Ground-state path integral Monte Carlo simulations of positive ions in He-4 clusters: Bubbles or snowballs?

The local order around alkali (Li+ and Na+) and alkaline-earth (Be+, Mg+, and Ca+) ions in He-4 clusters has been studied using ground-state path integral Monte Carlo calculations. The authors apply a criterion based on multipole dynamical correlations to discriminate between solidlike and liquidlike behaviors of the He-4 shells coating the ions. As it was earlier suggested by experimental measurements in bulk He-4, their findings indicate that Be+ produces a solidlike ("snowball") structure, similar to alkali ions and in contrast to the more liquidlike He-4 structure embedding heavier alkaline-earth ions.