Electron doping dependence of the anisotropic superconductivity in BaFe_2−xNi_xAs₂

The upper critical field H_c2 in superconducting BaFe_2−xNi_xAs₂ single crystals has been determined by magnetotransport measurements down to 0.6 K over the whole superconducting dome with 0.065 ≤ x ≤ 0.22 for both the interplane (H ‖ c, H^c _c2) and in-plane (H ‖ ab, H^ab _c2 ) field directions in static magnetic fields up to 16 T and pulsed magnetic fields up to 60 T. The temperature dependence of H^ab _c2 follows the Werthamer-Helfand-Hohenberg model incorporating orbital and spin paramagnetic effects, while H^c _c2(T ) can only be described by the effective two-band model with unbalanced diffusivity. The anisotropy of the upper critical fields, γ (T ) = H^ab _c2/H^c _c2, monotonically increases with increasing temperature for all dopings, and its zero-temperature limit γ (0) has an asymmetric doping dependence with a significant enhancement in the overdoped regime, where the optimally doped compound has the most isotropic superconductivity. Our results suggest that the anisotropy in the superconductivity of iron pnictides is determined by the topology of the Fermi surfaces together with the doping-induced impurity scattering.