Polaronic mechanism of Nagaoka ferromagnetism in Hubbard models

The Hubbard model, a key descriptor of strongly interacting electronic systems, typically features antiferromagnetic spin alignment. However, in certain idealized limits, the celebrated Nagaoka theorem predicts a ferromagnetic ground state. Here, the authors demonstrate how itinerant ferromagnetism arises in a host of real experimental systems without the infeasible constraints required by the Nagaoka theorem. In this universal mechanism, an individual dopant surrounds itself with a ferromagnetically aligned spin bubble, with the local polarons from multiple such dopants combining to create global ferromagnetic order.