Unusual topological polar texture in moir\'e ferroelectrics

TL;DR

This study provides experimental evidence of complex topological polar textures, including merons and antimerons, in twisted 2D ferroelectric materials, revealing their potential for advanced electronic applications.

Contribution

First experimental observation of polar merons and antimerons in twisted 2D ferroelectrics using vector PFM, supported by theoretical simulations.

Findings

Detection of alternating out-of-plane polarizations in twisted hBN.

Identification of in-plane vortex-like polarization patterns at domain walls.

Observation of three polarity reversals across domain walls.

Abstract

Topological polar textures in ferroelectrics have attracted significant interest for their potential for 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 (2D vdW) materials remains challenging. Here, we utilize vector piezoresponse force microscopy (PFM) to reconstruct the polarization fields in R-type marginally twisted hexagonal boron nitride (hBN). We observe alternating out-of-plane (OOP) polarizations at domain regions and in-plane (IP) vortex-like polarization patterns along domain walls (DWs), indicative of a network of polar merons and antimerons. Notably, the OOP polarization exhibits three polarity reversals across a DW. Similar polar textures are identified in marginally twisted MoSe2 and WSe2 homobilayers. Our theoretical simulations attribute these unusual polarization reversals near the DWs to the competition between moir\'e ferroelectricity and piezoelectricity. These results provide experimental evidence of complex polar textures in moir\'e ferroelectrics, offering new insights into the electronic band topology in twisted transition metal dichalcogenides (TMDCs).