D.N. Borisenko, P.M. Walmsley, A.I. Golov, N.N. Kolesnikov, Yu.V. Kotov, A.A. Levchenko, L.P. Mezhov-Deglin, and M.J. Fear, Nanotube-based source of charges for experiments with solid helium at low temperatures: 10th International Conference on Cryocrystals and Quantum Crystals (Final part), Low Temp. Phys. 41, (2015) [Fiz. Nizk. Temp. 41, 729-732, (2015)] DOI: https://doi.org/10.1063/1.4927049 .