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Electronic structure of newly synthesized single crystals of calcium iron arsenide doped with sodium with Tc ranging from 33 to 14 K has been determined by angle-resolved photoemission spectroscopy (ARPES). The measured band dispersion is in general agreement with theoretical calculations, nonetheless implies absence of Fermi surface nesting at antiferromagnetic vector. A clearly developing below Tc strongly band-dependant superconducting gap has been revealed for samples with various doping levels. BCS ratio for optimal doping, 2∆/kBTc = 5.5, is substantially smaller than the numbers reported for related compounds, implying a non-trivial relation between electronic dispersion and superconducting gap in iron arsenides.Iron-based high temperature superconductors form an increasingly growing subject for investigation. Unlike other types of high-temperature superconductors, ironbased compounds can be synthesized in form of various crystals, exhibiting a large variety of electronic band structures, magnetic properties, superconducting order parameters, and electronic properties in general [1][2][3]. On the other hand, experimental difficulties, connected to the limitations of each particular technique, and complexity of the electronic interactions in iron-based materials hinder accurate and precise determination of the electronic structure in many cases. Thus, despite such abundance of possible forms of iron-based superconductors, the electronic spectrum, including both electronic band structure and superconducting gap function, has been thoroughly addressed experimentally only for holedoped BaFe 2 As 2 [4-9] and LiFeAs [10][11][12]. Available information at hand is still insufficient to tell, which features are generic to all iron-based high-T c s and responsible for the effective electron pairing.Here we report detailed studies of the electronic structure of newly synthesized large single crystals of Ca 1−x Na x Fe 2 As 2 (CNFA), with doping level, x, up to 0.7 and T c between 14 and 33 K. Angle-resolved photoemission spectroscopy (ARPES) measurements were performed at 1 3 end station at BESSY II synchrotron in Berlin [13,14]. The sample surface was prepared by cleaving, and was shown to be highly suitable for ARPES experiments: sharp spectral features and a pronounced superconducting transition at nominal T c were observed, offering possibility for detailed studies of the electronic band structure and superconducting gap distribution with high resolution.The Fermi surface (FS) of CNFA consists of a propeller-shaped structure at the Brillouin zone (BZ) corner, alike other hole-doped 122 compounds, and holelike FS sheets at the BZ center of increased size [see Fig. 1(a,b)].

In this study, we investigate the electronic and magnetic properties of Rb 0.77 Fe 1.61 Se 2 (T c = 32.6 K) in normal and superconducting states by means of photoemission and μSR spectroscopies as well as band-structure calculations. We demonstrate that the unusual behavior of these materials is the result of separation into metallic (∼12%) and insulating (∼88%) phases. Only the former becomes superconducting and has a usual electronic structure of electron-doped FeSe slabs. Our results thus imply that the antiferromagnetic insulating phase is just a by-product of Rb intercalation and its magnetic properties have no direct relation to the superconductivity. Instead, we find that also in this class of iron-based compounds, the key ingredient for superconductivity is a certain proximity of a Van Hove singularity to the Fermi level.

We report a systematic study on the electronic structure and superconducting (SC) gaps in electron doped NaFe0.95Co0.05As superconductor using angle-resolved photoemission spectroscopy. Hole-like Fermi sheets are at the zone center and electron-like Fermi sheets are at the zone corner, and are mainly contributed by xz and yz orbital characters. Our results reveal a ∆ K B Tc in the range of 1.8-2.1, suggesting a weak-coupling superconductivity in these compounds. Gap closing above the transition temperature (Tc) shows the absence of pseudogaps. Gap evolution with temperature follow the BCS gap equation near the Γ, Z, and M high symmetry points. Furthermore, an almost isotropic superconductivity along kz direction in the momentum space is observed by varying the excitation energies.PACS numbers: 74.70. Xa, 74.25.Jb, I.

Strong electron interactions in solids increase effective mass, and shrink the electronic bands [1]. One of the most unique and robust experimental facts about iron-based superconductors [2][3][4] is the renormalization of the conduction band by factor of 3 near the Fermi level [5][6][7][8][9]. Obviously related to superconductivity, this unusual behaviour remains unexplained. Here, by studying the momentum-resolved spectrum of the whole valence band in a representative material, we show that this phenomenon originates from electronic interaction on a much larger energy scale. We observe an abrupt depletion of the spectral weight in the middle of the Fe 3d band, which is accompanied by a drastic increase of the scattering rate. Remarkably, all spectral anomalies including the low-energy renormalization can be explained by coupling to excitations, strongly peaked at about 0.5 eV. Such high-energy interaction distinguishes all unconventional superconductors from common metals.

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