Orbital selectivity of layer-resolved tunneling in the iron-based superconductor Ba0.6K0.4Fe2As2

We use scanning tunneling microscopy/spectroscopy to elucidate the Cooper pairing of the iron pnictide superconductor ${\mathrm{Ba}}_0.6{\mathrm{K}}_0.4{\mathrm{Fe}}₂{\mathrm{As}}₂$. By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer identity. By cross referencing with the angle-revolved photoemission results and the tunneling data of LiFeAs, we find that tunneling on each termination surface probes superconductivity through selecting distinct $\mathrm{Fe}\text{\ensuremath{-}}3d$ orbitals. These findings imply the real-space orbital features of the Cooper pairing in the iron pnictide superconductors, and propose a general concept that, for complex multiorbital material, tunneling on different terminating layers can feature orbital selectivity.