Self-propelled motion of a particle above a two-dimensional plasma crystal

Plasma crystals consist of ordered arrangements of charged microparticles that levitate in a weakly ionized gas. Besides confining the particles, the plasma also alters the microparticle interaction, making it nonreciprocal. The nonreciprocal interactions cause interesting effects like the mode-coupling instability and the formation of particle strings or spinning particle pairs. Here, we analyze in simulations and theory the self-propelled motion of an "extra" particle floating above a two-dimensional plasma crystal. Previous experimental observations [1] showed that the extra particle is confined in a channel of two neighboring rows of particles and moves persistently through the crystal. We use the simple model of a pointlike ion wake charge to reproduce this intriguing effect in simulations. We show that the nonreciprocity of the particle interaction is responsible for a broken symmetry of the channel that enables the self-propelled motion of the extra particle [2] . [1] C.-R. Du, V. Nosenko, S. Zhdanov, H. M. Thomas, and G. E. Morfill, Phys. Rev. E 89, 021101(R) (2014) [2] I. Laut, C. Räth, S. K. Zhdanov, V. Nosenko, G. E. Morfill, and H. M. Thomas, Physical Rev. Lett. 118, 07500