Melting and reentrant freezing of two-dimensional colloidal crystals in confined geometry

We study the melting behavior of a finite number (N less than or equal to 4 5) of paramagnetic colloidal spheres in a two-dimensional circular hard wal l cavity. The interaction strength between the particles is varied by apply ing 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 furthe r reduction of the external Li field, however, angular order is restored ag ain before the system melts completely. We propose a simple mechanism to ac count for this reentrance phenomenon. [S0031-9007(99)08972-3].