We report the experimental observation of a two-stage transition in a monolayer of active colloidal particles in plasma. Analysis of pair and orientational correlation functions reveals a transition in a particle monolayer from a crystalline state to a hexatic state and subsequently to an isotropic liquid state, which is fully consistent with Berezinskii‒Kosterlitz‒Thouless transition theory. In our experiments, an active two-dimensional structure far from thermodynamic equilibrium exhibits behavior similar to that of systems in thermodynamic equilibrium.
Zamorin et al. (Sun,) studied this question.