Arsenic-bridged antiferromagnetic superexchange interactions in LaFeAsO

Abstract

From first-principles calculations, we have studied the electronic and magnetic structures of the ground state of LaOFeAs. The Fe spins are found to be collinear antiferromagnetic ordered, resulting from the interplay between the strong nearest and next-nearest neighbor superexchange antiferromagnetic interactions. The structure transition observed by neutron scattering is shown to be magnetically driven. Our study suggests that the antiferromagnetic fluctuation plays an important role in the Fe-based superconductors. This sheds light on the understanding of the pairing mechanism in these materials. PACS numbers: 74.25.Jb, 71.18.+y, 74.25.Ha, Recently an iron-based material LaOFeAs was reported to show superconductivity with a transition temperature T c ∼ 26K by partial substitution of O with F atoms [1]. Soon after, other families of Fe-As oxyarsenides with La replaced by Sm[2], Ce[3], Pr[4] and other rare earth elements were found superconducting with T c more than 50K. Like cuprates, these iron arsenides have a layered structure. The superconducting pairing is believed to happen in the iron-based FeAs layers. The high transition temperature and the preliminary band structure calculation suggests that the superconductivity in these Fe-arsenide superconductors is not mediated by electronphonon interaction. It is commonly believed that the understanding of electronic structures of the parent compound LaOFeAs is the key to determine the underlying mechanism to make it superconducting upon doping.The early band structure calculations suggested that the pure LaOFeAs compound is a nonmagnetic metal but with strong ferromagnetic or antiferromangtic (AFM) instability [5,6,7]. Later, it was found that the antiferromagnetically ordered state [8,9] has a lower energy than the nonmagnetic one, probably due to the Fermi surface nesting [8]. Dong et al.[10] predicted that the AFM state should form a collinear-striped structure by breaking the rotational symmetry. This collinear ordered AFM state has indeed been observed by the neutron scattering experiment [11,12]. Furthermore, the neutron scattering measurement found that there is a structure transition with a monoclinic lattice distortion at ∼ 150K and the collinear order is formed about 15∼20K below this transition temperature. Without this distortion, the square AFM order induced purely by the Fermi surface nesting is expected to be more stable since there are two orthogonal but equivalent nesting directions (π, π) and (π, −π), which can lower the energy of the ground state by keeping its rotational symmetry [8].In this paper, we report the theoretical result on the electronic and magnetic structures of the ground state of LaOFeAs obtained from first-principles band struc-ture calculations. We find that there are strong nearest and next-nearest neighbor superexchange interactions in this material (similar conclusion was obtained by Yildirim [13]). The nearest and next nearest neighbor superexchange interactions have almost the same amplitude within error of calculation....

Electronic structures of ternary iron arsenides AFe2As2 (A = Ba, Ca, or Sr)

“…This indicates that the crystal field splitting imposed by As atoms is very small and the Fe 3d-orbitals hybridize strongly with each other. We can see that this is a universal feature for all iron pnictides, as we first found in LaFeAsO [19]. The strong polarization of Fe magnetic moments is thus due to the Hund's rule coupling.…”

“…This leads to a structural transition from tetragonal space group I4/mmm to orthorhombic space group F mmm, similar to that in LaFeAsO [19] and observed by the neutron scattering [10]. This lattice relaxation is energetically favorable because the direct ferromagnetic exchange favors a shorter Fe-Fe separation while The symmetrical k-points in the Brillouin zone are referred to Fig.…”

“…In iron-pnictide semimetals, the effective moment of a Fe ion (in a 2+ valency) will be reduced by its hybridization with other atoms and by the Coulomb screening of itinerant electrons. However, it will remain finite if the Hund's rule coupling and the on-site Coulomb repulsion is strong enough in comparison with the hybridization and other screening effects, as we found in LaFeAsO [19].…”

“…This antiferromagnetic interaction between the nearest neighboring FeAs layers is significantly larger than that in LaFeAsO [19]. In Ref.…”

“…This can help us to understand the mechanism or the interactions that drive the magnetic phase transition and the related structural transition [19].…”