First-principles calculations study of valley polarization in antiferromagnetic bilayer systems

Superior to ferromagnetic (FM) valleytronics, antiferromagnetic (AFM) counterparts exhibit ultradense and ultrafast potential due to their intrinsic advantages of zero stray field, terahertz dynamics, and compensated moments of antiferromagnets. However, the physics of spontaneous valley polarization is mainly rooted in FM hexagonal lattices and is rarely used to explore the simultaneous spin and valley polarizations in AFM materials. Here, we propose a general stacking method to achieve valley polarization in AFM bilayer systems. The hexagonal ferrovalley material is used as the basic building unit, and then the space-inversion centrosymmetric bilayer system with interlayer AFM ordering is constructed by horizontal mirror and twofold rotational operations, which can exhibit spontaneous valley polarization. In this construction process, the rarely explored layer-locked hidden valley polarization, hidden Berry curvature, and layer Hall effect are involved, and an out-of-plane electric field can be used to detect hidden valley polarization and to realize layer-locked anomalous valley Hall effect. We use three examples to illustrate our proposal. First, the Janus GdBrI is used to illustrate concepts and effects involved in our design process. Second, the RuBr2 is used to demonstrate other phenomena, including valley polarization transition and near-ideal quantum spin Hall insulators. Finally, we use our design principles to understand the valley polarization of experimentally synthesized MnSe from a new perspective. Our works establish a robust general scheme to achieve valley polarization in AFM bilayer systems, thereby opening up new avenues for AFM valleytronics.