Defects and particle motions in the nonuniform melting of a two-dimensional Coulomb cluster.

The defect excitation and nonuniform melting of a two-dimensional Coulomb cluster with 300 charged particles (interacting with 1/r type force) in a uniform neutralizing background are studied numerically. Intrinsic defects exist around the outer circular shells surrounding the inner triangular lattice. They are the source regions for anisotropic particle thermal vibrations and then cyclic hoppings with the increasing temperature. It leads to the nonuniform melting associated with the thermal motion of intrinsic defects, and then the thermal excitation of dislocation pairs and disclinations. The intrinsic defect free center core has the highest melting temperature. It shows the sequential losses of translational and then orientational orders.