| Materials with strong electronic correlations have long resisted abinitio modeling due to their complexity arising from non-perturbative strength of the interaction. The Dynamical Mean Field Theory in combination with the Density Functional Theory and the Continuous Time Quantum Monte Carlo impurity solver has recently changed this position, and enabled detailed modeling of the electronic structure of complex heavy fermions, transition metal oxides and iron pnictides and chalchogenides. The important issues which we address are the strength of the correlations and the nature of the magnetic state in the parent compounds of iron superconductors. Unlike the antiferromagnetic order in cuprates, which is well described by the spin Heisenberg model, the nature of magnetism and its underlying electronic state in the iron pnictide superconductors is more subtle. We modeled the magnetic state of the iron-pnictide parent compound, and obtained magnetic moment, optical conductivity, and the anisotropy of the electronic states, all in excellent agreement with experiment. We demonstrate that energy dependent spin and orbital polarizations are essential features of the magnetic state in iron superconductors. While the spin polarization is enhanced at high energy, the orbital polarization is strong only at low energy. The magnetic phase arises from the paramagnetic phase by a large gain of the Hund's rule coupling energy and a smaller loss of kinetic energy, indicating that iron pnictides represent a new class of compounds where the nature of magnetism is intermediate between the spin density wave of almost independent particles, and the antiferromagnetic state of local moments. |
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