Observation of a ubiquitous three-dimensional superconducting gap function in optimally doped Ba_0.6K_0.4Fe₂As₂

The iron-pnictide superconductors have a layered structureformed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional¹ (3D) electronic structure of these high-temperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the k_z capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on Ba_0.6K_0.4Fe₂As₂ samples. We found a marked k_z dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer Δ₁ and interlayer Δ₂ pairing. The anisotropy ratio Δ₁/Δ₂, determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling J_a/J_ab in the parent compound². The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrains the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.

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