Robust second-order topological insulator in 2D van der Waals magnet CrI.

CrI offers an intriguing platform for exploring fundamental physics and the innovative design of spintronics devices in two-dimensional (2D) magnets, and moreover has been instrumental in the study of topological physics. However, the 2D CrI monolayer and bilayers have long been thought to be topologically trivial. Here we uncover a hidden facet of the band topology of 2D CrI by showing that both the CrI monolayer and bilayers are second-order topological insulators (SOTIs) with a nonzero second Stiefel-Whitney number = 1. Furthermore, the topologically nontrivial nature can be explicitly confirmed the emergence of floating edge states and in-gap corner states. Remarkably, in contrast to most known magnetic topological states, we put forward that the SOTIs in 2D CrI monolayer and bilayers are highly robust against magnetic transitions, which remain intact under both ferromagnetic and antiferromagnetic configurations. These interesting predictions not only provide a comprehensive understanding of the band topology of 2D CrI but also offer a favorable platform to realize magnetic SOTIs for spintronics applications.