Determination of the superconducting order parameter of BaFe$_{1.92}$Ni$_{0.08}$As$_2$ weakly underdoped pnictides by two complementary techniques

The structure of the superconducting order parameter of weakly underdoped BaFe$_{1.92}$Ni$_{0.08}$As$_2$ pnictides with the critical temperature $T_c \approx 18.2$ K has been determined. Two microscopic superconducting order parameters — the small superconducting gap $\Delta_S(0)$ and, supposedly, the edges of the large gap with anisotropy in the $ab$ plane $\Delta_L^{\rm out}(0)$ and $\Delta_L^{\rm in}(0)$ — have been determined at $T \ll T_c$, together with their temperature dependences, using incoherent multiple Andreev reflection spectroscopy. It has been shown that the determined temperature dependence of the lower critical field $H_{c1}(T)$ can be described in the two-band approximation both within the so-called alpha model (using experimental $\Delta_S(0)$, $\Delta_L^{\rm out}(0)$, and $\Delta_L^{\rm in}(0)$ values) and using the temperature dependences $\Delta_S(T)$ and $\Delta_L^{\rm out,in}(T)$ obtained by multiple Andreev reflection spectroscopy under the assumption of a fairly strong anisotropy of the small superconducting gap, where the experimental $\Delta_S(0)$ value is taken as the outer edge of its angular distribution.