On the local density dependence of electronic stopping of ions in solids

We use time-dependent density functional theory to calculate the electronic stopping Se in binary Ni-Ni atomic collisions and for a Ni projectile along channeling directions in a Ni crystal. Our results show that when Se is reported as a function of the ground state target electronic density ρ0 the stopping is not a single-valued function of the local density, as assumed in formalisms that date back to the origins of quantum mechanics, but shows loops, suggesting that it is inaccurate to model stopping as a dissipative force of the type F=β(ρ0)v, as it is customarily done in non-adiabatic molecular dynamics simulations of ion-solid interactions. We compare our results with Se in a uniform electron gas where the above definition for the force holds, and conclude on the validity of using jellium as a crude approximation for more realistic inhomogeneous electron gases.