Magnetic second-order topological insulator from inverted p-d orbitals

Zhao Liu, Monash

The extension of second-order topological insulators (SOTIs) to magnetic materials has opened new avenues for exploration nowadays. Compared with the well-studied nonmagnetic SOTIs, the pursuit of magnetic SOTIs remains limited due to a lack of comprehensive understanding, particularly concerning the underlying orbitals.

This study addresses this knowledge gap by focusing on insulators with inverted p−d orbitals. By employing a tight-binding model, we illuminate the critical role of doubly inverted p − d orbitals in determining SOTI phase. To achieve this orbital inversion in magnetic materials, we turn our attention to ferromagnetic negative charge-transfer energy insulators.

Through a symmetry-based topological invariant analysis, density-functional theory, Wilson loop and tight-binding calculations, we identify the ferromagnetic CrAs monolayer as a representative ferromagnetic SOTI. Importantly, we demonstrate that SOTI can never be realized in this material without such p−d orbitals inversion. Our findings establish a novel and broadly applicable pathway to magnetic SOTIs.