Moir\'e superlattices of two-dimensional (2D) materials with a small twist angle are thought to exhibit appreciable flexoelectric effect, though unambiguous confirmation of their flexoelectricity is challenging due to artifacts associated with commonly used piezoresponse force microscopy (PFM). For example, unexpectedly small phase contrast (∼8∘) between opposite flexoelectric polarizations was reported in twisted bilayer graphene (tBG), though theoretically predicted value is 180∘. Here we developed a methodology to extract intrinsic moir\'e flexoelectricity using twisted double bilayer graphene (tDBG) as a model system, probed by lateral PFM. For small twist angle samples, we found that a vectorial decomposition is essential to recover the small intrinsic flexoelectric response at domain walls from a large background signal. The obtained three-fold symmetry of commensurate domains with significant flexoelectric response at domain walls is fully consistent with our theoretical calculations. Incommensurate domains in tDBG with relatively large twist angles can also be observed by this technique. Our work provides a general strategy for unraveling intrinsic flexoelectricity in van der Waals moir\'e superlattices while providing insights into engineered symmetry breaking in centrosymmetric materials.
@article{arxiv.2104.02401,
title = {Unraveling intrinsic flexoelectricity in twisted double bilayer graphene},
author = {Yuhao Li and Xiao Wang and Deqi Tang and Xi Wang and K. Watanabe and T. Taniguchi and Daniel R. Gamelin and David H. Cobden and Matthew Yankowitz and Xiaodong Xu and Jiangyu Li},
journal= {arXiv preprint arXiv:2104.02401},
year = {2022}
}