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 $2{\Delta }_L^{\mathrm{out}}(0)/k_B{T}_c\approx 6$ and $2{\Delta }_L^{\mathrm{in}}(0)/k_B{T}_c\approx 4$, almost isotropic small gap with $2{\Delta }_S(0)/k_B{T}_c\approx 2$, 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.

27 pages, 10 figures

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