Excitonic Chern insulator in a moiré lattice (part2)

Excitonic insulators form when the charge gap of insulators becomes smaller than the binding energy of an electron-hole pair (exciton); spontaneous emergence of an exciton fluid is expected. Near such band inversion point with small charge gap, topological bands with finite Chern numbers can also emerge. A natural question arises: is it possible to have a Chern insulator from an excitonic mechanism? The emergence of moiré materials with tunable band inversion and strong excitonic interactions provides a platform to address this question. Here, by combining local thermodynamic measurements and magneto-optical spectroscopy, we report experimental evidence of an excitonic Chern insulator in AB-stacked MoTe₂/WSe₂ moiré heterobilayers, which support electric-field-tuned band inversions. These combined measurements performed simultaneously on the same sample location are made possible by employing a new optical technique to readout the thermodynamic equations of states. At half-band-filling, spontaneous formation of interlayer excitons, accompanied by the emergence of a ferromagnetic Chern insulator, is observed before an electric-field-induced charge gap closure. The results provide direct evidence of an excitonic Chern insulator driven by Coulomb correlations, and open the door to realize topological exciton condensates.