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 the observation in density functional theory (DFT) calculations that around the M point in the Brillouin zone, 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 at the M point, 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 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. Orbital-selective correlations are crucial in understanding Fe-based superconductors, yet their origins remain debated. Here, the authors propose a band-theory-based mechanism for orbital-selective physics in monolayer FeSe, revealing that antiferromagnetic fluctuations selectively couple to x2 − y2 orbitals, offering broader insights for orbital-selective systems with four-fold and six-fold symmetry axes.