Spin-Orbit Coupling and Gapped Magnetic Excitations in Iron-Based Superconductors

Abstract

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.