Cleaved surfaces of topological crystalline insulators (TCIs) often exhibit step edges. For example, as highlighted by red and gray spheres, the rendered perspective STM image shows a double- (left) and a single-atomic step edge (right). The periodicity of the surface lattice remains unchanged for the double-atomic step edge (green box; top). In contrast, in the single-atomic case translational symmetry is broken such that pairs of red or grey atoms appear (purple box). This results in the appearance of a conductive channel (rendered image at the bottom) where spin-up and spin- down electrons move into opposite directions.more
