IntIntermittent melting of Coulomb clusters from nonreciprocal parametric pumping

Complex systems out of equilibrium often experience intermittent oscillations between quiescent and highly dynamic states. The type of intermittency depends on how energy is pumped into the system, and how it is dissipated. For systems of discrete particles, energy can come from the activity of each particle, external stochastic noise, or external fields.  However, energy can also be sourced from field-mediated interactions between particles, which are often nonreciprocal, violating Newton’s 3rd law. Here we demonstrate how intermittency emerges in clusters of charged particles confined in a rf plasma sheath, where interactions are nonreciprocal. We track the particle motion in 3D using a scanning laser sheet. The clusters are driven into small vertical oscillations due to fluctuations of the plasma environment. When there is parametric pumping from vertical to horizontal oscillation modes, the clusters can spontaneously transition from quiescent cluster state to a dynamic melted state. Using experiments and theory, we show that during this process the nonreciprocal interactions dramatically enhance the parametric coupling by pumping the second harmonic of the horizontal mode, which further increases the amplitude of the vertical mode, and leads to explosive melting of the levitated clusters. As a result, intermittency emerges with a timescale on the order of 10-100 seconds, and is not observable with noise-driven dynamics alone.