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

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 $\Delta_L \approx 3.2 \textendash 4.5$meV,$\Delta_S \approx 1.2 \textendash 1.6$,meV (the ranges correspond to the minimum and maximum value of the coupling energy in the $k_x{k}_y$-plane). The $25\text{textendash}30\mathrm{\%}$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with s-wave-like gaps ${\mathrm{\Delta }}_L\approx 3.2$,meV and ${\mathrm{\Delta }}_S\approx 1.6$,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.