Hot-electron mediated diffusion in proton-irradiated MgO

Ionizing radiation is known to give rise to enhanced defect diffusion in materials. However, existing models including "thermal spike'' and "ionization-enhanced diffusion'' focus on ion dynamics and neglect the influence of electronic excitations due to electronic stopping. We use time-dependent density functional theory (TDDFT) in Ehrenfest dynamics simulations to describe the first 30 fs after proton impact into MgO directly from first principles. Comparison to Born-Oppenheimer molecular dynamics clearly illustrates the importance of electronic excitations for the emerging ion dynamics. In order to quantify the effect of hot electrons and the resulting modified ion dynamics on migration barriers, we combine TDDFT with constrained DFT and the nudged-elastic band method. Our data shows that hot electrons need to be described explicitly and, during the early stages after particle impact, cannot be approximated by a Fermi temperature or an effective charge state of defects in the material. Due to the large computational cost, these simulations are restricted to short-time dynamics, however, we extract parameters that enable improved multi-scale simulations based on two-temperature molecular dynamics and migration barriers in kinetic Monte-Carlo studies.