Blue phosphorene hosts an orbital Chern insulator with an experimentally distinct orbital Hall effect
Topological insulators are insulators in the bulk and conductors on the surface. This behaviour is caused by spin-orbit coupling, a property that is stronger in heavier elements. Therefore, most topological insulators are made using heavy elements, such as bismuth selenide (Bi₂Se₃) and antimony telluride (Sb₂Te₃). In this research, the authors introduce orbital Chern insulators, a topological phase in which the orbital angular momentum of electrons, rather than their spin, drives the nontrivial topology. This allows topological behaviour to emerge in materials composed of much lighter elements, demonstrated using monolayer blue phosphorus, which was previously regarded as a trivial insulator.
The authors introduce a feature‑spectrum topology framework, a systematic method for identifying and characterizing materials with orbital‑driven topology. Using this approach, they show that phosphorene hosts the first pure orbital Chern insulator, where the orbital topology is fully disentangled from spin and valley degrees of freedom. As a result, the material exhibits a pure orbital Hall effect that can be experimentally distinguished from spin and valley Hall responses, unlike in transition‑metal dichalcogenides where spin-orbit coupling and valley physics are intertwined.
Because orbital Chern insulators do not rely on spin-orbit coupling, they are not constrained by the small band gaps typical of spin-orbit coupling driven topological insulators, and can potentially support larger band gaps in light‑element systems. The authors also show that orbital nontriviality is expected more broadly in Group 5A monolayers with buckled or puckered structures, expanding the landscape of candidate materials. This research opens a path for orbitronics, where currents of orbital angular momentum instead of spin currents used in spintronics, can be generated, controlled, and applied in future quantum and electronic devices.
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Interacting topological insulators: a review by Stephan Rachel (2018)