Buckling of two-dimensional plasma crystals with nonreciprocal interactions

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.

Square lattice formation in a monodisperse complex plasma

“…Due to this wake suppression, the vertical pairing in our experiments become insignificant, which ultimately paves the way for the formation of the square lattice in the complex plasma. In this situation, when the external confinement is decreased below a critical value via decreasing the discharge voltage, the out-of-plane mode of dust lattice wave becomes purely imaginary [21]. It induces a transverse instability in the system, making the structure unstable.…”

“…Ion wake forces give dusty plasmas a unique character and distinguish them from other systems like colloidal or conventional plasmas. They play an important role in the phenomena of crystal melting [20] as well as in structural transformations [21]. Experiments in 3-D dusty plasmas have revealed different structural phases like fcc and bcc [22,23] while in a monolayer dust crystal formed from monodisperse particles the predominant observed phase is that of hexagonal structures [24].…”

“…Square lattice formations have been experimentally observed in binary complex plasmas where two dust species with nearly identical charge to mass ratios are seen, under certain conditions, to form a narrowly spaced bilayer which exhibits square lattice patterns [25]. For a monodisperse dust layer square lattices have been observed in MD simulations by inducing a buckling in the crystal layer through a manipulation of the confinement potential and wakefield interactions [21]. However, to date, there has been no experimental demonstration of a square lattice formation in a monodisperse complex plasma.…”

“…A major requirement for transitioning to such a state is the creation of a stable bilayer (a quasi-2D state) from the monolayer hexagonal crystal structure. This can be done through buckling the monolayer by altering the harmonic component of the confining potential as was demonstrated in the MD simulations of Zampetaki et al [21]. Such a technique has been successfully tried out experimentally in different systems like laser cooled trapped ions and colloids.…”

“…This instability, known as the mode coupling instability (MCI), is a major deterrent for the formation of a square lattice structure in a monodisperse complex plasma. As discussed in [21], one needs to suppress the MCI through proper control of the wake field interactions as well as neutral damping to successfully attain the desired structural phase transition. In our experimental device the asymetric electrode configuration has a direct influence on the course of the streaming ions that helps in the stability and ease of formation of the crystalline state [26].…”