Observation of Three-dimensional Long-range Order in Smaller Ion Coulomb   Crystals in an rf Trap
  Three-dimensional long-range ordered structures in smaller and near-spherically symmetric Coulomb crystals of ^{40}Ca^+ ions confined in a linear rf Paul trap have been observed when the number of ions exceeds ~1000 ions. This result is unexpected from ground state molecular dynamics (MD) simulations, but found to be in agreement with MD simulations of metastable ion configurations. Previously, three-dimensional long-range ordered structures have only been reported in Penning traps in systems of ~50,000 ions or more. 
A. Mortensen, E. Nielsen, T. Matthey, M. Drewsen