Alkali ions in superfluid He-4 and structure of the snowball

We performed variational Monte Carlo simulations of an alkali ion impurity (Li+, Na, K+, and Cs+) in liquid He-4 at the equilibrium density and T=0 K using the shadow wave function technique. We calculated the chemical potential, the local order, the single-particle excitation spectrum, and the effective mass of the ions. In all cases the first shell of He atoms is ordered, forming what is usually called a snowball. The radial density profiles and angular correlations show that the microscopic structure of the snowball is remarkably different for each ion. Only in the case of Na+ and K+ are the atoms of the first shell essentially localized so that the snowball can be considered as a solid. In the case of Li+ and Cs+ these He atoms readily exchange with the others and this difference shows up dramatically in the values of the effective masses. The local order is simple cubic in the case of Li+ and face-centered cubic in the case of K+.