Enhanced stability of the square lattice of a classical bilayer Wigner crystal

The stability and melting transition of a single layer and a bilayer crystal consisting of charged particles interacting through a Coulomb or a screened Coulomb potential is studied using the Monte-Carlo technique. A new melting criterion is formulated which we show to be universal for bilayer as well as for single layer crystals in the case of (screened) Coulomb, Lennard--Jones and 1/r^{12} repulsive inter-particle interactions. The melting temperature for the five different lattice structures of the bilayer Wigner crystal is obtained, and a phase diagram is constructed as a function of the interlayer distance. We found the surprising result that the square lattice has a substantial larger melting temperature as compared to the other lattice structures. This is a consequence of the specific topology of the defects which are created with increasing temperature and which have a larger energy as compared to the defects in e.g. a hexagonal lattice.Comment: Accepted for publication in Physical Review

Slightly perturbed two-dimensional solid : crystal, hexatic or glass ?

Melting properties of a two-dimensional system comprised of ~ 1 µm diameter, monodispersed, polystyrene spheres are compared with those of a system containing a onepercent concentration of larger spheres which have a larger interparticle interaction. The spheres interact via a dipolar interaction which is controlled externally. The translational and bond-orientational correlation functions are examined as a function of the dipole strength. The monodispersed system exhibits two-stage melting. In the system with a one-percent concentration of larger spheres the correlation functions are characteristic of a hexatic phase at the largest interaction strengths. Dislocations are pinned to the larger particles