Systems composed of few particles can exhibit behaviors that strongly differ from the ones of the thermodynamic limit.
One of these is the orientational melting of a planar crystal composed of particles with long-range repulsion. Depending on system parameters, these particles can lose crystallization and delocalize only in the angular direction.
INRiM researchers have studied this transition with trapped Barium ions and observed dependence on particle number and impurities, which reveals the mesoscopic character of the orientational melting transition. The work paves the way to fundamental studies of thermodynamics of small systems and of quantum rotors, as well as rotational and magnetic field sensing.