Investigating Solid-Fluid Phase Coexistence in DC Plasma Bilayer Crystals: The Role of Particle Pairing and Mode Coupling

TL;DR

The paper addresses solid–fluid phase coexistence in bilayer dusty plasmas and why classical monolayer MCI fails to explain it. It combines phonon-spectrum analysis with particle-tracking under varying confinement ring bias in a DC glow discharge to probe interlayer coupling and wake-mediated effects. The study identifies dynamic interlayer particle pairing and wake-mediated nonreciprocity as primary drivers of melting, and introduces a pair-resolved nonreciprocity metric $R=\langle|F_{top}+F_{bottom}|\rangle$ that correlates with phase instability. The findings reveal a hybrid mechanism for bilayer phase transitions and provide a framework for diagnosing energy transport, defect dynamics, and structural stability in non-equilibrium layered plasmas.

Abstract

This article presents a detailed investigation of solid-fluid phase coexistence in a bilayer dusty plasma crystal subjected to varying confinement ring bias voltages in a DC glow discharge argon plasma. Melamine formaldehyde particles were employed to form a stable, hexagonally ordered bilayer crystal within a confinement ring electrically isolated from the grounded cathode. By systematically adjusting the confinement ring bias, a distinct phase coexistence emerged: it is characterized by a fluid-like melted core surrounded by a solid crystalline periphery. Crucially, analysis of the phonon spectra revealed frequency shifts that deviate significantly from the predictions of classical monolayer Mode-Coupling Instability (MCI) theory. Stability analysis further demonstrated that dynamic interlayer particle pairing and non-reciprocal interactions play a pivotal role in destabilizing the bilayer structure. These findings highlight previously underappreciated mechanisms driving the melting transition in bilayer dusty plasmas, offering a more comprehensive understanding of phase behavior in complex plasma systems. The results underscore the importance of interlayer coupling and confinement effects in tuning structural transitions.

Figures (8)

• Figure 1: Schematic of the experimental arrangement. L$_1$ and L$_2$ are green (532 nm) and red (650 nm) illumination lasers.
• Figure 2: Snapshot showing overlapped image of 20 consecutive frames from the top view camera at 103 V of confining ring voltage. Circular marked region shows the region of melting. The outside region is almost stationary with a small thermal fluctuation around the equilibrium position.
• Figure 3: View of the dust cloud from the side camera at different bias voltage. The layer gap decreases with reducing confinement potential from 140 V to 103 V.
• Figure 4: Longitudinal (top) and transverse (bottom) spectra for $0^\circ$ k-vector orientation
• Figure 6: Optical phonon modes for different bias voltages