Two-stage melting of paramagnetic colloidal crystals in two dimensions

A novel two-dimensional system of colloidal particles with absolutely calib rated magnetic interaction is used to investigate static and dynamic proper ties at the 2D crystal to liquid phase transition. We observe two successiv e transitions from the solid to the liquid phase with an intermediate hexat ic phase, in perfect agreement with the theory of Kosterlitz and Thouless. The absolute values of the transition temperatures are given, and we demons trate that they depend neither on the system size nor on the cooling rate. [S0031-9007(99)08831-6].