Connecting Higher-Order Topology with the Orbital Hall Effect in Monolayers of Transition Metal Dichalcogenides

TL;DR

This paper investigates the link between higher-order topological insulators and the orbital Hall effect in monolayers of transition metal dichalcogenides, revealing their coexistence and potential for spin-orbitronic applications.

Contribution

It demonstrates the correlation between HOTI phases and orbital Hall effects in TMD monolayers, identifying topological invariants and edge states through density functional theory and symmetry analysis.

Findings

1. 2H-TMDs are confirmed as HOTIs protected by $C_3$ symmetry.

2. 1T-TMDs possess a $ ext{Z}_4$ topological invariant and host conducting edge states.

3. HOTI phases are accompanied by an orbital Hall effect, with implications for spin-orbitronics.

Abstract

Monolayers of transition metal dichalcogenides (TMDs) in the 2H structural phase have been recently classified as higher-order topological insulators (HOTI), 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 spin-orbitronics.