Dependence of the electron spin scattering in the BiAg2 surface alloy on step density and its chemical composition

Resumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España).-- et al.The spin-orbit interaction (SOI) can dramatically influence electron scattering on the surface of crystalline compounds. A most striking consequence is the quenching of backscattering, which arises from the entanglement between the spin and crystal momentum in the surface states of topological insulators and other two-dimensional systems. The absence of backscattering makes an electric current less sensitive to defects and facilitates the generation of spin currents, which is attractive for applications in spintronics. Previous investigations of the BiAg2 surface alloy showed evidence of electron backscattering for both the occupied and unoccupied surface bands but with a considerable transmission. It has been reported that the electron scattering is strongly dependent on the chemical composition of the step edges, where spin-flip backward scattering can lead to intraband transitions between states of opposite spin projection. The spin-flip scattering amplitude depends on the chemical composition of the steps, leading to total confinement for pure Bi step edges (A-steps), and considerable leakage for mixed Bi-Ag step edges (B-steps). 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. We confirm that such remarkable differences of the electron interaction with the different step types exists when using non-local surface techniques. This study was carried out by ARPES in a synchrotron using two differently oriented curved crystals by scanning the photon beam along the crystal curvature. Curved crystals provide tunable step densities and, in this case, with two step orientations, thereby the different behaviour between A- and Bsteps can be unravelled. Our data shows strong differences in energy and momentum variation of the surface electrons between the two crystals that agree with the previous STM data, but it is striking that the effective step barriers that the electrons feel are so different for the same surface alloy.Peer reviewe