Positron states at a lithium-adsorbed Al(100) surface: Two-component density functional theory simulation

The positron surface state and the energetics for positron reemission are investigated using two-component density functional theory (TC-DFT) in the projector augmented-wave framework. Trapping of positrons by the surface image potential and the effect of the positron band-shift energy in the surface region are appropriately described by the corrugated mirror model and the ramp potential, respectively, without empirical parameters. The results obtained for various physical quantities of positron states on a clean Al(100) surface, i.e., the affinity, work function, life-time, binding energy, and activation energy, are in good agreement with the experimental results. The positron states on Li-adsorbed Al(100) surfaces are highly dependent on the Li coverage. In particular, the work function of positronium negative ions (Ps(-)) becomes negative at low Li coverage, which indicates the possible emission of Ps(-) from the adsorbed surface. The present study not only elucidates the key energetics that are responsible for positron reemission from the surface, but also emphasizes the excellent performance of TC-DFT for prediction of the positron state on real surfaces.