Iron-based superconductors offer an opportunity to study the physical processes that are responsible for high-temperature superconductivity. The quantum-mechanical processes in these materials lead to a large number of effects and allows for various stable phases. Theoretically, it seems that investigations are stuck because of a fundamental issue. It is unknown whether the electronic states should be treated in an itinerant approach or using the opposite picture where electrons are almost localized. Guided by recent experiments in the superconducting phase of iron-based superconductors, we discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based superconductors, particularly Fe chalcogenide systems. A key ingredient for an improved itinerant pairing theory is orbital selectivity, i.e. incorporating less coherent quasiparticles occupying specific orbital states into the pairing theory [1]. This approach, based on the Fermi liquid description of a correlated electron system, modifies the usual spin-fluctuation pairing. In detail, pair scattering processes involving more incoherent states are suppressed, which results in orbital selective Cooper pairing of electrons in the remaining states. We show that this paradigm yields remarkably good agreement with the experimentally observed anisotropic gap structures in 3 different materials: Bulk FeSe [2] and monolayer FeSe, as well as LiFeAs. The same paradigm also describes the scattering properties of the electrons in the normal state [3] and yields a spin susceptibility with properties in agreement to recent neutron scattering experiments [4]. All this indicates that orbital selective electrons play a key role in the more strongly correlated iron-based superconductors.

References:
[1] A. Kreisel, Brian M. Andersen, Peter O. Sprau, Andrey Kostin, J.C. Séamus Davis, P. J. Hirschfeld, Phys. Rev. B 95, 174504 (2017) "Orbital selective pairing and gap structures of iron-based superconductors"
[2] Peter O. Sprau, Andrey Kostin, Andreas Kreisel, Anna E. Böhmer, Valentin Taufour, Paul C. Canfield, Shantanu Mukherjee, Peter J. Hirschfeld, Brian M. Andersen, J.C. Séamus Davis, Science, 357, 75 (2017) "Discovery of Orbital-Selective Cooper Pairing in FeSe"
[3] Andrey Kostin, Peter O. Sprau, Andreas Kreisel, Yi Xue Chong, Anna E. Böhmer, Paul C. Canfield, Peter J. Hirschfeld, Brian M. Andersen, J.C. Séamus Davis, arXiv:1802.02266 "Visualizing Orbital-selective Quasiparticle Interference in the Hund's Metal State of FeSe"
[4] A. Kreisel, Brian M. Andersen, P. J. Hirschfeld, arXiv:1807.09482 "Itinerant approach to magnetic neutron scattering of FeSe: effect of orbital selectivity"