Observation of melting in a heated, two-dimensional complex plasma

The melting transition in a two-dimensional complex plasma has been studied experimentally. The complex plasma is heated by amplitude modulating the rf discharge power with a square wave at the vertical resonance frequency. The vertical motion couples to an in-plane dust-acoustic instability at one-half the driving frequency, thereby increasing the average in-plane kinetic energy (i.e., the effective ``temperature'') of the system. The ``thermodynamic'' phase of a complex plasma consisting of $\approx3900$ $9-\mu\rm{m}$ diameter particles has been characterized for increasing levels of amplitude modulation at constant neutral pressure (35 mtorr Ar) and constant average rf power using the bond-orientational correlation function, defect densities, the Lindemann ratio and the pair correlation function. A melting transition showing clear evidence for hexatic and liquid phases is observed.