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  Evtushinsky, D.V.; Zabolotnyy, V.B.; Harnagea, L.; Yaresko, A.N.; Thirupathaiah, S.; Kordyuk, A.A.; Maletz, J.; Aswartham, S.; Wurmehl, S.; Rienks, E.; Follath, R.; Buechner, B.; Borisenko, S.V.: Electronic band structure and momentum dependence of the superconducting gap in Ca_1-xNa_xFe₂As₂ from angle-resolved photoemission spectroscopy. Physical Review B 87 (2013), p. 094501/1-5

10.1103/PhysRevB.87.094501
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Abstract:
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 an antiferromagnetic vector. A clearly developing below Tc strongly band-dependant superconducting gap has been revealed for samples with various doping levels. The BCS ratio for optimal doping, 2Δ/kBTc=5.5, is substantially smaller than the numbers observed for related compounds.