Interlayer Coupling and Strain Localization in Small-Twist-Angle Graphene Flakes

TL;DR

This study uses a continuum finite element model to analyze how small-twist-angle graphene flakes experience localized strain and structural relaxation, revealing metastable states and shear strain solitons that influence their physical properties.

Contribution

It introduces a continuum modeling approach to simulate inhomogeneous deformations and strain localization in small-twist-angle graphene flakes, capturing metastable configurations and shear solitons.

Findings

Strain localization is prominent in large flakes at small twist angles.

Multiple metastable equilibrium states exist depending on twist angle and flake size.

Shear strain solitons form along domain boundaries, affecting physical properties.

Abstract

Twisted bilayer graphene (TBG) exhibits a wide range of intriguing physical properties, such as superconductivity, ferromagnetism, and superlubricity. Depending on the twist angle, periodic moir\'e superlattices form in twisted bilayer graphene, with inhomogeneous interlayer coupling and lattice deformation. For a small twist angle (typically <2{\deg}), each moir\'e supercell contains a large number of atoms (>10,000), making it computationally expensive for first-principles and atomistic modeling. In this work, a finite element method based on a continuum model is used to simulate the inhomogeneous interlayer and intralayer deformations of twisted graphene flakes on a rigid graphene substrate. The van der Waals interactions between the graphene layers are described by a periodic potential energy function, whereas the graphene flake is treated as a continuum membrane with effective elastic properties. Our simulations show that structural relaxation and the induced strain localization are most significant in a relatively large graphene flake at small twist angles, where the strain distribution is highly localized as shear strain solitons along the boundaries between neighboring domains of commensurate AB stacking. Moreover, it is found that there exist many metastable equilibrium configurations at particular twist angles, depending on the flake size. The nonlinear mechanics of twisted bilayer graphene is thus expected to be essential for understanding the strain distributions in the moir\'e superlattices and the strain effects on other physical properties.