Observation of a ubiquitous three-dimensional superconducting gap function in optimally doped Ba0.6K0.4Fe2As2

Abstract

The iron-pnictide superconductors have a layered structure formed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional 1 (3D) electronic structure of these hightemperature 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 0.6 K 0.4 Fe 2 As 2 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 ∆ 1 and interlayer ∆ 2 pairing. The anisotropy ratio ∆ 1 /∆ 2 , 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 2 . 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.Angle-resolved photoemission spectroscopy (ARPES) has played an important role in revealing the electronic structure of the pnictides. These measurements have typically been carried out at a fixed incident photon energy (hν) and varying incident angles that map out the planar band dispersion as a function of k x and k y . Thus far, four FS sheets have been observed with two hole pockets centred around the (0, 0) point and two electron pockets around the M (π,0) point in the unfolded 2D Brillouin zone. Below the superconducting transition temperature T c , nodeless SC gaps open everywhere on the FS sheets 3-6 , pointing to a pairing order parameter with an s-wave symmetry in the a-b plane, in agreement with a number of theoretical results 7-11 . However, there are other experiments that have indicated possible nodes in the superconducting gap of some pnictides, either line nodes in the a-b plane or nodes along the c axis [12][13][14] . It is well known that on tuning the incident photon energy hν, the allowed direct transitions will shift in energy and, consequently, in the momentum perpendicular to the a-b plane (k z ), which enables the determination of the electronic dispersion along the c axis. spectral intensity measured at 10 K plotted on a false-colour scale as a function of the in-plane momentum (k ) and binding energy along -X using 46-eV photons, which corresponds to k z = 0. Two hole-like bands (α (inner) and β (outer)) are observed. b, Second derivative of the spectral intensity plot as shown in a. c, A set of EDCs within the E-k range indicated by the red rectangle in a. The red EDC is at k F of the α band. d, Second derivative plot of the dispersion along Z-R (k z = π) measured using 32-eV photons. Three hole-like bands (α (inner), α (middle) and β (outer)) are observed. 198NATURE PHYSICS |...

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