Plasma crystal: Coulomb crystallization in a dusty plasma.

A macroscopic Coulomb crystal of solid particles in a plasma has been observed. Images of a cloud of 7-p,m "dust" particles, which are charged and levitated in a weakly ionized argon plasma, reveal a hexagonal crystal structure. The crystal is visible to the unaided eye. The particles are cooled by neutral gas to 310 K, and their charge is >9800e, corresponding to a Coulomb coupling parameter r > 20700. For such a larger value, strongly coupled plasma theory predicts that the particles should organize in a Coulomb solid, in agreement with our observations.