Electronic band structure and momentum dependence of the superconducting   gap in (Ca, Na)Fe2As2 from angle-resolved photoemission spectroscopy
  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, $2\Delta/k_{\rm B}T_{\rm c}=5.5$, is substantially smaller than the numbers reported for related compounds, implying a non-trivial relation between electronic dispersion and superconducting gap in iron arsenides. 
D. V. Evtushinsky, V. B. Zabolotnyy, L. Harnagea, A. N. Yaresko, S.   Thirupathaiah, A. A. Kordyuk, J. Maletz, S. Aswartham,...
cond-mat.supr-con