Doping Evolution of the Superconducting Order Parameter of Na(Fe, Co)As Alkali-Metal Pnictides

Using incoherent multiple Andreev reflection effect spectroscopy, we have studied the multiple-gap superconducting state of underdoped and overdoped Na(Fe, Co)As single crystals. We directly determined the magnitudes of the microscopic superconducting order parameters, their temperature dependences, and characteristic ratios $$2\Delta (0){\text{/}}{{k}_{B}}{{T}_{c}}$$ . The obtained large data statistics unambiguously proves a significant decrease in the degree of the large superconducting gap possible anisotropy in the overdoped compositions away from the antiferromagnetic and nematic regions of the doping phase diagram.