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

TL;DR

This study uses ground-state path integral Monte Carlo simulations to analyze the local structure around various ions in helium clusters, distinguishing solid-like from liquid-like behaviors.

Contribution

It introduces a criterion based on multipole dynamical correlations to identify solid-like versus liquid-like helium shells around ions, providing new insights into their structural nature.

Findings

Be$^+$ forms a solid-like 'snowball' structure

Alkali ions also produce solid-like structures

Heavier alkaline-earth ions tend to have more liquid-like helium shells

Abstract

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