Science https://doi.org/dmwz (2020)

The quantum Hall effect was originally observed in two-dimensional electron systems subjected to a magnetic field. Crucially, the magnetic field leads to nontrivial band topology and breaks time-reversal symmetry. A ferromagnetically ordered topological insulator simultaneously satisfies these two requirements so that the effect can occur even without an external field; this is referred to as the quantum anomalous Hall effect. Yujun Deng and co-workers have now reported the observation of this anomalous effect in an intrinsic magnetic topological insulator MnBi2Te4.

A quantized anomalous Hall response has been observed previously in a doped topological insulator. Doping endows the material itself with magnetism, but there is a caveat — the dopants act as impurities that limit the quality of the material. MnBi2Te4 is a topological insulator with intrinsic magnetic order from the Mn2+ ions, allowing the study of the quantum anomalous Hall effect in pristine crystals. Deng and co-workers showed that a quantized Hall response can be achieved under more relaxed conditions.