Creating anisotropic spin-split surface states in momentum space by molecular adsorption

Abstract

In this ab initio study we demonstrate that molecular adsorption on a surface Rashba system can be used to modulate the surface electronic structure in different momentum space directions, i.e., to create anisotropic spin splittings in $\mathbf{k}$ space. This effect is rooted in the asymmetric adsorption of the molecules on the surface in a hollow site which breaks the surface symmetry. More specifically, we demonstrate that the physisorbed ${\mathrm{NH}}_{3}$ has a small influence on the surface Rashba states and only gives rise to variations of the surface state Rashba parameters up to a factor of 1.4 over the surface Brillouin zone. In contrast, the chemisorption of ${\mathrm{BH}}_{3}$ leads to variations of the Rashba parameter by more than a factor of 2.5. Consequently, the anisotropy of the Rashba-split-surface states induced by molecular adsorption also gives rise to a modulation of the surface state spin texture, i.e., the out-of-plane spin polarization varies along different $\mathbf{k}$ directions by up to 70% for the occupied states. This offers the possibility to change the spin direction from in-plane to predominantly out-of-plane by modifying the electronic momentum by 90\ifmmode^\circ\else\textdegree\fi{}.