Ultra-flat twisted superlattices in 2D heterostructures

Moire-superlattices are ubiquitous in 2D heterostructures, strongly influencing their electronic properties. They give rise to new Dirac cones and are also at the origin of the superconductivity observed in magic-angle bilayer graphene. The modulation amplitude (corrugation) is an important yet largely unexplored parameter in defining the properties of 2D superlattices. The generally accepted view is that the corrugation monotonically decreases with increasing twist angle, while its effects on the electronic structure diminish as the layers become progressively decoupled. Here we found by lattice relaxation of around 8000 different Moire-superstructures using high scale Classical Molecular Simulations combined with analytical calculations, that even a small amount of external strain can substantially change this picture, giving rise to more complex behavior of superlattice corrugation as a function of twist angle. One of the most surprising findings is the emergence of an ultra-flat phase that can be present for arbitrary small twist angle having a much lower corrugation level than the decoupled phase at large angles. Furthermore, Moire-phase maps evidence that the state with no external strain is located in the close vicinity of a triple Moire-phase boundary, implying that very small external strain variations can cause drastic changes in the realized superlattice morphology and corrugation. This renders the practical realization of 2D heterostructures with large-area homogeneous superlattice morphology highly challenging.