Nearly free phonons in a weak soliton potential and the case of twisted bilayer graphene

Structural relaxation in slightly misaligned bilayer graphene leads to the formation of domains separated by topological solitons. The influence of such a soliton lattice on phonons is investigated in the framework of the Frenkel-Kontorova model, where a solution is derived in the continuum limit. A nearly free phonon model is developed to explain the characteristic features in the phonon band structure derived from the Frenkel-Kontorova model. Its 2D extension is discussed and applied on the specific case of twisted bilayer graphene. This model explains a number of computational results reported until now and can be used directly to investigate other misaligned two-dimensional networks as found in van der Waals heterostructures.