Symmetry-required Orbital Selectivity in Monolayer FeSe

Orbital-selective correlations have been observed to play an important role in Fe-based superconductors. Here, in contrast to previous site-local Mott transition-based origins, we present a band-theory-based mechanism for orbital-selective physics in monolayer FeSe, for which only electron pockets appear. Underlying our mechanism is our density functional theory (DFT)-based observation that, for the electron pockets, antiferromagnetic fluctuations are strongly coupled to electrons in x²-y² orbitals but weakly coupled to those in {xz,yz} orbitals. Symmetry arguments reveal that this orbital selective coupling originates from the different intertwined orbital and Fe-site sublattice Bloch wavefunctions for these two sets of orbitals, specifically, the x²-y² orbitals can be Fe-site localized. The strong coupling of electrons in x²-y² orbitals to the magnetic fluctuations enables orbital-selective electronic renormalizations that can account for important features of our angle-resolved photoemission spectroscopy (ARPES) measurements. Our symmetry-required mechanism for orbital selective physics can be generalized to a range of crystal space groups with four-fold and six-fold screw axes.