Theoretical prediction of valley spin splitting in two-dimensional Janus MSiGeZ₄ (M = Cr and W; Z = N, P, and As)

With the exploration of valleytronic materials in MA₂Z₄ structures, larger valley spin splitting has become a hot topic of research. Based on first-principles calculations, we predicted six valleytronic 2D (two-dimensional) Janus MSiGeZ₄ (M = Cr and W; Z = N, P, and As) materials. The valley spin splitting value of WSiGeZ₄ (Z = N, P, and As) can reach more than 400 meV, which is favorable for the practical application of valleytronics. Two-dimensional WSiGeZ₄ (Z = N, P, and As) materials are dynamically and mechanically stable and have an abundance of electronic properties. The two-dimensional Janus WSiGeZ₄ (Z = N, P, and As) structures comprise both direct and indirect bandgap semiconductor materials. Among them, WSiGeN₄ is an indirect bandgap semiconductor material with a bandgap of 1.654 eV and WSiGeP₄ is a direct bandgap semiconductor material. The valley spin splitting originates from the symmetry breaking of the material structure and the spin-orbit coupling effect of the transition metal, which is manifested as the Berry curvature. In particular, the Berry curvature of 2D Janus WSiGeP₄ and WSiGeAs₄ is as high as 300 Bohr², which is quite large. The W atom has more d-orbital electrons than the Cr atom, and the SOC (spin-orbit coupling) effect is stronger; thus, the valley spin splitting value CrSiGeZ₄ of WSiGeZ₄ is more than 300 meV, which is quite large. In addition, the bandgap and valley spin splitting of WSiGeZ₄ (Z = N, P, and As) can be significantly modulated by applying a biaxial strain. Our study shows that WSiGeZ₄ (Z = N, P, and As) has great potential for valleytronic applications.