Electronic correlations in Hund metals

Abstract

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...

Magnetic interactions in iron superconductors: A review

“…Hund's coupling was found to be responsible for the electronic mass enhancement [39,42]. At large J H the correlated metal, in the following Hund metal, is built from high-spin states with small overlap with single particle states [42,43,179,180], suppressing Z.…”

“…However, in the last years it is becoming clear that Hund's coupling plays a key role on the correlations of these materials and other multi-orbital systems [39,42,43,107,110,118,[177][178][179][180][181]. [42,43,[177][178][179][180]. Soon after the discovery of superconductivity in doped LaFeAsO, LDA+DMFT calculations [39,42,100] determined this parent compound to be a correlated metal far from the Mott transition and not well described by either atomic physics or band theory [100].…”

“…The simplest generalisation of these models allows the coexistence of itinerant weakly correlated orbitals and strongly localised ones. However, in the last years it is becoming clear that Hund's coupling plays a key role on the correlations of these materials and other multi-orbital systems [39,42,43,107,110,118,[177][178][179][180][181]. [42,43,[177][178][179][180].…”

“…But the complexity did not end here, and the effect of correlations in multi-orbital systems has become a research field on its own right. Hund's coupling is one of the main characters of the emergent story, leading to a correlated Hund's metal, [42,43] with a suppressed but non-zero quasiparticle weight. Different degrees of correlation for the orbitals (orbital differentiation), as observed in ARPES experiments [41], adds up to this richness.…”

Temperature and doping dependence of normal state spectral properties in a two-orbital model for ferropnictides

“…Our approach could also be useful to address the nature of correlations in Hund metals and its relationship with Mott-physics in other multi-orbital systems 86 , problems we plan to study in the future. As mentioned in section II B, for the model Hamiltonian given by Eqs.…”