Nature 579, 56–61 (2020)

The prototypical topological material — the topological insulator — can be in either topological or trivial states. However, there is a richer class of topological materials called Chern insulators, where there are many non-trivial states characterized by a Chern number that can take any integer value. Researchers from the USA and China have now demonstrated a state with a Chern number of two in trilayer graphene with a moiré pattern.

The team showed that at a particular electron density, a plateau appears in the transverse resistivity that is quantized in the same way as the quantum Hall effect with filling fraction equal to two. This indicates that a state has been formed with that Chern number, and its hysteresis in a magnetic field shows that it is ferromagnetic. The manifestation of these states in a material with strong electron interactions opens the possibility that this material can also host non-abelian ‘fractional’ Chern states.