Orbital loop currents in iron-based superconductors

We show that the antiferromagnetic state commonly observed in the phase diagrams of the iron-based superconductors necessarily triggers loop currents characterized by charge transfer between different Fe $3d$ orbitals. This effect is rooted on the glide-plane symmetry of these materials and on the existence of an atomic spin-orbit coupling that couples states at the $X$ and $Y$ points of the 1-Fe Brillouin zone. In the particular case in which the magnetic moments are aligned parallel to the magnetic ordering vector direction which is the moment configuration most commonly found in the iron-based superconductors these loop currents involve the $d_{xy}$ orbital and either the $d_{yz}$ orbital (if the moments point along the $y$ axis) or the $d_{xz}$ orbitals (if the moments point along the $x$ axis). We show that the two main manifestations of the orbital loop currents are the emergence of magnetic moments in the pnictide/chalcogen site and an orbital-selective band splitting in the magnetically ordered state, both of which could be detected experimentally. Our results highlight the unique intertwining between orbital and spin degrees of freedom in the iron-based superconductors, and reveal the emergence of an unusual correlated phase that may impact the normal state and superconducting properties of these materials.