Influence of Spin-Orbit Coupling in the Iron-Based Superconductors

In addition to the remarkable resilience of unconventional superconductivity, the iron-based superconductors (FeSCs) are characterized by a broad phenomenology across their phase diagram. This diversity has compounded the challenge of defining a minimal model for these complex materials. We explore the essential role of spin-orbit coupling (SOC) in the FeSCs via circularly-polarized spin- and angle-resolved photoemission spectroscopy (CPS-ARPES), a technique which provides direct access to both spin and orbital degrees of freedom [1]. Applying this technique to both LiFeAs and FeSe, we establish a strong momentum dependence of the spin-orbital entanglement near the Fermi surface imbued by SOC. Influenced strongly by the details of the crystal field, the relevance of SOC to the states near the Fermi level, and consequently to superconductivity and magnetism is found to be strongly material-dependent. Furthermore, in the case of hole-like carriers in FeSe, SOC is observed to be of comparable strength to orbital order, allowing spin-orbit mixing to persist into the nematic phase.

[1] R. P. Day et al, Phys. Rev. Lett. 127, 076401 (2018)