Notes on Barkas-Andersen effect

Stimulated by recent statements in the literature on electronic stopping of heavy ions in matter, we try to clarify some central theoretical aspects of the Barkas-Andersen effect, about which there does not seem to be unanimous agreement in the community. We address the role of inner versus outer target shells, of projectile screening by bound electrons, the interference between Lindhard's description and perturbation theory, as well as the equivalence between a single-electron versus Fermi-gas description of the effect.