  Electronic stopping of slow protons in ZnO, VO2 (metal and semiconductor
phases), HfO2, and Ta2O5 was investigated experimentally. As a comparison of
the resulting stopping cross sections (SCS) to data for Al2O3 and SiO2 reveals,
electronic stopping of slow protons does not correlate with electronic
properties of the specific material such as band gap energies. Instead, the
oxygen 2p states are decisive, as corroborated by density functional theory
calculations of the electronic densities of states. Hence, at low ion
velocities the SCS of an oxide primarily scales with its oxygen density.
