We compare the superconducting gap structure of underdoped and overdoped iron pnictides NaFe_1-xCo_xAs with x ≈ 0.02–0.045 in order to determine its evolution along the doping phase diagram. Using incoherent multiple Andreev reflection effect (IMARE) spectroscopy, we observed multiple-gap superconductivity and directly determined three superconducting order parameters D_L^out, D_Lⁱⁿ, and D_S, as well as their temperature dependences. Due to a number of indirect arguments, we attributed D_L^out and D_Lⁱⁿ to one and the same, anisotropic superconducting condensate, being the maximum and the minimum Cooper pair coupling energies in the related bands. The degree of the possible anisotropy A_L º 100%∙[1 - D_Lⁱⁿ/D_L^out] remains almost constant with temperature until T_c. We detected a significant increase in A_L in the underdoped region (up to 42%) as compared to overdoped one (about 22%) with similar critical temperatures T_c ≈ 18.5 K. Such increase in the gap anisotropy in the underdoped regime could be caused by an influence of the spin density wave and nematic orders at the superconducting characteristics.

S.Kuzmichev et al.,
JETP Letters 120, issue 2 (2024)