In two-dimensional (2D) systems, the coexistence of magnetism and nontrivial topological states can induce many novel physical phenomena. A typical example is the quantum anomalous Hall effect (QAHE), where the combination of ferromagnetism and topological insulator can generate a band gap in the bulk and gapless chiral edge states at boundaries. Owing to the dissipationless chiral edge states, QAHE is expected to have potential applications in ultralow-power consumption spintronic devices. QAHE was first proposed in a honeycomb lattice. Since then, people have made intensive studies on hexagonal and triangular lattices, and proposed some candidate materials to realize QAHE. In these materials, d orbitals of the transition metal usually play important roles in realizing QAHE. Two interesting questions then arise: whether can the QAHE be realized in other lattices, such as square lattice? Can the QAHE be obtained in materials with p orbitals?
Recently, a group led by Professor Gang Su from University of Chinese Academy of Sciences address these appealing issues by revealing a square lattice with the space group P/4n (No.85) that can accommodate three different p-orbital magnetic topological states, namely, the fully spin-polarized nodal loop semimetal, ferromagnetic semiconductor and QAHE. These three quantum states can be obtained by the symmetry and k·p model analysis, which can be implemented in 2D materials ScLiCl 5 , ScLiBr 5 and LiScZ 5 (Z=Cl, Br), respectively, because of their different spin-orbit coupling (SOC) parameters. It is shown that the ferromagnetism in these 2D materials is attributed to p x and p y orbitals of halogen elements. Unlike the p z orbital of graphene in low energy physics, where the SOC is between the next-nearest neighbor hopping with much small strength, p x and p y orbitals can provide large atomic spin-orbit coupling, leading to strong out-of plane magnetism. Therefore, these three systems have stable ferromagnetism with high Curie temperatures, which is calling for further experimental explorations.
The present study provides a new materials-independent mechanism of magnetic topological states from p-orbital electrons on square lattices and presents a novel family of 2D magnetic topological materials with high Chern number.
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See the article:
Jing-Yang You, Bo Gu and Gang Su
P-orbital magnetic topological states on square lattice
Natl Sci Rev , nwab214 (2021), doi: 10.1093/nsr/nwab114
https://doi.org/10.1093/nsr/nwab114
National Science Review