Magnetism and its microscopic origin in iron-based high-temperature superconductors

High-temperature superconductivity in the iron-based materials emerges from, or sometimes coexists with, their metallic or insulating parent compound states. This is surprising since these undoped states display dramatically different antiferromagnetic (AF) spin arrangements and N$\rm \acute{e}$el temperatures. Although there is general consensus that magnetic interactions are important for superconductivity, much is still unknown concerning the microscopic origin of the magnetic states. In this review, progress in this area is summarized, focusing on recent experimental and theoretical results and discussing their microscopic implications. It is concluded that the parent compounds are in a state that is more complex than implied by a simple Fermi surface nesting scenario, and a dual description including both itinerant and localized degrees of freedom is needed to properly describe these fascinating materials.Comment: 14 pages, 4 figures, Review article, accepted for publication in Nature Physic

Nature of magnetic excitations in superconducting BaFe1.9Ni0.1As2

Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors1–3, a central question has been whether magnetism in these materials arises from weakly correlated electrons4,5, as in the case of spin density wave in pure chromium6, requires strong electron correlations7, or can even be described in terms of localized electrons8,9 such as the AF insulating state of copper oxides10. Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe1.9Ni0.1As2 (Tc = 20 K), which allows us to obtain the size of the fluctuating magnetic moment 〈m2〉, and its energy distribution11,12. We find that superconducting BaFe1.9Ni0.1As2 and AF BaFe2As2 (ref. 13) both have fluctuating magnetic moments 〈m2 〉 ≈ 3.2 µ2B pe

Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides

In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, superconductivity occurs when electrons form coherent Cooper pairs below the superconducting transition temperature Tc. Although the kinetic energy of paired electrons increases in the superconducting state relative to the normal state, the reduction in the ion lattice energy is sufficient to give the superconducting condensation energy. For iron pnictide superconductors derived from electron or hole doping of their antiferromagnetic (AF) parent compounds, the microscopic origin for supercnductivity is unclear. Here we use neutron scattering to show that high-Tc superconductivity only occurs for iron pnictides with low-energy itinerant electron-spin excitation coupling and high energy spin excitations. Since our absolute spin susceptibility measurements for optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy, we conclude that the presence of both high-energy spin excitations giving rise to a large magnetic exchange coupling J and low-energy spin excitations coupled to the itinerant electrons is essential for high-Tc superconductivity in iron pnictides.Comment: 9 pages, 4 figures in the main article; 11 pages, 13 figures in the supplementary material