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We report on the Andreev spectroscopy and specific heat of high-qualitysingle crystals BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$. The intrinsic multiple Andreevreflection 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 energyin the $k_xk_y$-plane). The $25 \textendash 30 \%$ anisotropy shows the absenceof nodes in the superconducting gaps. Using a two-band model with s-wave-likegaps $\Delta_L \approx 3.2$\,meV and $\Delta_S \approx 1.6$\,meV, thetemperature dependence of the electronic specific heat can be well described. Alinear magnetic field dependence of the low-temperature specific heat offers afurther support of s-wave type of the order parameter. We find that a d-wave orsingle-gap BCS theory under the weak-coupling approach cannot describe ourexperiments.

Superconducting gap symmetry in BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ superconductor