Strained twisted bilayer graphene

Twisted bilayer graphene is a two-dimensional moire superlattice material formed by simple twisting of two stacked layers of graphene. This material exhibits a series of exotic physical properties dictated by the superlattice structure, such as superconducting, magnetic, and localized states. These properties have made it one of the most exciting recent developments in the research field of condensed matter physics and materials. However, designing and fabricating the two dimensional moire superlattice materials and devices remains challenging due to intrinsic disorders of twist angles and lattice strains. These disorders severely complicate the understanding of materials properties and may even degrade the device performance. In this review, we present an overview of recent theoretical and experimental progress in the study of strained twisted bilayer graphene systems. The interlayer superlubricity of twisted bilayer graphene arises from the surface flatness, charge neutrality, and weak van der Waals interactions, allowing easy rotation and translation of the layered structure from each other. Consequently, the intrinsic strain can be induced during the preparation of materials and fabrication of related devices. Significant lattice reconstruction occurs in the moire superlattice due to the shear and tensile strain, giving rise to uncertainty in the ultimate properties of the materials. Even moire superlattice materials and devices fabricated using state-of-the-art techniques suffer from low repeatability and intrinsic strain disorder, although applying external strains can somewhat homogenize the intrinsic strain. When the twist angle is small, strain generation is typically accompanied by the reconstruction of moire superlattice structures, known as the commensurate-incommensurate phase transition. This phase transition involves changes within and between the layers of the layered structure. Topological defects formed after the phase transitions are called soliton boundaries, and the interlayer distance changes depending on the local atomic stacking arrangement. These soliton boundaries create a triangular network with topologically protected chiral edge states, offering an ideal pathway for the ballistic transport of electrons. External strain can modulate the electronic band structure of twisted graphene, generate pseudomagnetic fields, and lead to photoelectric properties, potentially enabling a richer variety of functionalities such as superconductivity across a broader range of twist angles. In summary, research on moire superlattices has witnessed significant progress, with a particular focus on twisted graphene. However, several aspects remain to be investigated and refined to fully exploit their potential benefits. It is essential to develop simple and accurate characterization techniques for measuring local twist angles and strain. Straightforward and effective optical methods, such as Raman spectroscopy, could be a promising tool for future research. The emerging research field of moire superlattice opens great opportunities for exploration in multi-field coupling at low dimensionality, including straintronics. This paper reviews the progress and challenges in this field, aiming to facilitate the practical application of twisted graphene.