) constant. It was found that the phase velocity of the vertical waves exceeded its group velocity by a factor 70 and had the opposite direction as expected for an optical-like dispersion relation (backward wave). The complex plasma was in a strongly coupled (crystallized) state and had the following properties: the vertical wave group velocity V/sub gr/=4 mm/s, the vertical wave phase velocity V/sub ph/=-290 mm/s, and the compressional (dust-lattice) wave velocity C/sub DL/=35 mm/s. The theory describing the lattice motion is based on three-dimensional equations of motion and uses a long wave-length weak dispersion weak inhomogeneity approximation. It takes into account interaction of each particle with each other and is not limited by the nearest neighbour approximation. It was found that the constant wave packet with is provided by a balance of dispersion and weak lattice inhomogeneity. While the dispersion causes the wave packet to spread, lattice inhomogeneity focuses it. Neutral gas damping can also counteract packet spreading. The theory describes propagation of well formed wave packets (not their initial stages after the excitation) and shows a good agreement with the experiment. A new method of plasma - iagnostics was developed that is based on the ratio between vertical and dust-lattice waves speeds. This ratio is very sensitive to the screening parameter /spl kappa/ in a very useful range of /spl kappa//spl les/2. It was found that only a two-dimensional lattice model can provide a quantitative description of the vertical waves, while a linear chain model gives only a qualitative agreement.