Abstract
Laboratory realizations of two-dimensional (2D) plasma crystals typically involve monodisperse microparticles confined into horizontal monolayers in radio-frequency (rf) plasma sheaths. This gives rise to the so-called plasma wakes beneath the microparticles. The presence of wakes renders the interactions in such systems nonreciprocal, a fact that can lead to a quite different behavior from the one expected for their reciprocal counterparts. Here we examine the buckling of a hexagonal 2D plasma crystal, occurring as the confinement strength is decreased, taking explicitly into account the nonreciprocity of the system via a well-established point-wake model. We observe that for a finite wake charge, the monolayer hexagonal crystal undergoes a transition first to a bilayer hexagonal structure, unrealizable in harmonically confined reciprocal Yukawa systems, and subsequently to a bilayer square structure. Our theoretical results are confirmed by molecular dynamics simulations for experimentally relevant parameters, indicating the potential of their observation in state-of-the-art experiments with 2D complex plasmas.
Highlights
Monodisperse negatively charged microsized particles, levitating in a plasma sheath above a powered radio-frequency electrode, form, under sufficiently strong confinement, a monolayer hexagonal complex plasma crystal [1,2]
Our theoretical results are confirmed by molecular dynamics simulations for experimentally relevant parameters, indicating the potential of their observation in state-of-the-art experiments with 2D complex plasmas
We have investigated the buckling of 2D monodisperse complex plasma crystals as their vertical confinement weakens
Summary
Monodisperse negatively charged microsized particles, levitating in a plasma sheath above a powered radio-frequency (rf) electrode, form, under sufficiently strong confinement, a monolayer hexagonal complex plasma crystal [1,2] Such crystals offer the possibility to study complex kinetic phenomena in solids [3,4,5], such as crystal melting [2,6,7,8,9,10,11] and the dynamics of dislocations [12], on a particle-resolved level. In our theoretical and numerical investigation, we explicitly take into account the nonreciprocal character of pair interactions in the system, employing a simplified but successful model of wakes as pointlike positive charges located below the dust particles [15,28,54,55] Within this model, we show that for decreasing confinement strength, the hexagonal monolayer gives its place to a hexagonal triple or bilayer crystalline structure, depending on the value of the effective wake charge. Our detailed phase diagram for experimentally relevant values of the parameters extends the existing theoretical results to the case of nonreciprocal interactions and allows for estimating the conditions under which each of the investigated structures can be found, facilitating their observation in experiments of 2D complex plasma crystals
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