Helical network model for twisted bilayer graphene

In the presence of a finite interlayer displacement field, bilayer graphene has an energy gap that is dependent on stacking and largest for the stable AB and BA stacking arrangements. When the relative orientations between layers are twisted through a small angle to form a moire pattern, the local stacking arrangement changes slowly. We show that for nonzero displacement fields the low-energy physics of twisted bilayers is captured by a phenomenological helical network model that describes electrons localized on domain walls separating regions with approximate AB and BA stacking. The network band structure is gapless and has of a series of two-dimensional bands with Dirac band-touching points and a density of states that is periodic in energy with one zero and one divergence per period.