Surface- and edge-states in ultrathin Bi-Sb films

Employing first-principles calculations, we studied the electronic structure of ultrathin Bi-Sb films, focusing on the appearance of surface or edge states that are topologically protected. Our calculations show that in ordered structures the Bi-Sb bonds are quite strong, forming well-defined double layers that contain both elements. We find surface states appearing on the (111) surface of a thin film of layerwise ordered Bi-Sb compound, while thin films in (110) orientation are insulating. In the gap of this insulator, edge states can be found in a (110)-oriented ribbon in the A17 (black phosphorus) structure. While these states are strongly spin polarized, their topological properties are found to be trivial. In all structures, we investigate the influence of spin-orbit coupling and analyze spin polarization of the states at the boundaries of the material.