Promising valleytronic materials with strong spin-valley coupling in two-dimensional MN2X2 (M = Mo, W; X = F, H)

The valley degree of freedom of electrons in two-dimensional systems enables the unprecedented exploration of some physical properties as well as promising device applications. However, suitable two-dimensional valleytronic materials, especially with strong spin-valley coupling, are rare. In this work, based on first-principles calculations, we demonstrate a class of promising two-dimensional valleytronic materials in monolayer MN2X2 (M = Mo, W; X = F, H). Monolayer MN2X2 exhibits a semiconducting nature with the valence band maximum located at the K/K′ points, forming a pair of degenerate valleys. Importantly, upon including spin-orbital coupling, the valleys in these systems experience a significant spin splitting (601 meV), which gives rise to the long-sought strong spin-valley coupling. Such significant spin splitting is attributed to the strong spin–orbit coupling strength within the W- d x 2 − y 2/ d x y orbitals as well as the inversion asymmetry. Moreover, under hole doping and optical illumination, the valley Hall effect can be achieved in these systems.