Resonant-driving-induced ferromagnetism in the Fermi-Hubbard model

In this article we study quantum phases and the phase diagram of a Fermi-Hubbard model under periodic driving that has been realized in recent cold-atom experiments, in particular, when the driving frequency is nearly resonant with the interaction energy. Due to the driving, the effective Hamiltonian contains a correlated hopping term where the density occupation strongly modifies the hopping strength. Focusing on half filling, in addition to the charge- and spin-density wave phases, large regions of ferromagnetic phase and phase separation are discovered in the weakly interacting regime. The mechanism of this ferromagnetism is attributed to the correlated hopping, because the hopping strength within a ferromagnetic domain is normalized to a larger value than the hopping strength across the domain. Thus, the kinetic energy drives the system into a ferromagnetic phase. We note that this is a different mechanism, in contrast to the well-known Stoner mechanism for ferromagnetism where the ferromagnetism is driven by interaction energy.