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