Spin-Flip and Element-Sensitive Electron Scattering in the BiAg $_{2}$ Surface Alloy

Heavy metal surface alloys represent model systems to study the correlation between electron scattering, spin-orbit interaction, and atomic structure. Here, we investigate the electron scattering from the atomic steps of monolayer BiAg2 on Ag(111) using quasiparticle interference measurements and density functional theory. We find that intraband transitions between states of opposite spin projection can occur via a spin-flip backward scattering mechanism driven by the spin-orbit interaction. The spin-flip scattering amplitude depends on the chemical composition of the steps, leading to total confinement for pure Bi step edges, and considerable leakage for mixed Bi-Ag step edges. Additionally, the different localization of the occupied and unoccupied surface bands at Ag and Bi sites leads to a spatial shift of the scattering potential barrier at pure Bi step edges