Induced order and reentrant melting in classical two-dimensional binary clusters

– A binary system of classical charged particles interacting through a dipole repulsive potential and confined in a two-dimensional hardwall trap is studied by Brownian dynamics simulations. We found that the presence of small particles stabilizes the angular order of the system as a consequence of radial fluctuations of the small particles. There is an optimum in the increased rigidity of the cluster as function of the number of small particles. The small (i.e. defect) particles melt at a lower temperature compared to the big particles and exhibit a reentrant behavior in its radial order that is induced by the intershell rotation of the big particles. Introduction. – Melting and crystallization are fundamental processes in nature and have been widely studied. Charged particles systems like e.g. colloids [1] and dusty plasma’s [2] display similar phase behavior as atoms and molecules with the added advantage that the micrometer size of the particles and their slower dynamics make them accessible for real space imaging [3]. Most of the previous research was directed towards one-component systems. Recently, in a theoretical study in Ref. [4] the complexity of the system was increased by investigating systems with two types of particles of different radii and different effective charge confined in a parabolic trap. A recent experimental study [5] concentrated on oppositely charged colloidal particles confined in a cavity and found a remarkable diversity of new binary structures. In this letter we consider a finite size binary system of repulsive particles which are confined to move in two dimensions (2D). The circular hard wall confinement potential competes with the 2D Wigner crystal structure [6] and leads to ring like arrangements for the particles [7, 8]. Previously it was shown experimentally [9] and theoretically [10] that single component systems exhibit a remarkable re-entrant melting behavior. In the present binary system we assume a large difference in the size and ‘charge’ of the particles and therefore the smaller particles can be considered as ‘defects’ which disturb the order of the big particles [11]. We found that these defect particles have a pronounced effect on the melting behavior of the system and results in an unexpected stabilization of the ordered phase and a new reentrant