Realization of the Haldane Chern insulator in a moir lattice

The Chern insulator displays a quantized Hall effect without Landau levels. Theoretically, this state can be realized by engineering complex next-nearest-neighbour hopping in a honeycomb lattice-the so-called Haldane model. Despite its profound effect on the field of topological physics and recent implementation in cold-atom experiments, the Haldane model has not yet been realized in solid-state materials. Here we report the experimental realization of a Haldane Chern insulator in AB-stacked MoTe2/WSe2 moire bilayers, which form a honeycomb moire lattice with two sublattices residing in different layers. We show that the moire bilayer filled with two holes per unit cell is a quantum spin Hall insulator with a tunable charge gap. Under a small out-of-plane magnetic field, it becomes a Chern insulator with a finite Chern number because the Zeeman field splits the quantum spin Hall insulator into two halves with opposite valleys: one with a positive and the other with a negative moire band gap. We also demonstrate experimental evidence of the Haldane model at zero external magnetic field by proximity coupling the moire bilayer to a ferromagnetic insulator. The Haldane model is a paradigmatic example of topological behaviour but has not previously been implemented in condensed-matter experiments. Now a moire bilayer is shown to realize this model with the accompanying quantized transport response.