Imaging topological polar structures in marginally twisted 2D semiconductors

TL;DR

This paper uses advanced microscopy to visualize and differentiate topological polar structures in twisted bilayer WSe2, revealing how twist and strain influence the formation of non-trivial meron/antimeron nanostructures in 2D semiconductors.

Contribution

It provides the first experimental visualization of topologically non-trivial meron structures in marginally twisted 2D heterostructures, distinguishing effects of twist and strain.

Findings

Observation of both Bloch-type and Neel-type merons

Differentiation between twist-induced and strain-induced moire superlattices

Experimental proof of topologically non-trivial polar nanostructures

Abstract

Moire superlattices formed in van der Waals heterostructures due to twisting, lattice mismatch and strain present an opportunity for creating novel metamaterials with unique properties not present in the individual layers themselves. Ferroelectricity for example, arises due to broken inversion symmetry in twisted and strained bilayers of 2D semiconductors with stacking domains of alternating out-of-plane polarization. However, understanding the individual contributions of twist and strain to the formation of topological polar nanostructures remains to be established and has proven to be experimentally challenging. Inversion symmetry breaking has been predicted to give rise to an in-plane component of polarization along the domain walls, leading to the formation of a network of topologically non-trivial merons (half-skyrmions) that are Bloch-type for twisted and Neel-type for strained systems. Here we utilise angle-resolved, high-resolution vector piezoresponse force microscopy (PFM) to spatially resolve polarization components and topological polar nanostructures in marginally twisted bilayer WSe2, and provide experimental proof for the existence of topologically non-trivial meron/antimeron structures. We observe both Bloch-type and Neel-type merons, allowing us to differentiate between moire superlattices formed due to twist or heterogeneous strain. This first demonstration of non-trivial real-space topology in a twisted van der Waals heterostructure opens pathways for exploring the connection between twist and topology in engineered nano-devices.