We examine the magnetic phase diagram of iron pnictides using a five-band model. For the intermediate values of the interaction expected to hold in the iron pnictides, we find a metallic low moment state characterized by antiparallel orbital magnetic moments. The anisotropy of the interorbital hopping amplitudes is the key to understanding this low moment state. This state accounts for the small magnetization measured in undoped iron pnictides and leads to the strong exchange anisotropy found in neutron experiments. Orbital ordering is concomitant with magnetism and produces the large zx orbital weight seen at Γ in photoemission experiments.

We propose a five-band tight-binding model for the Fe-As layers of iron pnictides with the hopping amplitudes calculated within the Slater-Koster framework. The band structure found in DFT, including the orbital content of the bands, is well reproduced using only four fitting parameters to determine all the hopping amplitudes. The model allows to study the changes in the electronic structure caused by a modification of the angle α formed by the Fe-As bonds and the Fe-plane and recovers the phenomenology previously discussed in the literature. We also find that changes in α modify the shape and orbital content of the Fermi surface sheets.

Recent experiments on iron pnictides have uncovered a large in-plane resistivity anisotropy with a surprising result: The system conducts better in the antiferromagnetic x direction than in the ferromagnetic y direction. We address this problem by calculating the ratio of the Drude weight along the x and y directions, D(x)/D(y), for the mean-field Q=(π,0) magnetic phase diagram of a five-band model for the undoped pnictides. We find that D(x)/D(y) ranges between 0.2

To clarify the nature of correlations in Hund metals and its relationship with Mott physics we analyze the electronic correlations in multiorbital systems as a function of intraorbital interaction U , Hund's coupling JH and electronic filling n. We show that the main process behind the enhancement of correlations in Hund metals is the suppression of the double-occupancy of a given orbital, as it also happens in the Mott-insulator at half-filling. However, contrary to what happens in Mott correlated states the reduction of the quasiparticle weight Z with JH can happen on spite of increasing charge fluctuations. Therefore, in Hund metals the quasiparticle weight and the mass enhancement are not good measurements of the charge localization. Using simple energetic arguments we explain why the spin polarization induced by Hund's coupling produces orbital decoupling. We also discuss how the behavior at moderate interactions, with correlations controlled by the atomic spin polarization, changes at large U and JH due to the proximity to a Mott insulating state.PACS numbers: 74.70. Xa, 74.10.Fd, 71.30.+h The Mott transition is one of the most dramatic manifestations of electronic correlations [1,2]. In the single orbital Hubbard model at half-filling the system becomes insulating at a critical interaction U c to avoid the cost of doubly occupying the orbital. Away from half-filling metallicity is recovered. Nevertheless atomic configurations involving double occupancy are avoided inducing strong correlations between the electrons. Charge fluctuations are suppressed and bad metallicity is observed.In multiorbital systems the Mott transition happens not only at half-filling but at all integer fillings [3]. The crucial role of Hund's coupling J H on electronic correlations has been recognized only recently [4][5][6][7][8][9][10][11][12][13][14]. J H modifies U c in a doping dependent way [4,8] and promotes bad metallic behavior in a wide range of parameters [7,9].Within the context of iron superconductors, which accomodate 6 electrons in 5 orbitals when undoped, the term Hund metal was coined to name the correlated metallic state induced by Hund's coupling at moderate interaction U [15]. Originally Hund metals were described as strongly correlated but itinerant systems which are not in close proximity to a Mott insulating state and have physical properties distinctly different from doped Mott insulators [10]. On the other hand, a number of authors [16][17][18][19][20][21][22], have described iron superconductors as doped Mott insulators due to the doping dependence of correlations observed: there is both experimental and theoretical evidence of an enhancement of correlations with hole-doping as the half-filling Mott insulator, with 5 electrons in 5 orbitals, is approached [16][17][18][19][20][21][22][23][24][25][26].Orbital dependent correlations, named orbital differentiation, have been observed in some iron superconductors [16,19,21,23,[26][27][28] and are known to play an important role in ruthenates [29]. It has been...

