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.Comment: 12 pages, 11 figures, to appear in Phys. Rev.

The magnetic phase diagram of an extended J1-J2 model on a modulated square lattice and its implications for the antiferromagnetic phase of KyFexSe2

Motivated by the experimentally observed $\sqrt{5} \times \sqrt{5}$ iron vacancy order and a block spin antiferromagnetic phase with large magnetic moment in $\mathrm{K}_{0.8}\mathrm{Fe}_{1.6}\mathrm{Se}_2$, we study the magnetic phase diagram of an extended $J_1-J_2$ model on a 1/5-depleted square lattice with $\sqrt{5} \times \sqrt{5}$ vacancy order, using a classical Monte Carlo analysis. The magnetic phase diagram involves various antiferromagnetically ordered phases, and most of them have higher order commensuration. We find that the experimentally relevant block-spin state occupies a significant portion of the phase diagram, and we discuss the spin dynamics of this phase using a linear spin-wave analysis. Based on our spin wave calculations in different parameter regimes corresponding to the block spin antiferromagnetic phase, we show how spin-wave degeneracy along the high symmetry directions of the magnetic Brillouin zone can provide information regarding the underlying exchange couplings. We have also analyzed the magnetic phase diagram of a $J_1-J_2$ model on two different modulated square lattices relevant for $\mathrm{K}_y\mathrm{Fe}_{1.5}\mathrm{Se}_2$, which respectively exhibit 1/4-depleted $2\times2$ and $4\times2$ vacancy ordering.Comment: 9 pages, 8 figures, 3 table