Superconducting Order Parameters in overdoped BaFe$_{1.86}$Ni$_{0.14}$As$_2$ Revealed by Multiple Andreev Reflection Spectroscopy of Planar Break-Junctions

Abstract

Using multiple Andreev reflection (MAR) spectroscopy, we studied superconductor - normal metal - superconductor (SnS) semiballistic contacts with incoherent transport in overdoped pnictide superconductors BaFe$_{1.86}$Ni$_{0.14}$As$_2$ with critical temperature $T_c \approx 12$ K. We directly determined an anisotropic large gap with the characteristic ratios $2\Delta_L^{\rm out}(0)/k_BT_c \approx 6$ and $2\Delta_L^{\rm in}(0)/k_BT_c \approx 4$, almost isotropic small gap with $2\Delta_S(0)/k_BT_c \approx 2$, and the temperature dependence of the resolved superconducting order parameters. Above the large gap position $2\Delta_L$, a fine dI(V)/dV structure fully vanishing at $T_c$ and possibly caused by a resonant coupling with a bosonic mode was observed.