Designing the Rashba spin texture by adsorption of inorganic molecules

We present an approach in which we show that the spin texture of a surface Rashba system can be adjusted by the adsorption of molecules. By selecting physisorbed and chemisorbed inorganic molecules on the BiAg$_2$/Ag(111) surface alloy [1] we demonstrate from \emph{ab initio} that both the spin-orbit splitting and the spin direction of Rashba-split surface states can be controlled selectively. The physisorption of NH$_3$ gives rise to a slightly enhanced outward buckling of the surface Bi which enlarges the magnitude of the Rashba splitting. On the contrary, the weak chemisorption of BH$_3$ defines a strong inward relaxation of the surface Bi. This causes the occupied Rashba split state to shift into Ag bulk states. In addition a new Rashba splitting is created in an unoccupied state upon BH$_3$ adsorption. Most importantly, in contrast to the clean surface [1] in case of the BH$_3$-BiAg$_2$/Ag(111) system the out-of-plane spin polarization is significantly larger than the in-plane one. \newline [1] C. R. Ast, \emph{et al.}, PRL \textbf{98}, 186807 (2007).