Phonon spectra split in vertically aligned bilayer crystal of binary complex plasma

Abstract

Structural and dynamical properties of a bilayer binary strongly coupled dusty plasma system in a sheath of an rf discharge are experimentally investigated. Microparticles of two different sizes are aligned into vertical pairs due to ion focusing (plasma wakes) downstream from the upper particles in the ion flow toward the bottom electrode. The pairs are ordered into a 2D hexagonal crystalline structure. The collective mode dispersion relations are measured and analyzed. Splitting of the modes in the bilayer lattice is observed and theoretically described. The relative intensities of the mode branches reflect non-reciprocal features of the wake-mediated interaction between the microparticles. The conditions of stability of vertically aligned bilayer structure are discussed, and relevant plasma parameters are estimated.