Melting of 2D Coulomb clusters in dusty plasmas
  The melting of 2D dust clusters caused by one additional particle in the lower layer has experimentally been observed to undergo a two-step transition, which divides the phase of the cluster into three stages. The first transition is a jump of the dust kinetic energy due to the onset of an instability of the lower-layer particle, shifting the cluster from an ordinary to a hot crystalline state. The second transition is the actual phase transition into a liquid state, which occurs at a decisively lower gas pressure. The detailed dynamical properties of the system during the transition were determined in terms of the normal mode analysis. 
Ryuta Ichiki, Yuriy Ivanov, Matthias Wolter, Yoshinobu Kawai, Andre   Melzer