Comparison of the gap structure of underdoped and overdoped superconducting pnictides BaFe$_{2-x}$Ni$_{x}$As$_2$

We compare the structure of the superconducting order parameter of overdoped BaFe$_{1.88}$Ni$_{0.12}$As$_2$ and underdoped BaFe$_{1.92}$Ni$_{0.08}$As$_2$ pnictides with similar ${{T}_{c}} \approx 18.0{-} 18.3$ K. Using incoherent multiple Andreev reflection effect spectroscopy, we directly determined the magnitudes of the two microscopic superconducting order parameters: the small superconducting gap and probably anisotropic large gap, their characteristic ratios and temperature dependences. We discuss similarity and difference in the gap structure, as well as possible influence of antiferromagnetic phase to superconducting properties.