Mott transition in multiorbital models for iron pnictides

The bad-metal behavior of the iron pnictides has motivated a theoretical description in terms of a proximity to Mott localization. Since the parent compounds of the iron pnictides contain an even number of 3d electrons per Fe, it is important to determine whether a Mott transition robustly exists and clarify the nature of the possible Mott insulating phases. We address these issues in a minimal two-orbital model and a more realistic four-orbital model for the parent iron pnictides using a slave-spin approach. In the two-orbital model with two electrons per Fe, we identify a single transition from a metal to a Mott insulator, showing that this transition must exist as a result of orbital degeneracy. Depending on the ratio between the inter-and intraorbital Coulomb repulsions, the insulating state can be either a high-spin Mott insulator or a low-spin orbital-Mott insulator. In the four-orbital model with four electrons per Fe, we find a rich phase diagram for the metal-to-insulator transition. At strong Hund's couplings, a localization transition to a high-spin Mott insulator always occurs. At zero and weak Hund's couplings, on the other hand, we find a transition to an intermediate-spin insulating state. This transition can be viewed as an orbitally selective metal-to-insulator transition: the transition to a Mott insulator in the xz and yz orbitals takes place at the same critical coupling as the transition to either a band insulator at zero Hund's coupling or an orbitally polarized insulator at weak but finite Hund's coupling in the xy and x(2) - y(2) orbitals. The implications of our model studies for the physics of iron pnictides and iron chalcogenides are discussed.