title: Vertical wave packets observed in a crystallized hexagonal monolayer complex plasma.
abstract: Propagation of vertical wave packets was observed experimentally in a crystallized hexagonal monolayer complex plasma. It was found that the phase velocity exceeded the group velocity by a factor 65 and was directed into the opposite direction as expected for an inverse optical-like dispersion relation. The wave packets propagated keeping their width constant. The explanation of this behavior is based on three-dimensional equations of motion and uses a long-wavelength weak dispersion weak inhomogeneity approximation. While the wave dispersion causes the wave packet to spread, lattice inhomogeneity and neutral gas drag counteract spreading. A plasma diagnostic method was developed that is based on the ratio between vertical and dust-lattice wave speeds. This ratio is very sensitive to the lattice parameter kappa (ratio of the particle separation to the screening length) in a very useful range of kappa < or = 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.
authors.affiliation: Max-Planck-Institut für Extraterrestrische Physik, D-85740 Garching, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Samsonov
authors.forename: S
authors.initials: S
authors.lastname: Zhdanov
authors.forename: G
authors.initials: G
authors.lastname: Morfill