Interlayer interaction, shear vibrational mode, and tribological properties of two-dimensional bilayers with a commensurate moiré pattern
The potential energy surface (PES) of the interlayer interaction of infinite twisted bilayer graphene is calculated for a set of commensurate moiré patterns using the registry-dependent Kolmogorov-Crespi empirical potential. The calculated PESs have the same shape for all considered moiré patterns, and the unit cell size of the PESs is inversely related to the unit cell size of the moiré pattern. The amplitude of PES corrugations is found to decrease exponentially upon increasing the size of the moiré pattern unit cell. An analytical expression for such a PES including the first Fourier harmonics compatible with the symmetries of both layers is derived. It is shown that the calculated PESs can be approximated by the derived expression with an accuracy within 1%. This means that different physical properties associated with relative in-plane motion of graphene layers are interrelated and can be expressed analytically as functions of the amplitude of PES corrugations. In this way, we obtain the shear mode frequency, the shear modulus, the shear strength, and the barrier for relative rotation of the commensurate twisted layers to a fully incommensurate state for the considered moiré patterns. This barrier may possibly lead to robust macroscopic superlubricity for a twisted graphene bilayer with a commensurate moiré pattern. The conclusions drawn should be valid for diverse two-dimensional systems of twisted commensurate layers.