Orbital-spin ordering in the striped antiferromagnetic state of iron-based superconductors

The magnetic properties of iron-arsenides are investigated using the LSDA+U approach. In addition to one high moment state, we find that a positive U also produces two low moment states with m~0.4 $\mu_{B}$ and m~1.0 $\mu_{B}$. The electronic structures indicate that the low moment states originate in the strong orbital hybridization between antiferromagnetic Fe neighbors. Therefore, the geometry of the FeAs4 tetrahedron is crucial to the low moment states, which is the key to understand the negative pressure coefficient of Tc in LiFeAs. Finally, our theory suggests that the superconducting phase is an orbital-spin ordered state, where the orbital spin-moments cancel each other.