Interplay of orbital ordering and magnetism in the parent compounds of the iron pnictides

The neutron scattering experiments on the parent compounds of the 122 family of the iron pnictide superconductors show a pronounced orhthorhombic anisotropy in the spin wave spectra [1], also observed in resistivity measurements on detwinned crystals [2,3]. Orbital ordering of the d$_{xz}$ and d$_{yz}$ orbitals may be a possible explanation for this behaviour [4]. In this work, we establish the effect of orbital ordering on the magnetism and study their interplay through a combination of the first-principles band theory calculations and phenomenological analysis in the framework of the Landau theory. This enables us to establish the coupling between the orbital and magnetic degrees of freedom in these materials. Consequences for the symmetry of the superconducting order parameter are discussed.\\[4pt] [1] J. Zhao et al., Nature Phys. {\bf 5}, 555 (2009) \\[0pt] [2] M.A. Tanatar at al., Phys. Rev. B {\bf 81}, 184508 (2010).\\[0pt] [3] J.-H. Chu et al., Science {\bf 329}, 824 (2010). \\[0pt] [4] C.-C. Chen et al., Phys. Rev. B {\bf 82}, 100504(R) (2010).