Multiple Andreev Reflection Effect Spectroscopy of Underdoped NaFe1 – xCoxAs Single Crystals

We study the structure of the superconducting order parameter in underdoped NaFe1 – xCoxAs pnictides with Tc ≈ 19–21 K related to the 111 family. Using incoherent multiple Andreev reflection effect spectroscopy of planar break junctions, we directly determine the magnitudes of the two microscopic superconducting order parameters: the small superconducting gap ΔS(0), and possible edges of the large gap ΔL(0) having an anisotropy in the $$ab$$ plane at T ≪ Tc, the corresponding characteristic ratios, as well as their temperature dependences. Additionally, we detect features of the tunneling $$dI(V){\text{/}}dV$$ spectra in NaFe1 – xCoxAs at $$eV\text{*} > 2{{\Delta }_{{\text{L}}}}(0)$$ , those irrelative to superconducting state, and discuss their origin.