Origin of the surface-state band-splitting in ultrathin Bi films: from a Rashba effect to a parity effect

Following our previous work (Hirahara T et al 2007 Phys. Rev. B 76 153305), we have performed spin- and angle-resolved photoemission measurements at similar to 120K on ultrathin Bi films grown on a Si substrate, focusing on the split surface-state bands near the Fermi level. We found clear experimental evidence that these states show Rashba-type spin-split behavior near (Gamma) over bar, but the splitting is lost near (M) over bar M where they overlap with the bulk band projection. This can be explained as a change in the origin of the splitting from a Rashba effect to a parity effect as revealed by ab initio calculations.