Spin-orbit coupling in iron-based superconductors gives rise to anisotropy in spin space for the
magnetism, which may in turn help establish a rich variety of electronic phases. In this talk, I will
present our latest progress in using inelastic neutron scattering (INS) to determine low-energy spin
excitations in Sr(1-x)Na(x)Fe2As2 and FeSe(1-x)S(x) superconductors. Upon cooling Sr(1-x)Na(x)
Fe2As2 into its double-Q tetragonal magnetic phase, a relatively large spin excitation gap develops,
which we attribute to pronounced spin-space anisotropy that echoes with the spin reorientation
transition upon entering this phase. The existence of such a gap appears to preclude the development
of any spin resonant mode in the superconducting phase at lower temperatures, hence it explains
why the double-Q phase strongly suppresses the superconductivity. In FeSe(1-x)S(x) which exhibits
nematicity but no magnetic order at low temperatures, we show that the spin excitations are also
gapped at low energies, which by itself is consistent with the notion that the spins are in a "quantum
paramagnetic" state with entanglement established along one of the in-plane Fe-Fe directions.
However, our spin-polarized INS experiments further reveal that the excitations above the gap are
strongly anisotropic in spin space. We expect our results to help unveil the intriguing interplay
between the spin and the orbital physics in the iron-based superconductors.
Spin-Orbit Coupling and Gapped Magnetic Excitations in Iron-Based Superconductors.
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