A.A. Pelmenev, I.N. Krushinskaya, I.B. Bykhalo, and R.E. Boltnev, On charged impurity structures in liquid helium, Low Temp. Phys. 42, (2016) [Fiz. Nizk. Temp. 42, 289-296, (2016)] DOI: https://doi.org/10.1063/1.4942760 .