Electrons in Magic-Angle Twisted Bilayer Graphene Behave as Both Heavy and Light Particles

Introducing a small twist angle between two graphene layers stacked on top of each other can change the material's properties. When a twist angle of about 1.1 degrees is used, the resulting material known as magic-angle twisted bilayer graphene exhibits unusual properties including unconventional superconductivity.

These properties originate from the formation of flat bands in which the electron's energy is almost independent of its momentum. As a result, electrons in the material move slowly and interact strongly with each other. They behave as if they have a large mass.

@Nature reported that this picture is incomplete. For some momenta, electrons in magic-angle twisted bilayer graphene are heavy and interact strongly. For others, they are light and mobile. The findings come from a paper published in Nature.

Graphene consists of two-dimensional sheets of carbon atoms. Stacking two such layers with a small twist angle between them produces the material known as magic-angle twisted bilayer graphene when the angle is approximately 1.1 degrees. The material has drawn attention for its ability to display unconventional superconductivity.

The flat bands in the material cause electrons to move slowly. This leads to strong interactions between electrons. Earlier studies had established this heavy-electron behavior as central to the material's properties.

The latest research shows that electron behavior in the material varies by momentum. Electrons act as heavy particles with strong interactions at certain momenta. At other momenta, the same electrons behave as light and mobile particles. This dual character indicates that the standard description of the material relying solely on heavy electrons does not fully capture its electronic structure.

The study provides new details on how interactions reshape the flat bands in magic-angle twisted bilayer graphene.