Melting transition in a two-dimensional complex plasma heated by driven acoustic instability

The melting transition in a two-dimensional complex (dusty) plasma is studied experimentally. A system consisting of ≅3900 microspheres 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 modulation frequency, thereby increasing the complex plasma's effective temperature. The ''thermodynamic'' phase of the system is characterized for increasing levels of amplitude modulation at constant neutral pressure (35 mTorr Ar) and average rf power using the Lindemann ratio, defect density, bond-orientational correlation function, and pair correlation function. A melting transition showing evidence for an intermediate hexatic phase is observed.