Metal-insulator transition in a two-band model for the perovskite nickelates

Motivated by recent Fermi-surface and transport measurements on LaNiO3, we study the Mott metal-insulator transitions of perovskite nickelates, with the chemical formula RNiO3, where R is a rare-earth ion. We introduce and study a minimal two-band model, which takes into account only the e g bands. In the weak to intermediate correlation limit, a Hartree-Fock analysis predicts charge and spin order consistent with experiments on R = Pr, Nd, driven by Fermi surface nesting. It also produces an interesting semimetallic electronic state in the model when an ideal cubic structure is assumed. We also study the model in the strong-interaction limit and find that the charge and magnetic order observed in experiment exist only in the presence of very large Hund's coupling, suggesting that additional physics is required to explain the properties of the more insulating nickelates, R = Eu, Lu, Y. Next, we extend our analysis to slabs of finite thickness. In ultrathin slabs, quantum confinement effects substantially change the nesting properties and the magnetic ordering of the bulk, driving the material to exhibit highly anisotropic transport properties. However, pure confinement alone does not significantly enhance insulating behavior. Based on these results, we discuss the importance of various physical effects and propose some experiments.