Enhanced Interlayer Shear Locking in ABC Three-Layer Graphene.

Rhombohedral graphene is one of the most promising material candidates for quantum computing applications. Although Bernal (ABA) is the most common stacking order, only rhombohedral (ABC) graphene readily exhibits correlated phenomena, such as superconductivity and magnetism. This exotic behavior arises from its unique electronic structure and the strong interlayer mechanical coupling characteristic of the ABC stacking order. Despite this importance, the interlayer mechanical properties of ABC graphene remain unexplored. Here, we measure the interlayer transverse shear stiffness of ABA and ABC three-layer epitaxial graphene using a novel approach that enables mapping of local variations in shear modulus with sub-10 nm resolution. Epitaxial graphene grown on SiC forms ABA and ABC stacking domains that are robust enough to persist even under strong shear, allowing a reliable comparison of their mechanical responses. The mapping reveals that the interlayer shear modulus is on average equal to 3.5 GPa in ABC graphene, against 2.6 GPa in ABA graphene. Atomistic simulations also show a greater interlayer shear modulus in three-layer ABC graphene than in ABA, and with a pronounced directional dependence. The higher elastic shear modulus in ABC suggests enhanced interlayer shear locking, a key factor in determining strong electron-phonon coupling.