Multiple Andreev Reflection Effect Spectroscopy of Underdoped NaFe 1 – x Co x As Single Crystals

We study the structure of the superconducting order parameter in underdoped NaFe 1 – x Co x As pnictides with T c ≈ 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 Δ S (0), and possible edges of the large gap Δ L (0) having an anisotropy in the ab plane at T ≪ T c , the corresponding characteristic ratios, as well as their temperature dependences. Additionally, we detect features of the tunneling dI(V)/dV spectra in NaFe 1 – x Co x As at eV* > 2Δ_L(0) , those irrelative to superconducting state, and discuss their origin.