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 $k_xk_y$-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.