Giant spin-layer locking in novel two-dimensional X₂Bi₂ (X=Si, Ge, or Sn)

In contrast to the general view in spin-orbit interaction, it was reported that even in materials with centrosymmetry may have Rashba spin-orbit coupling (the R-2 effect) resulting in spatial spin splitting. However, not only its underlying fundamental physics been still unclear, but the search for materials emerging the R-2 interaction has also been very limited. Here, we propose novel two-dimensional materials X₂Bi₂ (X=Si, Ge, or Sn), which exhibit spin-layer locking with giant Rashba parameter value (α_R≤12.3 eV nm). Our first principles calculations show the top and bottom atomic sublayers in X₂Bi₂ produce opposite out-of-plane dipole moments leading to the spin-layer locking phenomenon for in-plane spin moment. We also find that the local orbital angular momentum (OAM) plays a crucial role in determining the R-2 spin splitting. We further suggest possible manipulation strategies for securing the R-2 spin splitting: 1) Strain reinforces the R-2 effect via changing OAM distribution, and 2) layer stacking ensures further spatial spin splitting.