Switchable large-gap quantum spin Hall state in the two-dimensional MSi2Z4 class of materials

Quantum spin Hall (QSH) insulators exhibit spin-polarized conducting edge states that are topologically pro-tected from backscattering and offer unique opportunities to address fundamental science questions and device applications. Finding viable materials that host such topological states, however, remains a continuing challenge. Here, by using in-depth first-principles theoretical modeling, we predict large band gap QSH insulators in the recently synthesized bottom-up two-dimensional MSi2Z4 (M = Mo or W and Z = P or As) material family with 1T' structure. A structural distortion in the 2H phase drives a band inversion between the metal (Mo/W) d and p states of P/As to realize spinless Dirac states without spin-orbit coupling. When spin-orbit coupling is included, a hybridization gap as large as similar to 204 meV opens up at the band-crossing points, realizing spin-polarized conducting edge states with nearly quantized spin Hall conductivity. We also show that the inverted band gap can be tuned with a vertical electric field, which drives a topological phase transition from the QSH to a trivial insulator with Rashba-like edge states. Our study identifies the two-dimensional MSi2Z4 material family in the 1T' structure as large band gap, tunable QSH insulators with protected spin-polarized edge states and large spin Hall conductivity.