We analyze the metallic (π, 0) antiferromagnetic state of a five-orbital model for iron superconductors. We find that with increasing interactions the system does not evolve trivially from the pure itinerant to the pure localized regime. Instead we find a region with a strong orbital differentiation between xy and yz, which are half-filled gapped states at the Fermi level, and itinerant zx, 3z 2 − r 2 and x 2 − y 2 . We argue that orbital ordering between the yz and zx orbitals arises as a consequence of the interplay of the exchange energy in the antiferromagnetic x direction and the kinetic energy gained by the itinerant orbitals along the ferromagnetic y direction with an overall dominance of the kinetic energy gain. We indicate that iron superconductors may be close to the boundary between the itinerant and the orbital differentiated regimes and that it could be possible to cross this boundary with doping.There is a strong interrelation between the orbital degree of freedom, the magnetization, and the lattice structure in the Fe-superconductors. Unveiling the nature of these connections would define the landscape from which superconductivity emerges in these materials. One of the important issues is the determination of the strength of the interactions. Unlike the cuprates, which are antiferromagnetic Mott insulators when undoped, the Fesuperconductors are antiferromagnetic metals, highlighting the relevance of the itinerancy of the conduction electrons. Undoped materials have to accomodate six electrons in the five Fe-d orbitals, with an average filling of 1.2, close to the one of doped Mott insulators. 1-3 The itinerant 4-7 versus localized 8,9 origin of the magnetization has been discussed since the discovery of superconductivity in these systems.On the other hand there is increasing evidence for orbital differentiation and a possible coexistence of itinerant and localized electrons. 10-13 Angle Resolved Photoemission Spectroscopy (ARPES) measurements report different renormalization values for the various bands close to the Fermi energy depending on their orbital character. 14,15 Similar qualitative conclusions may be inferred from Dynamical Mean Field Theory (DMFT) and slave-spin calculations. 1,[16][17][18] The possible role of orbital ordering in the magnetism is of present interest. The current debate is focused on whether it is the leading instability driving the magnetism 19 or it appears as a consequence of the magnetic ordering, 20-23 as well as its possible relation to the observed anisotropic properties. 17,[24][25][26][27][28][29][30][31][32][33][34][35][36] In particular, the resistivity in the (π, 0) antiferromagnetic state was measured to be larger in the ferromagnetic y-direction than in the antiferromagnetic x-direction 24,25 with a change in sign upon hole doping. 37 In order to shed light on the role of the different orbitals on the magnetic state of Fe-superconductors, we analyze the metallic (π, 0) antiferromagnetic state as a function of the interactions treated within mean-field.Clos...

The properties of nanoscopic superconducting structures fabricated with a scanning tunnelling microscope are reviewed, with emphasis on the effects of high magnetic fields. These systems include the smallest superconducting junctions which can be fabricated, and they are a unique laboratory where to study superconductivity under extreme conditions. The review covers a variety of recent experimental results on these systems, highlighting their unusual transport properties, and theoretical models developed for their understanding.

High temperature superconductivity in iron pnictides and chalcogenides emerges when a magnetic phase is suppressed. The multi-orbital character and the strength of correlations underlie this complex phenomenology, involving magnetic softness and anisotropies, with Hund's coupling playing an important role. We review here the different theoretical approaches used to describe the magnetic interactions in these systems. We show that taking into account the orbital degree of freedom allows us to unify in a single phase diagram the main mechanisms proposed to explain the (π, 0) order in iron pnictides: the nesting-driven, the exchange between localized spins, and the Hund induced magnetic state with orbital differentiation. Comparison of theoretical estimates and experimental results helps locate the Fe superconductors in the phase diagram. In addition, orbital physics is crucial to address the magnetic softness, the doping dependent properties, and the anisotropies.

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