# On the multi-orbital band structure and itinerant magnetism of iron-based superconductors - Condensed Matter > Superconductivity

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Abstract: This paper explains the multi-orbital band structures and itinerant magnetismof the iron-pnictide and chalcogenides. We first describe the generic bandstructure of an isolated FeAs layer. Use of its Abelian glide-mirror groupallows us to reduce the primitive cell to one FeAs unit. Fromdensity-functional theory, we generate the set of eight Fe $d$ and As $p$localized Wannier functions for LaOFeAs and their tight-binding TBHamiltonian, $hk$. We discuss the topology of the bands, i.e. allowed andavoided crossings, the origin of the d6 pseudogap, as well as the role of theAs $p$ orbitals and the elongation of the FeAs$ {4}$ tetrahedron. We thencouple the layers, mainly via interlayer hopping between As $p {z}$ orbitals,and give the formalism for simple and body-centered tetragonal stackings. Thisallows us to explain the material-specific 3D band structures. Due to the highsymmetry, several level inversions take place as functions of $k {z}$ orpressure, resulting in linear band dispersions Dirac cones. The underlyingsymmetry elements are, however, easily broken, so that the Dirac points are notprotected, nor pinned to the Fermi level. From the paramagnetic TB Hamiltonian,we form the band structures for spin spirals with wavevector $q$ by coupling$hk$ and $h k+q$. The band structure for stripe order is studied as afunction of the exchange potential, $\Delta$, using Stoner theory. Gapping ofthe Fermi surface FS for small $\Delta $ requires matching of FS dimensionsnesting and $d$-orbital characters. The origin of the propeller-shaped FS isexplained. Finally, we express the magnetic energy as the sum overband-structure energies, which enables us to understand to what extent themagnetic energies might be described by a Heisenberg Hamiltonian, and theinterplay between the magnetic moment and the elongation of the FeAs4tetrahedron.

Author: ** Ole Krogh Andersen, Lilia Boeri Max-Planck-Institute for Solid State Research, Germany**

Source: https://arxiv.org/