Abstract Studies of ground-state topology in quantum materials have revealed the discovery of topological phases with novel Hall responses. Recently, the orbital Hall effect has drawn growing attention; however, the hidden origin behind large orbital Hall conductivity in insulators remains elusive. Here, we introduce the concept of orbital Chern insulators (OCIs), a previously unexplored topological phase in which orbital angular momentum drives nontrivial topology and hosts the orbital Hall effect in insulating systems. We establish a comprehensive orbital-topology-based framework for systematically characterizing OCIs, and identify monolayer blue phosphorene, a material previously regarded as a trivial insulator, hosting the first pure OCI with robust topological boundary states. We demonstrate that OCI is entirely orbital driven, fully disentangled from the spin and valley degrees of freedom, resulting in an orbital Hall effect that can be experimentally distinguished from the spin and valley Hall effects in insulating materials. Our work suggests a new avenue for exploring orbital topology in materials and advancing orbitronics-based technologies.
Yao et al. (2025) studied this question.