Superconducting gap symmetry in BaFe${}_{1.9}$Ni${}_{0.1}$As${}_2$ superconductor

Abstract

We report on the Andreev spectroscopy and specific heat of high-quality single crystals BaFe${}_{1.9}$Ni${}_{0.1}$As${}_2$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps $Δ_L \approx 3.2 \textendash 4.5$\,meV, $Δ_S \approx 1.2 \textendash 1.6$\,meV (the ranges correspond to the minimum and maximum value of the coupling energy in the $k_x{k}_y$-plane). The $25 \textendash 30 \%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with s-wave-like gaps ${\Delta }_L\approx 3.2$\,meV and ${\Delta }_S\approx 1.6$\,meV, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers a further support of s-wave type of the order parameter. We find that a d-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

12 pages, 5 figures