Structural phase transition in a monodisperse 2D complex plasma

We present experimental observations of a structural phase transition in a monodisperse complex plasma wherein a 2-Dimensional plasma crystal with inherent hexagonal configuration transitions to a bilayer square configuration. The experiments were carried out in the Dusty Plasma Experimental (DPEx-II) device in DC glow discharge Argon plasma environment. We have found that mitigating particle wake interactions in the dust system is a crucial prerequisite for the attainment of transition. The unique electrode configuration of our device and the specific discharge conditions are helpful in providing such a mitigation. When the vertical confinement of the dust particles is decreased, subsequent to suppressing the particle-wake interaction, the 2D dust system buckles and triggers a transverse instability in the system which results in the structural phase transition. Local bond order and structure static factor are used to distinguish different structural phases of the system. The formation of a square lattice is accompanied by the bifurcation of the monolayer to a bilayer system where the interlayer distance increases with decrease of confinement strength. Our results also demonstrate a clear hysteresis in the system. Molecular dynamics (MD) simulation is also performed by incorporating the various forces which are dominant in our experimental conditions. The MD results reproduce the main features of our experimental observations.