Electronic stopping of slow H and He atoms in gold from first principles

In spite of a long history, the quantitative understanding of non-adiabatic processes in condensed matter and our ability to perform predictive theoretical simulations of processes coupling many adiabatic energy surfaces is very much behind what accomplished for adiabatic situations, for which first-principles calculations provide predictions of varied properties within a few percent accuracy. We will present here high-accuracy results for the electronic stopping power of H and He moving through gold, using time-evolving density-functional theory, thereby conveying usual first-principles accuracies to strongly coupled, continuum non-adiabatic processes in condensed matter. The two key unexplained features of what observed experimentally have been reproduced and understood: ($i$) The non-linear behavior of stopping power versus velocity is a gradual crossover as excitations tail into the $d$-electron spectrum; and ($ii$) the higher stopping for He than for H at low velocities is explained by the substantial involvement of the $d$ electrons in the screening of the projectile even at the lowest velocities where the energy loss is generated by $s$-like electron-hole pair formation only.