Proximity-induced spin-orbit coupling in phosphorene on a WSe2 monolayer

We investigate, using first-principles methods and effective-model simulations, the spin-orbit coupling proximity effects in a bilayer heterostructure comprising phosphorene and ${\mathrm{WSe}}₂$ monolayers. We specifically analyze holes in phosphorene around the $\mathrm{\ensuremath{\Gamma}}$ point, at which we find a significant increase of the spin-orbit coupling that can be attributed to the strong hybridization of phosphorene with the ${\mathrm{WSe}}₂$ bands. We also propose an effective spin-orbit model based on the ${\mathbf{C}}_{1\mathrm{v}}$ symmetry of the studied heterostructure. The corresponding spin-orbit field can be divided into two parts: the in-plane field, present due to the broken nonsymmorphic horizontal glide mirror plane symmetry, and the dominant out-of-plane field triggered by breaking the out-of-plane rotational symmetry of the phosphorene monolayer. Furthermore, we also demonstrate that a heterostructure with ${60}^{\ensuremath{\circ}}$ twist angle exhibits an opposite out-of-plane spin-orbit field, indicating that the coupling can effectively be tuned by twisting. The studied phosphorene/${\mathrm{WSe}}₂$ bilayer is a prototypical low common-symmetry heterostructure in which the proximity effect can be used to engineer the spin texture of the desired material.