We develop an understanding of the anomalous metal state of the parent compounds of recently discovered iron-based superconductors starting from a strong-coupling viewpoint, including orbital degrees of freedom. On the basis of an intermediate-spin (S=1) state for the Fe2+ ions, we derive a Kugel-Khomskii spin-orbital Hamiltonian for the active t2g orbitals. It turns out to be a highly complex model with frustrated spin and orbital interactions. We compute its classical phase diagrams and provide an understanding for the stability of the various phases by investigating its spin-only and orbital-only limits. The experimentally observed spin-stripe state is found to be stable over a wide regime of physical parameters and can be accompanied by three different types of orbital orders. Of these the orbital-ferro and orbital-stripe orders are particularly interesting since they break the in-plane lattice symmetry—a robust feature of the undoped compounds. We compute the magnetic excitation spectra for the effective spin Hamiltonian, observing a strong reduction in the ordered moment, and point out that the proposed orbital ordering pattern can be measured in resonant x-ray diffraction.
|Number of pages||15|
|Journal||Physical review B: Condensed matter and materials physics|
|Publication status||Published - 2009|
Kruger, F., Kumar, S., Zaanen, J., & van den Brink, J. (2009). Spin-orbital frustrations and anomalous metallic state in iron-pnictide superconductors. Physical review B: Condensed matter and materials physics, 79(5), 054504/1-054504/15. https://doi.org/10.1103/PhysRevB.79.054504