Wave dispersion in a three-dimensional complex plasma solid under microgravity conditions.

An analysis of lattice wave spectra in a three-dimensional dusty plasma structure formed in a direct current gas discharge with alternating polarity under microgravity conditions is reported. The spectra are determined using the Fourier transform of microparticle velocities, measured by tracking microparticles with subpixel resolution. Both longitudinal and transverse modes are detected and analyzed. The absence of a "k-gap" in the long-wavelength domain of the transverse mode strongly suggests that the microparticles form a solid structure. Therefore, the experimental spectra are compared with the spectra obtained from molecular dynamics simulations for different lattice structures and their orientation. This comparison yields important dusty plasma parameters, such as the particle charge and the plasma screening length. The measured longitudinal and transverse sound velocities allow us to estimate the elastic moduli of the particle component. These are rather small in the absolute magnitude, but when normalized by the number density and the interaction energy of the particles resemble those in conventional matter.