Localization of lattice dynamics in low-angle twisted bilayer graphene

Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain(1-3). The resulting superlattice modulates the vibrational(3,4) and electronic(5,6) structures within the material, leading to changes in the behaviour of electron-phonon coupling(7,8) and to the observation of strong correlations and superconductivity(9). However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for experimental techniques to accurately resolve the relevant energy levels and too large for theoretical models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope(10), of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points(1) causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local density of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topological points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at atomic and nanometric scales, such as Jahn-Teller effects(11) and electronic Cooper pairing(12-14), and may help to improve device characterization(15) in the context of the rapidly developing field of twistronics(16). Nano-Raman spectroscopy reveals localization of some vibrational modes in reconstructed twisted bilayer graphene and provides qualitative insights into how electron-phonon coupling affects the vibrational and electronic properties of the material.