First-principles study of the threshold effect in the electronic stopping power of LiF andSiO2for low-velocity protons and helium ions

Abstract

Nonadiabatic dynamics simulations are performed to investigate the electronic stopping power of LiF and ${\text{SiO}}_{2}$-cristobalite-high crystalline thin films when protons and helium ions are hyperchanneling in the $\ensuremath{\langle}$001$\ensuremath{\rangle}$ axis. In this theoretical framework, ab initio time-dependent density-functional theory calculations for electrons are combined with molecular dynamics simulations for ions in real time and real space. The energy transfer process between the ions and the electronic subsystem of LiF and ${\text{SiO}}_{2}$ nanostructures is studied. The velocity-proportional stopping power of LiF and ${\text{SiO}}_{2}$ for protons and helium ions is predicted in the low-energy range. The measured velocity thresholds of protons in LiF and ${\text{SiO}}_{2}$, and helium ions in LiF are reproduced. The convergence of the threshold effect with respect to the separation of grid points is confirmed. The underlying physics of the threshold effect is clarified by analyzing the conduction band electron distribution. In addition, the electron transfer processes between the projectile ions and solid atoms in hyperchanneling condition are studied, and its effects on the energy loss is investigated.