Emerging topology in Bi(trimer)/Ge(111) subsurface from first-principles

We employ first-principles simulations to investigate the electronic and topological properties of the Bi(trimer)/Ge(111) system. Our results show that the electronic states near the electronic gap, primarily derived from subsurface Ge atoms, display a non-trivial topological phase. These states exhibit in-plane orbital character in the valence bands and out-of-plane orbital character in the conduction bands, along with Rashba splitting and non-trival topology, absent in reconstructed Ge(111). This behavior emerges from the broken inversion symmetry at the surface combined with the spin–orbit coupling from Bi atoms. Because topology resides in the subsurface states, it possess further protection compared to conventional surface localized topological states. The Bi(trimer)/Ge(111) system is therefore a promising platform for spintronic applications, offering enhanced spin injection efficiency and stability.