Observation of three-dimensional long-range order in small ion Coulomb crystals in an rf trap

Three-dimensional long-range ordered structures in smaller and near-spherically symmetric Coulomb crystals of Ca-40(+) ions confined in a linear rf Paul trap have been observed when the number of ions exceeds similar to 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 similar to 50000 ions or more.