Ising and Spin orders in Iron-Based Superconductors

Abstract

Motivated by recent neutron scattering experiments, we study the ordering of spins in the iron-based superconductors La(O\(_{1-x}\)F\(_x\))FeAs, assuming them in proximity to a Mott insulator in the phase diagram. The ground state of the parent system with x = 0 is a spin density wave with ordering wave vector \(\vec{Q}\) = (0, \(\pi\)) or (\(\pi\), 0). Upon raising the temperature, we find the system to restore SU(2) symmetry, while an Ising symmetry remains broken, explaining the experimentally observed lattice distortion to a monoclinic crystal structure. Upon further temperature increase, the spins finally disorder at a second transition. The phase transition driven by doping with charge carriers similarly splits into an O(3) transition, and an Ising transition with z = 3 at larger doping.