Observation of a ubiquitous three-dimensional superconducting gap function in optimally-doped Ba${}_{0.6}$K${}_{0.4}$Fe${}_2$As${}_2$

Abstract

The iron-pnictide superconductors have a layered structureformed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional \cite{3D_SC} (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 \KZ-capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on \BKFAOP samples. We found a marked \KZ 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 ${\Delta }_1$ and interlayer ${\Delta }_2$ pairing. The anisotropy ratio ${\Delta }_2/{\Delta }_1$, determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling $J_c/J_{ab}$ in the parent compound \cite{NeutronParent}. The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrain the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.

10 pages, 4 figures

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