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.

A note on strong edge-colorings of 2-degenerate graphs

We show that every 2-degenerate graph with maximum degree $\Delta$ has a strong edge-coloring with at most $8\Delta-4$ colors

U(1) Slave-spin theory and its application to Mott transition in a multi-orbital model for iron pnictides

A U(1) slave-spin representation is introduced for multi-orbital Hubbard models. As with the $Z_2$ form of L. de'Medici et al. (Phys. Rev. B 72, 205124 (2005)), this approach represents a physical electron operator as the product of a slave spin and an auxiliary fermion operator. For non-degenerate multi-orbital models, our U(1) approach is advantageous in that it captures the non-interacting limit at the mean-field level. For systems with either a single orbital or degenerate multiple orbitals, the U(1) and $Z_2$ slave-spin approachs yield the same results in the slave-spin-condensed phase. In general, the U(1) slave-spin approach contains a U(1) gauge redundancy, and properly describes a Mott insulating phase. We apply the U(1) slave-spin approach to study the metal-to-insulator transition in a five-orbital model for parent iron pnictides. We demonstrate a Mott transition as a function of the interactions in this model. The nature of the Mott insulating state is influenced by the interplay between the Hund's rule coupling and crystal field splittings. In the metallic phase, when the Hund's rule coupling is beyond a threshold, there is a crossover from a weakly correlated metal to a strongly correlated one, through which the quasiparticle speactral weight rapidly drops. The existence of such a strongly correlated metallic phase supports the incipient Mott picture of the parent iron pnictides. In the parameter regime for this phase and in the vicinity of the Mott transition, we find that an orbital selective Mott state has nearly as competitive a ground state energy.Comment: 7 pages, 2 figure

Orbital-selective Mott phase in multiorbital models for iron pnictides and chalcogenides

There is increasing recognition that the multiorbital nature of the 3d electrons is important to the proper description of the electronic states in the normal state of the iron-based superconductors. Earlier studies of the pertinent multiorbital Hubbard models identified an orbital-selective Mott phase, which anchors the orbital-selective behavior seen in the overall phase diagram. An important characteristics of the models is that the orbitals are kinetically coupled -- i.e. hybridized -- to each other, which makes the orbital-selective Mott phase especially nontrivial. A U(1) slave-spin method was used to analyze the model with nonzero orbital-level splittings. Here we develop a Landau free-energy functional to shed further light on this issue. We put the microscopic analysis from the U(1) slave-spin approach in this perspective, and show that the intersite spin correlations are crucial to the renormalization of the bare hybridization amplitude towards zero and the concomitant realization of the orbital-selective Mott transition. Based on this insight, we discuss additional ways to study the orbital-selective Mott physics from a dynamical competition between the interorbital hybridization and collective spin correlations. Our results demonstrate the robustness of the orbital-selective Mott phase in the multiorbital models appropriate for the iron-based superconductors.Comment: 10 pages, 2 figure

Orbital-selective Mott Phase in Multiorbital Models for Alkaline Iron Selenides K(1-x)Fe(2-y)Se2

We study a multiorbital model for the alkaline iron selenides K(1-x)Fe(2-y)Se2 using a slave-spin method. With or without ordered vacancies, we identify a metal-to-Mott-insulator transition at the commensurate filling of six 3d electrons per iron ion. For Hund's couplings beyond a threshold value, this occurs via an intermediate orbital-selective Mott phase, in which the 3d xy orbital is Mott localized while the other 3d orbitals remain itinerant. This phase is still stabilized over a range of carrier dopings. Our results lead to an overall phase diagram for the alkaline iron selenides, which provides a unified framework to understand the interplay between the strength of vacancy order and carrier doping. In this phase diagram, the orbital-selective Mott phase provides a natural link between the superconducting K(1-x)Fe(2-y)Se2 and its Mott-insulating parent compound.Comment: 6 pages, 5 figures, including supplementary informatio