Channel-dependent anomalous low-velocity stopping power in proton-irradiated graphene

The behavior of electronic stopping power, the drag force induced on an incident ion by electrons, is rife with material-specific effects at low ion velocities. For example, semiconductors and insulators have a threshold velocity below which an ion cannot excite electrons across the band gap and electronic stopping vanishes. Also, directional bonding in these materials makes electronic stopping sensitive to the ion's trajectory even for slow ions, when core electrons are negligible. Graphene presents a highly interesting case with directional bonding but no band gap. Our real-time time-dependent density functional theory simulations of proton-irradiated graphene reveal a shoulder in the low-velocity stopping of channeling protons which does not occur for protons traversing a centroid path. From analyzing the post-impact band occupations and projectile charge state, we infer that resonant charge capture from certain valence bands by channeling protons is responsible for this feature. This prediction of a new form of anomalous low-velocity stopping has implications for ion beam imaging, where such trajectory-dependent behavior could be exploited to achieve high resolution.