Melting and Reentrant Freezing of Two-Dimensional Colloidal Crystals in Confined Geometry

We study the melting behavior of a finite number ( N≤45) of paramagnetic colloidal spheres in a two-dimensional circular hard wall cavity. The interaction strength between the particles is varied by applying a magnetic field B. At high B, i.e., strong interaction, the particles are arranged in a highly ordered shell-like structure. With decreasing B we observe first a loss of angular order between adjacent shells. Upon further reduction of the external B field, however, angular order is restored again before the system melts completely. We propose a simple mechanism to account for this reentrance phenomenon.