Non-reciprocity in quasi-two-dimensional complex plasmas

Complex plasmas, which are composed of a weakly ionized gas and charged microparticles, represent the plasma state of soft matter. Along with colloidal dispersions, they have been widely used as a natural model system of various generic phenomena occurring in liquids and solids. The dynamics of individual microparticles in experiments with quasi-two-dimensional (q2D) complex plasmas levitating above a horizontal electrode can be easily visualized and measured. These systems have two remarkable features distinguishing them from other soft-matter systems. First, since the surrounding gas is dilute, the short-time dynamics of individual microparticles in strongly coupled complex plasmas is practically undamped. Second, the effective interparticle interactions in q2D complex plasmas violate the action-reaction symmetry: the vertical electric field near the electrode induces a plasma flow, which generates plasma wakes around the levitating microparticles and thus makes their effective electrostatic interactions nonreciprocal.

In a monolayer system composed of monodisperse particles, under certain conditions the non-reciprocity triggers the so-called mode-coupling instability, accompanied by an explosive growth of the particle kinetic temperature. Furthermore, in a (stable) binary system of different particles levitating at slightly different heights, the non-reciprocal interactions lead to a remarkable kind of detailed dynamical equilibrium, where different particle sorts have different kinetic temperatures.

The degree of non-reciprocity can be easily controlled in experiments with q2D complex plasmas, which makes such systems perfectly suited for studying collective out-of-equilibrium phenomena. In my talk I will present experiments illustrating the non-equilibrium behavior, explain the underlying mechanisms, and discuss interesting analogies with non-equilibrium phenomena occurring in regular liquids and solids.