Comparing the Extrinsic Orbital Hall Effect in Centrosymmetric and Noncentrosymmetric Systems: Insights from Bilayer Transition Metal Dichalcogenides

Abstract: Both intrinsic and extrinsic orbital Hall effects (OHE) in bilayer transition metal dichalcogenides (TMDs) are investigated in the presence of short-range disorder using quantum kinetic theory. Bilayer TMDs provide an ideal platform to study the effects of inversion symmetry breaking on transport properties due to their unique structural and electronic characteristics. While bilayer TMDs are naturally inversion symmetric, applying a finite gate voltage to create a bias between the layers effectively breaks this symmetry. Our findings reveal that slightly away from the band edges, the extrinsic OHE becomes the dominant contribution in both inversion-symmetric and asymmetric cases, with its prominence increasing significantly as the Fermi energy rises. Furthermore, we demonstrate that breaking inversion symmetry greatly enhances the extrinsic OHE. This enhancement arises from the fundamentally distinct behavior of orbital angular momentum (OAM) in centrosymmetric systems, where intraband components vanish due to symmetry constraints. As a result, in centrosymmetric systems, only the off-diagonal components of the density matrix contribute to the extrinsic OHE. In contrast, in noncentrosymmetric systems, both diagonal and off-diagonal components play a role. Our study suggests that in experimentally relevant, highly doped systems, the OHE is predominantly extrinsic in nature, regardless of whether the system is centrosymmetric or noncentrosymmetric. Importantly, we infer that even a weakly breaking of inversion symmetry can lead to a dramatic enhancement of the OHE, a finding with significant implications for experimental investigations.