Intermittent dynamics in Coulomb clusters

Complex systems away from equilibrium often experience intermittent oscillations between quiescent and highly dynamic states. Here we show how this phenomenon can be experimentally studied in a system as small as 2 interacting particles. Our system is a dusty plasma, where micron-sized charged particles are levitated in an argon plasma and form crystalline structures due to the competition between a confining potential and screened Coulomb interactions between particles. We investigate rotating clusters of 2-100 particles, and under certain gas pressures and rf power conditions, the clusters exhibit intermittency, characterized by spontaneous transitions between a quiescent crystal state and a dynamic, gas-like state. Mode analysis reveals that parametric coupling between vertical oscillation and horizontal "breathing" mode induces the intermittent melting of the crystal structure. The vertical oscillation amplitude typically exceeds that expected from thermal Brownian motion. We consider several mechanisms to explain the energy input responsible for the large vertical oscillation amplitude, including ion wake-mediated non-reciprocal interactions, finite particle charging time, ion-drag force, and particle inhomogeneity.