Two-dimensional hydrogenated molybdenum and tungsten dinitrides MN2H2 (M = Mo, W) as novel quantum spin hall insulators with high stability

Based on first-principles calculations, we predict the existence of the quantum spin Hall (QSH) effect in hydrogenated transitionmetal nitrides MN2H2 (M = Mo, W), showing high structural stability. MN2H2 monolayers are identified to be intrinsic topological insulators (TIs) with protected Dirac type topological helical edge states, and show robust topological features against the large stretching strain. Besides, sizeable intrinsic nontrivial band gaps (70-124 meV) ensure the QSH effect in MN2H2 at room temperature. The pure d-d band inversion was revealed. More interestingly, the topological phase transition between a QSH phase and a topological semimetallic phase can be induced by applying in-plane strain.