Orbital ordering induces structural phase transition and the resistivity anomaly in iron pnictides

We attribute the structural phase transition (SPT) in the parent compounds of the iron pnictides to orbital ordering. Due to the anisotropy of the dxz and dyz orbitals in the $xy$ plane, a ferro-orbital ordering makes the orthorhombic structure more energetically favorable, thus inducing the SPT. In this orbital-ordered system, the sites with orbitals that do not order have higher energies. Scattering of the itinerant electrons by these localized two-level systems causes a resistivity anomaly upon the onset of the SPT. The proposed orbital ordering also leads to stripelike antiferromagnetism and anisotropy of the magnetic exchange couplings. This model is quantitatively consistent with available experimental observations.