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 L ≈
3.2−4.5 meV, S ≈ 1.2−1.6 meV (the ranges correspond to the minimum and maximum value of the coupling
energy in the kxky plane). The 25%−30% anisotropy shows the absence of nodes in the superconducting gaps.
Using a two-band model with s-wave-like gaps L ≈ 3.2 meV and S ≈ 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.