Experimental observation of two-stage melting in a classical two-dimensional screened Coulomb system

We have observed a two-stage melting transition in a two-dimensional (2D) colloidal suspension of highly charged, submicron-sized spheres in water using optical microscopy and digital imaging. Our system is confined between two flat glass plates, has a lateral extent of at least 1000 spheres, and has equilibrated for over 10⁵ time steps. Direct observation of topological defects and computed correlation functions from real-space images gives insight into the nature of the 2D melting process.