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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...

Since their discovery, it has been suggested that pairing in pnictides can be mediated by spin fluctuations between hole and electron bands. In this view, multiband superconductivity would substantially differ from other systems like MgB 2 , where pairing is predominantly intraband. Indeed, interband-dominated pairing leads to the coexistence of bonding and antibonding superconducting channels. Here, we show that this has profound consequences on the nature of the low-energy superconducting collective modes. In particular, the so-called Leggett mode for phase fluctuations is absent in the usual two-band description of pnictides. On the other hand, when also the repulsion between the hole bands is taken into account, a more general three-band description should be used, and a Leggett mode is then allowed. Such a model, which has been proposed for strongly hole-doped 122 compounds, can also admit a low-temperature s + is phase that breaks the time-reversal symmetry. We show that the (quantum and thermal) transition from the ordinary superconductor to the s + is state is accompanied by the vanishing of the mass of Leggett-like phase fluctuations, regardless the specific values of the interaction parameters. This general result can be obtained by means of a generalized construction of the effective action for the collective degrees of freedom that allows us also to deal with the nontrivial case of dominant interband pairing.

We derive the effective action for superconducting fluctuations in a four-band model for pnictides, discussing the emergence of a single critical mode out of a dominant interband pairing mechanism. We then apply our model to calculate the paraconductivity in two-dimensional and layered three-dimensional systems and compare our results with recent resistivity measurements in SmFeAsO(0.8)F(0.2)

We derive the effective action for the collective spin modes in iron-based superconductors. We show that, due to the orbital-selective nature of spin fluctuations, the magnetic and nematic instabilities are controlled by the degrees of orbital nesting between electron and hole pockets. Within a prototypical three-pockets model the hole-electron orbital mismatch is found to boost spin-nematic order. This explains the enhancement of nematic order in FeSe as compared to 122 compounds, and its suppression under pressure, where the emergence of the second hole pocket compensates the orbital mismatch of the three-pockets configuration.

The origin of the nematic state is an important puzzle to be solved in iron pnictides. Iron superconductors are multiorbital systems and these orbitals play an important role at low energy. The singular C4 symmetry of dzx and dyz orbitals has a profound influence at the Fermi surface since the Γ pocket has vortex structure in the orbital space and the X/Y electron pockets have yz/zx components respectively. We propose a low energy theory for the spin-nematic model derived from a multiorbital Hamiltonian. In the standard spin-nematic scenario the ellipticity of the electron pockets is a necessary condition for nematicity. In the present model nematicity is essentially due to the singular C4 symmetry of yz and zx orbitals. By analyzing the (π, 0) spin susceptibility in the nematic phase we find spontaneous generation of orbital splitting extending previous calculations in the magnetic phase. We also find that the (π, 0) spin susceptibility has an intrinsic anisotropic momentum dependence due to the non trivial topology of the Γ pocket.

FeSe is an intriguing iron-based superconductor. It presents an unusual nematic state without magnetism and can be tuned to increase the critical superconducting temperature. Recently it has been observed a noteworthy anisotropy of the superconducting gaps. Its explanation is intimately related to the understanding of the nematic transition itself. Here we show that the spin-nematic scenario driven by orbital-selective spin-fluctuations provides a simple scheme to understand both phenomena. The pairing mediated by anisotropic spin modes is not only orbital selective but also nematic, leading to stronger pair scattering across the hole and X electron pocket. The delicate balance between orbital ordering and nematic pairing points also to a marked kz dependence of the hole-gap anisotropy.

The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments calls for a prominent role of local correlations and place iron superconductors at the entrance of a Hund metal state, the effect of the electronic correlations on the nematic state has been theoretically poorly investigated. In this work we study the nematic phase of iron superconductors accounting for local correlations, including the effect of the Hund's coupling. We show that Hund's physics strongly affects the nematic properties of the system. It severely constraints the precise nature of the feasible orbital-ordered state and induces a differentiation in the effective masses of the zx=yz orbitals in the nematic phase. The latter effect leads to distinctive signatures in different experimental probes, so far overlooked in the interpretation of experiments. As notable examples the splittings between zx and yz bands at Gamma and M points are modified, with important consequences for ARPES measurements.Comment: The title, the text and the figures have been modified with respect to the first version. 8 pages (6 pages + references), 3 pdf figure