Three-dimensional nodal superconducting gap in single crystals Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$

The London penetration depth, λ, is directly related to the density, ns, of the Cooper pairs (λ ∝ 1/ns) and its variation with temperature provides valuable insight into the pairing mechanism. Here we study the evolution with doping of the temperature dependence of the in-plane (λab) and out-of-plane (λc) penetration depths in single crystals of electron-doped Ba(Fe1−xNix)2As2. As is the case for other pnictides, λ(T ) ∼ T n over the whole doping range and this behavior extends down to at least T = Tc/100, setting a very small upper limit on the gap minimum. Furthermore, in the overdoped regime: 1) the exponent n becomes substantially smaller than 2, which is incompatible with the models that explain power-law behavior to be due to scattering; 2) the exponent n becomes anisotropic, with λc(T ) showing a clear T -linear behavior over a large temperature interval. These findings suggest that in the overdoped regime the superconducting gap in iron-based pnictide superconductors develops nodal structure, which unlike in the cuprates, cannot be understood within a two-dimensional picture.