The Fermi and Rashba energies of surface states in the ${\text{Bi}}_x{\text{Pb}}_{1-x}/\text{Ag}\left(111\right)$ alloy can be tuned simultaneously by changing the composition parameter $x$. We report on unconventional Fermi surface spin textures observed by spin and angle-resolved photoemission spectroscopy that are correlated with a topological transition of the Fermi surface occurring at $x=0.5$. We show that the surface states remain fully spin polarized upon alloying and that the spin-polarization vectors are approximately tangential to the constant energy contours. We discuss the implications of the topological transition for the transport of spin.

• Received 27 February 2009

DOI:https://doi.org/10.1103/PhysRevB.79.241408