Low temperature specific heat in BaFe1.9Ni0.1As2 single crystals

Low-temperature specific heat was measured on the BaFe1.9Ni0.1As2 single crystals with critical transition temperature Tc = 20.1 K. A clear specific heat jump with the value ΔC/T|Tc ≈ 23 mJ/mol K2 was observed. In addition, a roughly linear magnetic field dependence of the electronic specific heat coefficient Δγ(H) was found in the zero-temperature limit, suggesting that at least one Fermi pocket, probably the hole derivative one, was fully gapped with a small anisotropy in the present sample. A slight curvature of the curve Δγ(H) may suggest a complex gap structure (anisotropic gap or nodes) at other Fermi surfaces.