Transition of a 2-D crystal to a non-equilibrium two-phase coexistence state in a dusty plasma *

The transition of a 2D monolayer dust crystal to a nonequilibrium solid-liquid phase coexistence is investigated in a glow discharge Argon plasma.  Initially, a monolayer crystal of MF particles is formed at a discharge voltage of 400 V and neutral gas pressure of 6 Pa. which gets converted to a two-phase coexistence state of a molten liquid at the center surrounded by a crystalline periphery when the gas pressure is reduced below a threshold value.  Prior to this transition, the particles exhibit a self-excited horizontal oscillation of frequency ~30 Hz at the center of the monolayer. When the pressure is reduced further below another threshold value, the system shows a multilayer cold liquid-like behavior. Various structural and thermodynamic quantities are seen to differ significantly after a phase transition. The average kinetic energy of the central region increases and the inter-particle distance in both the coexistence phases increases with the decrease in gas pressure. Similar behaviour is seen when the discharge voltage is reduced gradually below a threshold value at a given pressure. The formation of an ion wake resulting in a  Schweigert instability may be responsible for the formation of phase coexistence.