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

Abstract

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.