Flat band magnetism in electride monolayer LaBr

Bulk LaBr is a layered material that is weakly bounded by interlayer van der Waals interactions. It has been predicted to be a ferromagnetic topological material that is also an electride, i.e., has electrons localized at interstitial sites of the crystal lattice.


In this work, we focus on LaBr in monolayer form. Using first-principles density-functional theory calculations, we predict monolayer LaBr to be a stable, magnetic material that also has the hallmarks of being an electride under ambient conditions. We show that its low energy electronic structure has flat bands and can be described by a simple tight-binding model with frustrated hoppings. The flat bands and magnetic ground state are sensitive to and can be controlled via external strain and doping (e.g., by means of an electrical gate). Extending the tight-binding model to include Hubbard interactions self-consistently, we find that electron-electron interactions play a critical role in determining both the low-energy bandstructure and the magnetic order of the ground state of monolayer LaBr.