Preferred Magnetic Excitations in the Iron-Based Sr1-xNaxFe2As2 Superconductor

With spin-orbit coupling, both local-moment magnetism and itinerant electrons are expected to behave anisotropically in spin space, but such effects' influence on the formation of unconventional super-conductivity has been hitherto unexplored. Here, in an iron-based superconductor, Sr1-xNaxFe2As2, we report spectroscopic evidence that itinerant electrons prefer to be assisted by c-axis polarized magnetic excitations in their formation of superconducting Cooper pairs, against the polarization of the local-moment excitations. Our result naturally explains why the superconductivity competes strongly with the tetragonal magnetic phase in this material, and provides a fresh view on how to make a good superconductor out of a magnetic Hund's metal.