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 |...

We report a systematic angle-resolved photoemission spectroscopy study on Ba(Fe1−xRux)2As2 for a wide range of Ru concentrations (0.15 ≤ x ≤ 0.74). We observed a crossover from twodimension to three-dimension for some of the hole-like Fermi surfaces with Ru substitution and a large reduction in the mass renormalization close to optimal doping. These results suggest that isovalent Ru substitution has remarkable effects on the low-energy electron excitations, which are important for the evolution of superconductivity and antiferromagnetism in this system. PACS numbers: 74.70.Xa, 71.18.+y, 74.25.Jb, Superconductivity in the iron-based materials usually emerges from a magnetic state by several kinds of routes leading to very similar phase diagrams of magnetism and superconductivity. In Ba 1−x K x Fe 2 As 2 [1] and Ba(Fe 1−x Co x ) 2 As 2 [2], the introduction of extra hole or electron carriers shifts the chemical potential so that the sizes of the hole and electron Fermi surface (FS) pockets evolve oppositely [3], which eventually suppresses the nesting between the hole and electron FS pockets that play a role in the formation of spin-density-wave (SDW) with exotic Dirac cone dispersion [4] in the parent compound. While it is generally believed that external pressure also changes the FS topology by modifying the chemical bonds [5], the role of isovalent element substitution is still debated. Various scenarios, for example changes of the FS topology by chemical pressure [6][7][8], the reduction of electron correlations [8,9], magnetic dilution [10], and even the addition of extra hole carriers [11], have been suggested to explain the suppression of the SDW order with isovalent element substitution in the BaFe 2 (As 1−x P x ) 2 and Ba(Fe 1−x Ru x ) 2 As 2 systems. Surprisingly, only little attention has been devoted to answer the reversed but somehow similarly important question: how does superconductivity is suppressed by increasing the substitution further than the optimal concentration?Since single-crystals can be grown for the entire phase diagram, the Ba(Fe 1−x Ru x ) 2 As 2 system is ideal to investigate the suppression of the SDW order, the emergence of superconductivity and its disappearance with isovalent-substitution. We expect that the electronic structure near the Fermi level (E F ) be substantially modified by the Ru substitution. Indeed, the isovalent Ru substitution at the Fe site leads to an anisotropic lattice distortion, resulting in a strong increase of the As-Fe(Ru)-As bond angle and a decrease of the As height from the Fe(Ru) plane [6,12,13]. The Hall coefficient, which is always negative and decreases with decreasing temperature in the parent compound BaFe 2 As 2 , increases with decreasing temperature in the Ru-substituted samples and even changes sign for large Ru concentrations [13]. With its capacity to resolve dispersive electronic states in the vicinity of E F , angleresolved photoemission spectroscopy (ARPES) is a powerful tool to determine which parameters drive the system from a SDW orde...

The superconducting gap is the fundamental parameter that characterizes the superconducting state, and its symmetry is a direct consequence of the mechanism responsible for Cooper pairing. Here we discuss about angle-resolved photoemission spectroscopy measurements of the superconducting gap in the Fe-based high-temperature superconductors. We show that the superconducting gap is Fermi surface dependent and nodeless with small anisotropy, or more precisely, a function of the momentum location in the Brillouin zone. We show that while this observation seems inconsistent with weak coupling approaches for superconductivity in these materials, it is well supported by strong coupling models and global superconducting gaps. We also suggest that a smaller lifetime of the superconducting Cooper pairs induced by the momentum dependent interband scattering inherent to these materials could affect the residual density of states at low energies, which is critical for a proper evaluation of the superconducting gap