The form and the origin of the orbital ordering in the electronic   nematic phase of the Iron-based superconductors
  We investigated the form of the orbital ordering in the electronic nematic phase of the Iron-based superconductors by applying a group theoretical analysis on a realistic five-band model. We find the orbital order can be either of the inter-orbital s-wave form or the intra-orbital d-wave form. From the comparison with existing ARPES measurements of band splitting, we find the orbital ordering in the 122 system is dominated by an intra-orbital d-wave component, while that in the 111 system is dominated by an inter-orbital s-wave component. We find both forms of orbital order are strongly entangled with the nematicity in the spin correlation of the system. The condensation energy of the magnetic ordered phase is found to be significantly improved(by more than 20 percents) when the degeneracy between the $(\pi,0)$ and $(0,\pi)$ ordering pattern is lifted by the orbital order. We argue there should be large difference in both the scattering rate and the size of the possible pseudogap on the electron pocket around the $\mathrm{X}=(\pi,0)$ and $\mathrm{Y}=(0,\pi)$ point in the electronic nematic phase. We propose this as a possible origin for the observed nematicity in resistivity measurements. 
Yuehua Su, Haijun Liao and Tao Li
cond-mat.supr-con