Switchable large-gap quantum spin Hall state in the two-dimensional M Si2Z4 class of materials

Quantum spin Hall (QSH) insulators exhibit spin-polarized conducting edge states that are topologically protected 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 M Si₂Z₄ (M = Mo or W and Z = P or As) material family with 1 T^' structure. A structural distortion in the 2 H 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 ∼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 M Si₂Z₄ material family in the 1 T^' structure as large band gap, tunable QSH insulators with protected spin-polarized edge states and large spin Hall conductivity.