Unusual topological polar texture in moiré ferroelectrics

Abstract

Abstract Topological polar textures in ferroelectrics have attracted significant interest for their potential applications in energy-efficient and high-density data storage and processing. Among these, polar merons and antimerons are predicted in strained and twisted bilayers of inversion symmetry broken systems. However, experimental observation of these polar textures within twisted two-dimensional van der Waals materials remains elusive. Here, we utilize vector piezoresponse force microscopy to reconstruct the polarization fields in R-type marginally twisted hexagonal boron nitride. We observe alternating out-of-plane polarizations at domain regions and in-plane vortex-like polarization patterns along domain walls, indicative of a network of polar merons and antimerons. Notably, the out-of-plane polarization exhibits three polarity reversals across a domain wall. Similar polar textures are identified in marginally twisted WSe2 and MoSe2 homobilayers. Our theoretical simulations attribute these unusual polarization reversals near the domain walls to the competition between moiré ferroelectricity and piezoelectricity. These results provide the experimental evidence of complex polar textures in moiré ferroelectrics, which may offer additional insights into the electronic band topology in twisted transition metal dichalcogenides.