Quantitatively Mapping Lattice Reconstruction and Strain Fields in Moiré Materials

Moiré superlattices, formed by stacking two-dimensional van der Waals layers with an interlayer rotation and/or lattice constant mismatch, have electronic band structures that are highly sensitive to structural modifications. For example, twisted bilayer graphene exhibits unconventional superconductivity and ferromagnetism at a ‘magic’ interlayer twist angle of 1.1°, associated with formation of flat electronic bands. At the same time, natural lattice deformations, strain, and disorder can also dramatically influence the behavior observed in these systems. Visualizing the structure and strain fields of moiré materials is therefore paramount to understanding and controlling their emergent electronic behavior. In this talk, I will present work on the development of a technique termed Bragg interferometry, based on four-dimensional scanning transmission electron microscopy (4D-STEM), for directly and quantitatively mapping interlayer atomic displacements and strain fields in moiré superlattices, including twisted bilayer graphene and twisted bilayer transition metal dichalcogenides. This work sheds light on the structural changes underpinning the twist angle dependent electronic properties of moiré systems and provides a new framework for directly visualizing lattice reconstruction mechanics, disorder, and strain in a wide array of moiré materials.