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 ≈ 18.2 K has been determined. Two microscopic superconducting order parameters—the small superconducting gap Δ S (0) and, supposedly, the edges of the large gap with anisotropy in the ab plane Δ _L^out(0) and Δ _L^in(0) —have been determined at T ≪ 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 Δ S (0), Δ _L^out(0) , and Δ _L^in(0) values) and using the temperature dependences Δ S ( T ) and Δ _L^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 Δ S (0) value is taken as the outer edge of its angular distribution.