Double-Q spin-density wave in iron arsenide superconductors

TL;DR

This paper provides conclusive evidence for a double-Q spin-density wave in iron arsenide superconductors, revealing a novel magnetic ground state with non-uniform magnetization and emphasizing the itinerant nature of their magnetism.

Contribution

It introduces Mossbauer data confirming a double-Q magnetic structure with non-magnetic iron sites, advancing understanding of the magnetic ground state in these superconductors.

Findings

Half of the iron sites are non-magnetic in the tetragonal phase.

The magnetic state results from interference between two spin-density waves.

The magnetism is primarily itinerant, not localized.

Abstract

Elucidating the nature of the magnetic ground state of iron-based superconductors is of paramount importance in unveiling the mechanism behind their high temperature superconductivity. Until recently, it was thought that superconductivity emerges only from an orthorhombic antiferromagnetic stripe phase, which can in principle be described in terms of either localized or itinerant spins. However, we recently reported that tetragonal symmetry is restored inside the magnetically ordered state of a hole-doped BaFe2As2. This observation was interpreted as indirect evidence of a new double-Q magnetic structure, but alternative models of orbital order could not be ruled out. Here, we present Mossbauer data that show unambiguously that half of the iron sites in this tetragonal phase are non-magnetic, establishing conclusively the existence of a novel magnetic ground state with a non-uniform magnetization that is inconsistent with localized spins. We show that this state is naturally explained as the interference between two spin-density waves, demonstrating the itinerant character of the magnetism of these materials and the primary role played by magnetic over orbital degrees of freedom.