Determination of the Superconducting Order Parameter of BaFe1.92Ni0.08As2 Weakly Underdoped Pnictides by Two Complementary Techniques.

The structure of the superconducting order parameter of weakly underdoped BaFe_1.92Ni_0.08As₂ 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 and —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), , and values) and using the temperature dependences Δ_S(T) and 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. [ABSTRACT FROM AUTHOR]