Mott Transition in Multi-Orbital 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 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 transition from metal to Mott insulator. The critical coupling, $U_c$, is greatly reduced by the Hund's coupling. Depending on the ratio between the inter- and intra-orbital Coulomb repulsions, the insulating state can be either a spin-Mott insulator or an orbital-Mott insulator. In the four-orbital model with four electrons per Fe, we find an orbitally selective metal-to-insulator transition in the case of zero Hund's coupling; the transition to a Mott insulator in the $xz$ and $yz$ orbitals takes place at the same critical coupling as the transition to a band insulator in the $xy$ and $x^2-y^2$ orbitals. In the presence of a finite Hund's coupling, however, the localization transition is into a spin-Mott state.