Consequences of Orbital Selectivity for Magnetism and Superconductivity in Fe-based Superconductors

Recently, it has been observed that electronic correlations in iron pnictides and chalcogenides affect electrons in different d-orbitals quite differently. The resulting reduced coherence of the quasiparticle states has consequences for the normal state properties and affects the superconducting state. The renormalization of the d_xy orbital is known to be largest, thus its quasiparticle weight smallest. In the Fe based systems, this leads to a reduction of Néel type (π,π) magnetic fluctuations and makes stripe fluctuations relatively stronger. Within a modified spin-fluctuation pairing theory, this makes the sign-changing s-wave state more competitive. In this work, we investigate these effects of orbital selectivity with a focus on the FeSe system, which allows us to study the effect of nematicity due to the breaking of tetragonal symmetry without magnetic order at low temperatures. Consequences include different renormalization of the d_yz and d_xz orbital states, leading to an anisotropic superconducting order parameter and enhancements of the (π,0) magnetic fluctuations, an effect seen in neutron scattering experiments. The strongest effects of the reduced coherence are observed in the KFe₂As₂, system where we discuss implications for the superconducting order parameter.