A two-dimensional (2D) melting transition has been studied in a nonequilibrium experimental model system. The system used was a complex or dusty plasma consisting of microspheres suspended in a glow-discharge plasma, where we have mapped the topological defects during the transition. The role of the defects in the melting transition is evaluated and the arrangement of the defects in the lattice is quantified in a new way. It is found that defect density increases dramatically during the melting; at all stages the defects tend to be clustered together rather than widely dispersed; the clustering tends to take the form of chain or string-like structures. We compare these results for the defect structure with the assumptions of the popular 2D melting theory of Halperin and Nelson, rather than the predictions, as is more common.

• Received 8 November 2000

DOI:https://doi.org/10.1103/PhysRevE.64.051404