Stability of two-dimensional complex plasma monolayers in asymmetric capacitively-coupled radio-frequency discharges

TL;DR

This study investigates the stability and phase transitions of two-dimensional complex plasma monolayers in asymmetric RF discharges, highlighting threshold pressures for crystallization and melting influenced by sheath profiles and ion wakes.

Contribution

It integrates experimental control parameters with theoretical modeling to identify pressure thresholds for phase changes in plasma monolayers, advancing understanding of plasma crystal stability.

Findings

Existence of two threshold pressures p_cryst and p_MCI for phase transitions.

Recrystallization occurs when pressure increases past p_cryst.

Melting occurs when pressure decreases below p_MCI due to mode-coupling instability.

Abstract

In this article, the stability of a complex plasma monolayer levitating in the sheath of the poweredelectrode of an asymmetric capacitively coupled radio-frequency argon discharge is studied. Com-pared to earlier studies, a better integration of the experimental results and theory is achieved byoperating with actual experimental control parameters such as the gas pressure and the dischargepower. It is shown that for a given microparticle monolayer at a fixed discharge power there existtwo threshold pressures: (i) above a specific pressure p cryst , the monolayer always crystallises; (ii)below a specific pressure p MCI , the crystalline monolayer undergoes the mode-coupling instability andthe two-dimensional complex plasma crystal melts. In-between p MCI and p cryst , the microparticlemonolayer can be either in the fluid phase or the crystal phase: when increasing the pressure frombelow p MCI , the monolayer remains in the fluid phase until it reaches p cryst at which it recrystallises;when decreasing the pressure from above p cryst , the monolayer remains in the crystalline phase untilit reaches p MCI at which the mode-coupling instability is triggered and the crystal melts. A simpleself-consistent sheath model is used to calculate the rf sheath profile, the microparticle charges and themicroparticle resonance frequency as a function of power and background argon pressure. Combinedwith calculation of the lattice modes the main trends of p MCI as a function of power and backgroundargon pressure are recovered. The threshold of the mode-coupling instability in the crystalline phaseis dominated by the crossing of the longitudinal in-plane lattice mode and the out-of plane latticemode induced by the change of the sheath profile. Ion wakes are shown to have a significant effecttoo.