Spin-resolved photoemission spectroscopy of FeSe thin films

The iron-based superconductor FeSe hosts rich intertwined behavior including superconductivity, nematic order, and strong magnetic fluctuations. Angle resolved photoemission has proven to be an invaluable tool to probe these intertwined states, revealing the effects of nematic order on the electronic structure and probing the superconducting gap structure. Here we report spin and angle resolved photoemission spectroscopy (SR-ARPES) measurements of FeSe thin films grown via molecular beam epitaxy. In spite of the fact that bulk FeSe preserves both time reversal and inversion symmetry which ensures fully spin degenerate bands, we detect observable spin asymmetry in the emitted photoelectrons. This effect is present even in thicker samples (~10 unit cells) where interfacial effects and associated inversion symmetry breaking from the film-substrate interface are presumed to be negligible. The spin texture of the photoelectrons is fully in the 2D FeSe plane and evolves nontrivially as a function of energy and momentum, with the measured spin switching sign across the Γ point and exhibiting spin-momentum locking. In this talk I will outline the experimental phenomenology of this effect and discuss its implications and possible origins.