Experiments on melting in two dimensions: Desperately seeking the hexatic phase

The melting transition in a two-dimensional complex plasma is studied in the DONUT experiment (Dusty Ohio Northern University experimenT). An initially strongly-coupled crystal made of $n\approx 3900$ monodisperse microspheres is ``heated'' by amplitude modulating the rf discharge power with a square wave at the vertical resonance frequency. The vertical motion is found to excite ``random'' acoustic waves in the plane of the crystal (i.e., the horizontal direction), effectively heating the crystal. As the ``temperature'' of the complex plasma increases, we observe a melting transition in qualitative agreement with the Kosterlitz, Thouless, Halperin, Nelson and Young (KTHNY) scenario. Translational order is initially lost as the dislocation density grows, and a hexatic phase is observed with short-range translational order and long-range orientational order. Further heating destroys the orientational order, resulting in a liquid.