Experimental investigation of a triple point in a dusty plasma

Abstract

In this work, we present the first experimental observation of a triple point in a strongly coupled complex plasma medium. Initially, a 2D crystalline structure of the dust component consisting of monodisperse micrometer-sized particles is created. The evolution of this monolayer as a function of the discharge voltage is then investigated and, in the course of this evolution, the co-existence of three distinct phases of the system consisting of a liquid region and two crystalline regions with square and hexagonal lattice configurations is observed. Two different transition sequences are identified in the system, depending on the initial dust density of the monolayer. The particles in the fluid region have high kinetic energy. Before the emergence of a triple state, it is found that the particles in the different lattice geometries show different in-plane or out-of-plane oscillations. It is conjectured that a complex or mixed instability that originates in the system in a localized manner gives rise to the formation of the triple point in our experiments.