Microscopic polarization in bilayer graphene

Bilayer graphene contains, compared to single-layer graphene, additional states related to the symmetry of the layers. These states can lead to the opening of a bandgap, which is highly desirable for device applications. The gap can be either tunable through an external electric field or spontaneously formed through an interaction-induced symmetry breaking. Here, we report scanning tunnelling microscopy measurements that reveal that the microscopic nature of the bilayer gap is very different from what has been observed in previous macroscopic measurements, and from what is expected from current theoretical models. The potential difference between the layers, which is proportional to charge imbalance and determines the gap value, shows a strong dependence on the disorder potential and varies spatially in both magnitude and sign on a microscopic level. Additional interaction-induced effects are observed on applying a magnetic field, as a subgap opens once the zero-orbital Landau level is placed at the Fermi energy.