Stacking various atomically-thin crystals on top of one another can strongly modify their overall properties. When two materials are stacked with a twist angle, a geometric interference pattern (a moiré pattern) emerges. At special twist angles, the moiré pattern can result in new electronic states dominated by strong correlations between electrons.
We have made devices in which a monolayer and a bilayer graphene sheet are stacked and twisted by around 1 degree. We found that strongly correlated states emerge in these devices, and can be controlled by applying an electric field. Additionally, we discovered a unique type of ferromagnetism known as “orbital magnetism” in these devices. Combined with nontrivial band topology, this results in a large anomalous Hall effect that is nearly quantized. These highly tunable correlated and topological states are promising for the development of future quantum technologies.