Emergent Bistable Switching in a Nonequilibrium Crystal*

Multistability is an unifying characteristic of a wide variety of physical, chemical and biological systems which are driven far from equilibrium. In nonequilibrium systems made of bistable elements, global bistability is a common feature, so that the entire system switches behavior. Nevertheless, here we experimentally demonstrate that bistable elements are not required for the global bistability of a collective system. We observe temporal switching between a crystalline, condensed state and a gas-like, excited state in a spatially-extended, quasi-two-dimensional layer of charged microparticles. The switching occurs over a broad range of time scales. Nevertheless, a dominant time scale is set by an external, periodic signal, such as a rotation of the crystal due to an external magnetic field. Increasing the number of particles in the system results into more complex, avalanche-like dynamics - only part of the system switches from crystalline to gaseous phase, and the size of the switching region varies with each avalanche event. We confirm our results using molecular dynamics simulations which show that conservative forces, damping, and stochastic noise are sufficient to induce switching.