PLASMA CRYSTAL

Plasma crystal is the term used to describe the recently discovered or dered state that a colloidal plasma may assume under certain condition s. This state resembles metals, to some extent, with the ''atoms'' rep resented by the highly negatively charged and highly ordered colloidal particles and the ''electrons'' by the mobile plasma ions and electro ns-perhaps ''heavy metal'' might be an appropriate description. Plasma crystals are formed in a colloidal plasma if two conditions are met: (1) The Coulomb coupling parameter (the ratio of the Coulomb energy be tween neighboring particles to their kinetic energy) exceeds a certain threshold and (2) the lattice parameter (the ratio of the particle se paration to the Debye length) is smaller than unity. These conditions are easy to generate in rf discharge plasmas and plasma crystallizatio n then proceeds spontaneously. Plasma crystals have some unique proper ties, which make them exciting systems to study. (1) In their own righ t, as a hitherto unknown form of condensed plasma, they may provide ma ny insights into basic plasma physical processes and transport effects . (2) As model systems for the detailed investigation of phase transit ions, lattice defects, annealing, doping, etc., they may provide new i nformation for a better understanding of solid state physics. (3) As t est systems they may be useful for investigating nonlinear effects in ''nanocrystals'' (crystals with less than approximately 100 lattice pl anes). The unique properties, which enable these investigations and po ssibly many more, are (1) global charge neutrality, (2) very fast resp onse, (3) very little damping, and (4) easy experimental control and d iagnostics, i.e., detailed imaging and high temporal resolution of the dynamics of individual particles (''atoms''). We present here an over view of recent developments in this new research field. (C) 1996 Ameri can Vacuum Society.