Electrically switchable valley polarization and an anomalous valley Hall effect in monolayer and bilayer NbS2

Abstract

Achieving electrically controlled valley polarization in ferrovalley materials is critical for their energy-efficient valleytronic applications, yet direct electrical controllability of valley polarization remains elusive in most systems. In this work, we investigate the switchable valley polarization in monolayer and bilayer NbS₂ using first-principles calculations. We propose a sliding-based mechanism as a promising route towards valley polarization switching. Specifically, we find that monolayer NbS₂ exhibits a significant valley polarization (an energy difference of 183 meV at K and K′ valleys in the valence band). The nonzero Berry curvature at K and K′ valleys indicates an observable anomalous valley Hall effect (AVHE). Crucially, this polarization can be switched by intralayer S atom sliding driven by an in-plane electrical field, where the valley index of electrons participating in transport is exchanged, while the electron deflection directions remain unchanged. To reduce the switching barrier for practical electrical control and make the AVHE tunable, we introduce a bilayer NbS₂ scheme exploiting A-type interlayer anti-ferromagnetism and sliding ferroelectricity. In the bilayer system, layer-locked half-metallic transport and the AVHE can be modulated by carrier doping concentration. Furthermore, the layer- and spin-locked valley polarization and the valley Hall effect can be controlled by interlayer sliding also driven by an electrical field. These findings reveal the sliding mechanism of atom sliding in monolayers and layer sliding in bilayers as a fundamental approach for manipulating valley polarization and the AVHE. This work provides a crucial theoretical basis for the design of low-energy-consumption, electrically controllable valleytronic devices based on sliding ferrovalley materials.