Imaging topological polar structures in marginally twisted 2D semiconductors.

Moiré superlattices formed in van der Waals heterostructures by twisting, lattice mismatch and strain present an opportunity for creating metamaterials with unique properties absent in the individual layers. Ferroelectricity for example, arises from broken inversion symmetry in twisted and strained bilayers of 2D semiconductors with stacking domains of alternating out-of-plane polarization. However, the individual contributions of twist and strain to the formation of topological polar nanostructures remain unclear and experimentally challenging to resolve. Inversion symmetry breaking is predicted to generate in-plane polarization along the domain walls, forming topologically non-trivial Bloch-type merons (half-skyrmions) in twisted systems and Néel-type merons in strained systems. Here we utilize angle-resolved vector piezoresponse force microscopy to spatially resolve polarization components and topological polar nanostructures in marginally twisted bilayer WSe2, providing experimental evidence of topologically non-trivial meron/antimeron structures. This approach can be used to distinguish Bloch-type, Néel-type and hybrid structures, allowing us to quantify the separate contributions of strain and twist in a moiré superlattice, opening pathways for exploring twist-induced topology in engineered nano-devices.