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

Abstract

The structure of the superconducting order parameter of weakly underdoped BaFe1.92Ni0.08As2 pnictides with the critical temperature Tc ≈ 18.2 K has been determined. Two microscopic superconducting order parameters—the small superconducting gap ΔS(0) and, supposedly, the edges of the large gap with anisotropy in the ab plane $$\Delta _{{\text{L}}}^{{{\text{out}}}}(0)$$ and $$\Delta _{{\text{L}}}^{{{\text{in}}}}(0)$$—have been determined at T ≪ Tc, 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 Hc1(T) can be described in the two-band approximation both within the so-called alpha model (using experimental ΔS(0), $$\Delta _{{\text{L}}}^{{{\text{out}}}}(0)$$, and $$\Delta _{{\text{L}}}^{{{\text{in}}}}(0)$$ values) and using the temperature dependences ΔS(T) and $$\Delta _{{\text{L}}}^{{{\text{out}}{\text{,in}}}}(T)$$ obtained by multiple Andreev reflection spectroscopy under the assumption of a fairly strong anisotropy of the small superconducting gap, where the experimental ΔS(0) value is taken as the outer edge of its angular distribution.