Multiple magnetic states, valley electronics, and topological phase transitions in two-dimensional Janus XYZH (X = Sc, Y, La, Y = Cl, Br, I, and Z = S, Se, Te): From monolayers to bilayers

TL;DR

This study predicts and analyzes various magnetic, valley, and topological phases in 2D Janus XYZH monolayers and bilayers, revealing their potential for quantum device applications through strain, stacking, and twisting manipulations.

Contribution

It introduces 27 stable ferromagnetic Janus XYZH monolayers with unique valley and topological properties, and explores bilayer stacking, ferroelectricity, and topological phase transitions.

Findings

All monolayers exhibit spontaneous valley polarization and anomalous valley Hall effect.

Bilayer systems show interlayer antiferromagnetism and multiferroic characteristics.

Strain and stacking control induce topological phase transitions and novel quantum effects.

Abstract

Exploring the coupling between layer, magnetism, valley, and topology in two-dimensional (2D) materials is an important approach to deepen our understanding of materials properties. We propose 27 stable ferromagnetic semiconductor monolayers of Janus XYZH (X = Sc, Y, La, Y = Cl, Br, I, and Z = S, Se, Te). All these monolayers exhibit spontaneous valley polarization, forming ferrovalley (FV) monolayers, showing anomalous valley Hall (AVH) effect. By applying external strain, the topological phase transitions including quantum anomalous Hall (QAH) effect can be introduced. In the ScBrSH bilayer system, AA and AB stacking configurations were constructed through interlayer sliding and rotational operation. The bilayer system exhibits interlayer antiferromagnetic (AFM) ordering with spontaneous valley polarization differing from the FV observed in monolayers. The sliding ferroelectricity observed in the AA stacking indicates that the system exhibits significant multiferroic characteristics. Further analysis shows that interlayer sliding can introduce a layer polarization anomalous valley Hall (LPAVH) effect, which can be precisely controlled by tuning the direction of the ferroelectric polarization. Upon applying external strain, the quantum layer spin Hall (QLSH) effect observed during the topological phase transition in the bilayer can be regarded as the superposition of two QAH monolayers. Furthermore, applying a twisting operation to the bilayer induces unexpected altermagnetism. Our study systematically reveals the rich physical properties of 2D XYZH materials, providing important theoretical foundations and guidance for the design and development of next-generation quantum devices.