For a single band superconductor, the superconducting pairing can be normally described using a single pairing channel with a specified symmetry. However, for iron-based superconductors whose Fermi surface consists of multi-bands with multi-orbital characters, the pairing interaction could be complex and consist of multiple pairing channels. Here, we show that the superconducting pairing cannot be described by a single s± pairing channel, but instead consists of at least two pairing channels. One pairing channel follows the s± pairing symmetry and originates from the pair scattering on the β and η pockets. The other pairing channel contributes stronger pairing interactions and originates from the pair scattering on the α and δ pockets.
According to theories, the pair scattering of electrons in iron-based superconductors could be mediated by a large Q scattering that connects the Fermi pockets at the Γ and M points28 − 31. On one hand, it has been proposed that the effectivity of the pair scattering is determined by the nesting condition of Fermi surface. Here, the outer hole/electron pockets are similar in size and the inner hole/electron pockets are well nested. The nesting of Fermi pockets naturally separates out two pair scattering channels, which locate respectively on the outer and inner Fermi pockets. On the other hand, it has been proposed that the pair scattering is most effective when the nesting portions of Fermi surface have the same orbital character29 − 32. Here, both the β and η pockets are constructed by the dxy orbital, while the α and δ pockets are constructed by the dxz/dyz orbitals. The intra-orbital pair scattering leads to a separation of two pairing channels respectively from the dxz/dyz and dxy orbitals. Based on above discussions, our results suggest that the intra-orbitals pair scattering between two nested Fermi pockets play dominating roles in the superconducting pairing of iron-based superconductors. As a result, the superconducting pairing is pocket- or orbital-selective32, 33, which explains the deviation from a single s± gap function observed here.
Finally, it is intriguing to discuss how the potassium atoms play roles on the surface of BaFe2(As1 − xPx)2. There are several possible scenarios. First, electrons transfer from the potassium atoms to the sample surface, which neutralizes the charge-non-neutral surface, leading to a suppression of the surface broadening effect. Second, the potassium atoms could act as a catalyzer which causes the redistribution of alkaline-earth metal atoms on the sample surface in a more homogeneous way. Third, the potassium atoms scatter electrons at the sample surface. The surface electronic states then turn into an incoherent and continuous background that is inconspicuous in photoemission spectra. To understand the mechanism of potassium deposition, further experimental and theoretical studies are required. Nevertheless, our results highlight the surface complicacy of 122 iron-based superconductors, which implies that the previous photoemission data taken on 122 should be carefully revisited.
For superconducting gap measurements, previous ARPES studies observed double peak features on both hole and electron pockets in Ba1 − xKxFe2As2, whose origin remains contraversal34–37. Some attribute it to the band degeneracy34, 35, while others believe it is originated from the electron-bosonic coupling36 or in-gap impurity state37. Here, we show that the surface effect is an alternative explanation of the double peak features and could be verified using the potassium deposition. Improving the accuracy of the superconducting gap measurement could help for refining the theoretical models of iron-based superconductors. On the other hand, for band structure measurements, previous ARPES studies observed complex band structures in BaFe2As2, SrFe2As2, CaFe2As2, etc38–40. The number of bands is apparently much larger than the expected number of bands in band calculations. Using potassium deposition, we could distinguish the surface bands from the bulk bands. The results could clarify controversies and help to get a unified picture of the band structure of 122 iron-based superconductors.
In summary, we measured the superconducting gap anisotropy of potassium-coated BaFe2(As0.7P0.3)2 using ARPES. We not only show that the superconducting pairing is pocket- or orbital-selective, but also highlight that the surface complicacy of 122 needs to be seriously considered. Using potassium deposition, we could suppress the surface-broadening effect of 122 and reveal its intrinsic electron properties. Together with all the advantages of 122, the studies of surface-neutralized 122 could provide accurate and crucial clues for uncovering the pairing mechanism of iron-based superconductors.