Tunable Sliding Ferroelectricity, Valley Polarization, and Layer-Polarized Anomalous Valley Hall Effect in the RuO₂Si₂N₂ Bilayer: Insights from First-Principles

The septuple-atomic-layer MoSi2N4-like crystals are a novel type of two-dimensional family of materials with no natural bulk counterparts. In this work, we perform a first-principles study of the RuO2Si2N2 material, an isostructural analogue of MoSi2N4. It is found that the RuO2Si2N2 monolayer possesses intrinsic ferromagnetism with a high Curie temperature and exhibits a ferrovalley semiconducting behavior with a remarkable valley polarization. The RuO2Si2N2 bilayers prefer the A-type antiferromagnetic state, and the electronic properties depend on the stacking order. The 3R-type stacking permits an out-of-plane electric polarization, which can be reversed via interlayer sliding, yielding sliding ferroelectricity. Accompanied with the change of electric polarization, the valley polarization is also modulated through sliding. More importantly, a layer-polarized anomalous valley Hall (LP-AVH) effect is present in the 3R-type RuO2Si2N2 bilayer. Conversely, the ferroelectricity is absent in the 2H-type stacking, where the band structure resembles the monolayer counterpart, and the valley polarization is robust against sliding. However, the LP-AVH effect could be recovered in the 2H-type stacking by a small vertical electric field. These interesting phenomena could be explained well by an effective k·p model. Our results demonstrate that the RuO2Si2N2 material is a promising candidate for potential applications in valleytronics and layertronics.