Magnetism and charge dynamics in iron pnictides

Unconventional superconductivity occurs in close proximity to a magnetically ordered state in many materials(1,2). Uncovering the character of the proximate magnetic phase is a crucial step towards understanding the mechanism of superconductivity. Unlike the case in the cuprate superconductors, the nature of the magnetism and its underlying electronic state in the iron pnictide superconductors(3) is still not well understood. Many low-energy probes such as transport(4), scanning tunnelling microscopy(5) and angle-resolved photoemission spectroscopy(6) have measured strong in-plane anisotropy of the electronic states, but there is no consensus on its physical origin. Using a first-principles theoretical approach, we modelled the magnetic state of the BaFe2As2 parent compound and obtained the magnetic moment, optical conductivity and anisotropy of the electronic states, all in excellent agreement with experiments. We demonstrate that energy-dependent spin and orbital polarizations are essential features of the magnetic state in iron pnictides. Although the spin polarization is enhanced at high energy, the orbital polarization is strong only at low energy. A gain of Hund's coupling energy rather than Hubbard repulsion energy compensates the loss in kinetic energy, thereby stabilizing the low-temperature magnetic phase.