Electromechanical response of saddle points in twisted hBN moir\'e superlattices

TL;DR

This study uses piezo force microscopy to investigate the electromechanical behavior of twisted hexagonal boron nitride, revealing narrow saddle point polarizations and complex 3D polarization fields in moiré superlattices.

Contribution

It provides the first comprehensive experimental analysis of saddle point polarizations and in-plane components in twisted hBN moiré structures.

Findings

Narrow saddle point polarizations (~20 nm) observed in twisted hBN.

Localized polarizations present in both parallel and anti-parallel stacking.

Visualization of moiré superlattice related to strain at saddle points.

Abstract

In twisted layered materials (t-LMs), an inter-layer rotation can break inversion symmetry and create an interfacial array of staggered out-of-plane polarization due to AB/BA stacking registries. This symmetry breaking can also trigger the formation of edge in-plane polarizations localized along the perimeter of AB/BA regions (i.e., saddle point domains). However, a comprehensive experimental investigation of these features is still lacking. Here, we use piezo force microscopy to probe the electromechanical behavior of twisted hexagonal boron nitride (t-hBN). For a parallel stacking alignment of t-hBN, we reveal very narrow (width ~ 20 nm) saddle point polarizations, which we also measure in the anti-parallel configuration. These localized polarizations can still be found on a multiply-stacked t-hBN structure, determining the formation of a double moir\'e. We also visualize a t-hBN moir\'e superlattice in the topography maps with atomic force microscopy, related to the strain accumulated at the saddle point domains. Our findings imply that polarizations in t-hBN do not only point in the out-of-plane direction, but also show an in-plane component, giving rise to a much more complex 3D polarization field.