Competing interlayer interactions in twisted monolayer-bilayer graphene: From spontaneous electric polarization to quasi-magic angle

Abstract: The family of moir\'e materials provides a powerful platform for tuning interlayer couplings via the twist angle in systems with large spatial periodicity. In trilayer graphene systems, interlayer couplings at the two interfaces can possibly be tuned separately, and the competition between these interactions can therefore influence the electronic structure in a significant way. In this study, we investigate the electronic properties of twisted monolayer-bilayer graphene (aAB) beyond the continuum model, using first-principles calculations combined with an accurate tight-binding model. We find that at large twist angles, the electronic features of aAB are well described by the interaction between the parabolic bands of the Bernal AB-bilayer and the Dirac bands of the twisted monolayer, resulting in a spontaneous electric polarization in the former that splits the parabolic bands. As the twist angle decreases, the coupling between adjacent layers at the twisted interface becomes dominant, which makes aAB look like twisted bilayer graphene (TBG) interacting with the outer Bernal layer. A moir\'e potential emerges in the TBG-like layers, leading to charge localization, while the outer Bernal layer exhibits charge delocalization with substantial sublattice polarization at the atomic scale. Furthermore, we identify narrow bands with a minimum width at a quasi-magic angle of 1.16 degrees, closely matching the magic angle of TBG. The enhanced electron correlation expected in these narrow bands suggests that aAB is a promising platform for exploring correlated electronic phenomena.