Hydrogenated group-IV binary monolayers: a new family of inversion-asymmetric topological insulators

Band topology and Rashba spin splitting (RSS) are two extensively explored exotic properties in condensed matter physics. However, the coexistence has rarely been reported in simplest stoichiometric films so far. Here, by using first-principles calculations, we demonstrate that a series of inversion-asymmetric group-IV XYH2 monolayers (X, Y = Si, Ge, Sn, Pb) allow for the simultaneous presence of topological order and large RSS that derives from their peculiarly atomic structure. The topological bulk gaps and RSS energies of PbSnH2, PbGeH2, and PbSiH2 are tunable over a wide range of strains (−8 to 8%), even the maximum value can be enhanced to 0.68 eV and 0.24 eV under achievable strain, but another three configurations transform from trivial to nontrivial phases under tensile strain. Furthermore, we find that the Te(111)-terminated BaTe surface is an ideal substrate for the growth of these monolayers, without destroying their intrinsic band topology. Our findings provide a possible route to future applications of inversion-asymmetric topological insulators in spintronic devices.