Monolayers of transition metal dichalcogenides (TMDs) in the $2H$ structural phase have been recently classified as higher-order topological insulators (HOTIs), protected by $C_3$ rotation symmetry. In addition, theoretical calculations show an orbital Hall plateau in the insulating gap of TMDs, characterized by an orbital Chern number. We explore the correlation between these two phenomena in TMD monolayers in two structural phases: the noncentrosymmetric $2H$ and the centrosymmetric $1T$. Using density functional theory, we confirm the characteristics of $2H$ TMDs and reveal that $1T$ TMDs are identified by a ${\mathbb{Z}}_4$ topological invariant. As a result, when cut along appropriate directions, they host conducting edge states, which cross their bulk energy-band gaps and can transport orbital angular momentum. Our linear response calculations thus indicate that the HOTI phase is accompanied by an orbital Hall effect. Using general symmetry arguments, we establish a connection between the two phenomena with potential implications for orbitronics and spin orbitronics.

• Received 10 May 2022
• Revised 18 November 2022
• Accepted 13 February 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.116204