Charge Motion in Solid Helium

We overview and discuss several advances in the understanding of the motion of ions through solid helium. Positive and negative ions in solid helium are microscopic complexes with internal structures embedded into the host crystal lattice. Their low-field mobilities normally decrease with cooling in both bcc and hcp crystals of either isotope of helium (\(^3\)He and \(^4\)He). Depending on the density of solid helium (as well as, in the case of positive ions in hcp \(^4\)He, on the crystal orientation), the corresponding activation energies for the mobility of the two species of ions may or may not coincide, and they are found to be typically either equal to or about twice the vacancy creation energy. In strong electric fields, the field dependence of the drift velocity of ions is often nonlinear but monotonic. However, for positive ions in hcp \(^4\)He, non-monotonic anomalies in both temperature and field dependences of the drift velocity were observed. We discuss these features within the framework of the theory of ion motion via inelastic scattering of low-energy vacancions put forward by Alexander Andreev and co-workers.