Large local spin-sublattice locking in an iron-based superconductor

We present spin-resolved angle-resolved photoemission (SR-ARPES) measurements of the Fe-based superconductor FeSe, a material which has a crystal structure and electronic ground state which preserve both time reversal and spatial inversion symmetry. In spite of these symmetries, these measurements reveal a nontrivial spin polarization of the electronic energy bands for both monolayer and bulk-like thin films grown by molecular beam epitaxy, as well as bulk single crystals. This spin-texture changes sign across the Γ point respecting time-reversal symmetry and is locked perpendicular to the momentum reminiscent of a Rashba-like mechanism, although no splitting of the bulk bands is observed. We show that this spin-texture is the result of a local spin-sublattice locking driven by spin orbit coupling and the local structural environment of the two inequivalent Fe-Se sublattice units of the unit cell, generating local spin textures of opposite signs in the different sublattices. We furthermore discuss the possible implications of this local inversion symmetry breaking and spin texture for the superconductivity in bulk and monolayer FeSe / SrTiO₃ as well as Fe-based superconductors in general.