Electronic response to high-velocity nuclei through matter from first principles

An atomic nucleus travelling through condensed matter at a velocity comparable to that of valence electrons produces significant electronic excitation in the scale of eV to keV per Angstrom along the trajectory, which is key for the understanding of many radiation damage processes. The nucleus produces a strong perturbation, giving rise to nanoscopic, far from equilibrium non-adiabatic processes hard to describe beyond linear response. Dynamical simulations from first principles using real-time time-dependent density-functional theory are being used to simulate them, achieving remarkable success. Important challenges remain, however, both theoretically and experimentally, towards understanding such a fundamental problem in non-equilibrium quantum physics. Building from the ground up, the focus is now on the characterisation of stationary states in time-periodic situations for constant velocity nuclei along space periodic directions in crystals, a model situation that allows looking at (stroboscopically) stationary states in terms of Floquet modes.