Itinerant Double-Q Spin-Density Wave in Iron Arsenide Superconductors*

The recent observation of a tetragonal magnetic ($C_4$) phase in hole-doped iron arsenide superconductors has provided evidence of a magnetic origin for the electronic nematicity in the $C_2$ phase of these compounds. Now, M\"ossbauer data shows that the new phase also establishes the itinerant character of the antiferromagnetism of these materials and the primary role played by magnetic over orbital degrees of freedom. Neutron diffraction had shown that the magnetic order in the $C_4$ phase was compatible with a double-Q structure arising from a collinear spin-density wave along both the X and Y directions simultaneously. The coherent superposition of the two modulations produces a non-uniform magnetic structure, in which the spin amplitudes vanish on half of the sites and double on the others, a uniquely itinerant effect that is incompatible with local moment magnetism. M\"ossbauer spectra in the $C_4$ phase confirm this double-Q structure, with 50\% of the spectral weight in a zero-moment peak and 50\% with double the magnetic splitting seen in the $C_2$ phase.