Crystallization of ion clouds in octupole traps: Structural transitions, core melting, and scaling laws

Abstract

The stable structures and melting properties of ion clouds in isotropic octupole traps are investigated using a combination of semi-analytical and numerical models, with a particular emphasis at finite size scaling effects. Small-size clouds are found to be hollow and arranged in shells corresponding approximately to the solutions of the Thomson problem. The shell structure is lost in clusters containing more than a few thousands of ions, the inner parts of the cloud becoming soft and amorphous. While melting is triggered in the core shells, the melting temperature unexpectedly follows the rule expected for three-dimensional dense particles, with a depression scaling linearly with the inverse radius.

Structure and dynamics of ion clusters in linear octupole traps: Phase diagrams, chirality, and melting mechanisms

“…This combination roughly explains the structural features found by Okada and co-workers based on the structures known theoretically for ion crystals in isotropic traps [6,7,20]. The dense versus hollow clouds predicted for the quadrupolar and octupolar traps, respectively, immediately suggest that the crystals in linear octupole traps should consist of a dense arrangement along a single axis with a hollow structure along the two other axes, which is nothing else but a cylinder.…”

A double ion trap for large Coulomb crystals

“…As mentioned above, for higher order geometries, the density is expected to increase with the distance from the center of the trap, leading to a tube-like self-organized structure. This prediction in the mean-field approach is confirmed by molecular dynamics simulations for ion numbers above a few hundred [10].…”

Stability and dynamics of ion rings in linear multipole traps

“…One can define a plasma parameter Γ(r) = q 2 n(r) 1/3 /(4π 0 k B T ), which results to be position dependent, and is hence smaller at the center of the trap, reaching a maximum at the radius where the density is maximum. This feature indicates that crystallization is first expected in the outer shells of the crystals, and only at sufficiently low temperatures the ions form ordered clusters in the entire trap volume where they are distributed [8,10].…”

“…The distribution of charges at equilibrium in a multipole trap has been numerically evaluated in a series of works, which have reported the appearance of hollow structures [6][7][8][9][10][11] . This phenomenon can already be understood by calculating the charge density distribution of a weakly correlated plasma [14].…”

“…Recent works have studied the formation of ordered structures [6][7][8][9][10][11] and crystallization in octupole traps has been observed by means of laser cooling [7]. These studies show that ions in multipole traps tend to form hollow structures at very low temperatures.…”