Bilayer graphene is supposed to have a bandgap, but experiments showed that this material cannot be turned into a real insulator. Now researchers from Berkeley Lab's Advanced Light Source (ALS) institute discovered that this is caused by tiny twists in the bilayer material, caused by subtle misalignments of the two layers. This twist can lead to surprisingly strong changes in the bilayer graphene's electronic properties.
The graphene layers twist produces massive and massless Dirac fermions. This structure prevents bilayer graphene from becoming fully insulating even under a very strong electric field. The researchers explain that Massless Dirac fermions are essentially electrons that act as if they were photons. As a result, they are not restricted by the same band gap constraints as conventional electrons. These new massless Dirac fermions move in a completely unexpected way governed by the symmetry twisted layers.