Properties of the multiorbital Hubbard models for the iron-based superconductors

Abstract

A brief review of the main properties of multiorbital Hubbard models for the Fe-based superconductors is presented. The emphasis is on the results obtained by our group at the University of Tennessee and Oak Ridge National Laboratory, Tennessee, USA, but results by several other groups are also discussed. The models studied here have two, three, and five orbitals, and they are analyzed using a variety of computational and mean-field approximations. A "physical region" where the properties of the models are in qualitative agreement with neutron scattering, photoemission, and transport results is revealed. A variety of interesting open questions are briefly discussed such as: what are the dominant pairing tendencies in Hubbard models? Can pairing occur in an interorbital channel? Are nesting effects of fundamental relevance in the pnictides or approaches based on local moments are more important? What kind of magnetic states are found in the presence of iron vacancies? Can charge stripes exist in iron-based superconductors? Why is transport in the pnictides anisotropic? The discussion of results includes the description of these and other open problems in this fascinating area of research.

Influence of s± symmetry on unconventional superconductivity in pnictides above the Pauli limit – two-band model study

“…is an artifact of the two-orbital approximation. The true BZ can be obtained by folding the effective BZ -the result of such downfolding is given in Figure 1c [62][63][64]. It is evident that the FS obtained in this way are in qualitative agreement with the result of LDA calculations (only the third, less relevant, small hole-like pocket at the center is absent) [29][30][31][32][33][34].…”

Magnetic orders in pnictide superconductors: the effect of biquadratic interaction

“…Both itinerant electrons and localized spins are included in the SF model, and the calculations have demonstrated the great success of this model for explaining complicated physics in these typical correlated electron systems. For example, not only the magnetic and spectral properties of the ground state [19], but also the in-plane resistivity anisotropy observed in BaFe 2 As 2 [20,21] can be qualitatively reproduced in MC simulations using the three-orbital SF model [22]. In addition, the lattice degrees of freedom have been introduced into this model to study the nematic state of pnictides [23].…”

Normal state electronic properties ofLaO1−xFxQuerales-Flores1, Ventura2, Citro3 et al. 2016Physica B: Condensed Matter2060View full textAdd to dashboardCite

“…We find that the effect of increasing V is opposite to that of increasing U , effectively tending to screen it: the main effect of increasing V is a monotonous decrease of the spectral weight of the DOS peak located slightly below the Fermi level, evolving the resulting DOS towards the non-interacting one (U = 0 = V ). Therefore, due to the differences in the relevant non-interacting effective bandstructure of LaO 1−x F x BiS 2 [13] and ferropnictides [31], and the corresponding differences in location of the Fermi levels, we predict a striking difference in the effect of the inter-orbital electron correlation V : which is relevant in BiS 2 -based superconductors, while largely irrelevant for ferropnictides [30,36,37]. This is a fact already pointed out in connection with other correlated systems, [38,39], where the presence of two or more bands close in energy to E F , with relatively low density of electrons ( as would be the case for LaO 1−x F x BiS 2 [13]), would amplify the effect of inter-orbital electron correlations.…”