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 $T_c \approx 19-21$ 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 $\Delta_{\mathrm S}(0)$, and possible edges of the large gap $\Delta_{\mathrm L}(0)$ having an anisotropy in the $ab$ plane at $T \ll T_c$, 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 $eV^{\ast} > 2\Delta_{\mathrm L}(0)$, those irrelative to superconducting state, and discuss their origin.