Impurity modes in two-dimensional strongly coupled complex plasma crystals

Complex plasma fluctuation processes have been extensively studied in many aspects, especially lattice waves in strongly coupled plasma crystals, which are of great significance for understanding fundamental physical phenomena. A challenge of experimental investigations in two-dimensional strongly coupled complex plasma crystals is to keep the main body and foreign particles of different masses on the same horizontal plane. To solve the problem, we have proposed a potential well formed by two negatively biased grids to bind the negatively charged particles in a two-dimensional (2D) plane, thus achieving a 2D plasma crystal in the microgravity environment. The study of such phenomena in complex plasma crystals under microgravity environment then becomes possible. In this paper, we focus on the continuum spectrum, including both phonon and optic branches of the impurity mode in a 2D system in microgravity environments. The results show the dispersion relation of the longitudinal and transverse impurity oscillation modes and their properties. Considering the macroscopic visibility of complex mesoscopic particle lattices, theoretical and experimental studies on this kind of complex plasma systems will help us further understand the physical nature of a wide range of condensed matters.