  Laboratory realizations of 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
non-reciprocal, a fact that can lead to a quite different behaviour 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 non-reciprocity of the system via
a well-established point-particle wake model. We observe that for a finite wake
charge, the monolayer hexagonal crystal undergoes a transition first to a
bilayer hexagonal structure, unrealisable 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.
