Ising and Spin orders in Iron-based Superconductors

TL;DR

This paper investigates spin ordering in iron-based superconductors, revealing a sequence of magnetic and structural phase transitions driven by temperature and doping, consistent with experimental observations.

Contribution

It provides a theoretical framework explaining the sequence of magnetic and structural phase transitions in iron-based superconductors near a Mott insulator.

Findings

Parent compound exhibits spin density wave order with specific wave vectors.

Temperature increase restores SU(2) symmetry but breaks Ising symmetry, causing lattice distortion.

Doping induces separate magnetic and Ising phase transitions.

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 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.