Superconducting gap symmetry in the superconductor BaFe1.9Ni0.1As2

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of BaFe1.9Ni0.1As2. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps Delta(L) approximate to 3.2-4.5 meV, Delta(S) approximate to 1.2-1.6 meV (the ranges correspond to the minimum and maximum value of the coupling energy in the k(x)k(y) plane). The 25%-30% anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with s-wave-like gaps Delta(L) approximate to 3.2 meV and Delta(S) approximate to 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 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.