Direct observations of the structural phases of crystallized ion plasmas

Laser-cooled Be-9(+) ions confined in a Penning trap were directly observed , and the images were used to characterize the structural phases of the ion s. With the ions in two-dimensionally extended lattice planes, five differe nt stable crystalline phases were observed, and the energetically favored s tructure could be sensitively tuned by changing the areal density of the co nfined ions. Qualitatively similar structural phase transitions occur or ar e predicted to occur in other planar single-component systems with a variet y of interparticle interactions. Closed-shell structures were observed with small ion clouds that were spherical or prolate, and crystals with long-ra nge order were observed in the centers of clouds with large numbers of ions . These experimental results are in good agreement with theoretical predict ions for the strongly coupled one-component plasma. [S1070-664X(99)92605-8] .