In moiré TMD heterostructures, such as WSe2/WS2, Mott-Hubbard insulating behaviour has been observed, where strong repulsive interactions localize electrons on the sites of the emergent moiré lattice. Crucially, this has been found to occur not only at half-filling (one electron per site), but also at certain other fractional fillings.

A crucial question concerns lifting the (pseudo-)spin degeneracy of these localized electrons. Whereas a textbook argument leads to antiferromagnetic interactions between nearest-neighbor sites at half-filling, the situation becomes more involved at fractional fillings: in this case, exchange interactions between charges become suppressed due to their exponentially decaying orbital overlap.

In our work, we predict that adding a few additional electrons (i.e. doping the system) allows for parametrically stronger interactions between the doped charges and local moments, thereby becoming the dominant mechanism for lifting the spin degeneracy.

We showed that the doped charges bind magnetic excitations (magnons) to form spin-polaron bound states. This is energetically optimal if all remaining moments are in a ferromagnetically aligned state, allowing us to predict that the system becomes ferromagnetic (with a small density of spin-polarons) upon doping [1].

Remarkably, during the review process of our paper, an experimental study on AB-stacked MoTe2/WSe2 reported results that are largely consistent with our predictions.