Direct observations of the structural phases of crystallized ion plasmas

Laser-cooled ${}^9{\mathrm{Be}}^{+}$ ions confined in a Penning trap were directly observed, and the images were used to characterize the structural phases of the ions. With the ions in two-dimensionally extended lattice planes, five different stable crystalline phases were observed, and the energetically favored structure could be sensitively tuned by changing the areal density of the confined ions. Qualitatively similar structural phase transitions occur or are predicted to occur in other planar single-component systems with a variety of interparticle interactions. Closed-shell structures were observed with small ion clouds that were spherical or prolate, and crystals with long-range order were observed in the centers of clouds with large numbers of ions. These experimental results are in good agreement with theoretical predictions for the strongly coupled one-component plasma.