Spin-orbital frustrations and anomalous metallic state in iron-pnictide superconductors

TL;DR

This paper develops a strong coupling model incorporating orbital degrees of freedom to explain the anomalous metallic state and magnetic properties of parent iron-pnictide superconductors, revealing complex frustrated spin-orbital interactions.

Contribution

It introduces a Kugel-Khomskii spin-orbital Hamiltonian for Fe^{2+} ions, analyzing its phase diagram and stability of various magnetic and orbital orders, including experimentally relevant spin-stripe states.

Findings

Stable spin-stripe phase over a wide parameter range

Multiple orbital orders breaking in-plane lattice symmetry

Reduced magnetic moments consistent with experiments

Abstract

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 Fe^{2+} ions, we derive a Kugel-Khomskii spin-orbital Hamiltonian for the active t_{2g} 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 of the ordered moment, and point out that the proposed orbital ordering pattern can be measured in resonant X-ray diffraction.