Even more strikingly, strong effects are observed if two adjacent graphene layers are twisted with respect to each other. The beautiful moiré structures of the lattice are reflected also in the electronic structure (in presence of magnetic field resulting in the Hofstadter butterfly). Most notably, in 2018, Cao et al. showed that at a magic twist angle of 1.05°, correlation is strongly enhanced and a Mott insulating state is formed, which results in a superconducting state. This effect was shown initially in 2011 by Bistritzer and MacDonald, who reported that the Dirac-point velocity vanishes at the twist angle of 1.05° and that this effect is accompanied by a very flat moiré band, which results in a sharp peak in the Dirac-point density of states (see Figure ). The difference between flat, conjugated bands and dispersionless bands should be noted here: the former result from the symmetry of the lattice (e.g., kagome) with extended  conjugation, while the latter indicate localized, non-interacting states without electron conjugation. This short summary of electronic properties of graphene, bilayer graphene, and graphite illustrates the importance of the proximity effect: even though the graphene layers are subject to only weak van-der-Waals forces, which leave the atomistic structure essentially unaffected, the interlayer interaction has significant impact on the electronic structure, which is so strong that it can even cause the electronic phase transition from a zero-gap semiconductor to a superconductor. Similar stackings are present in layered COFs and in MOFs with suitable geometries. The impact of the proximity effect in these materials will be discussed in the next chapters.