Strain-engineering quantized spin Hall conductivity in sliding multiferroic RuCl2 bilayers

Abstract

In quantum layertronics, the layer degree of freedom in two-dimensional (2D) van der Waals (vdW) materials will couple with the electron's charge or spin, leading to layer-coupled new physics and possible future quantum device applications. In our work, we investigate the layer-coupled topological properties in RuCl₂ bilayers by the strain effect. Firstly, the AA-type and AB-type stacking patterns of RuCl₂ bilayers are proposed by interlayer sliding or rotation operations. Using first-principles calculations, the interlayer magnetism is confirmed to be antiferromagnetic (AFM) for both stacking patterns. Moreover, the sliding ferroelectric and ferrovalley properties could coexist with the interlayer AFM order, which indicates the multiferroic properties in RuCl₂ bilayers. By consideration of the spin-layer coupling in the sliding bilayer, the properties of valley polarization could be manipulated by the strain effect. Most importantly, the strain-engineering quantized spin Hall conductivity (SHC) could be realized along with band inversions at K and K′ valleys, which can be termed as the quantum layer spin Hall (QLSH) effect. Different mechanisms for topological phase transitions in AA-type and AB-type stacking patterns are proposed by analyzing the layer-resolved band structures, SHC, topological edge states, Wannier charge centers (WCCs) and spin Berry curvature. Our work reveals that 2D vdW magnetic materials could exhibit new physics in layer-coupled topological properties.