Evidence from anisotropic penetration depth for a three-dimensional nodal superconducting gap in single-crystalline Ba(Fe_1−xNi_x)₂As₂

The London penetration depth, , is directly related to the density, n_s, of the Cooper pairs (²1/n_s) 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 (_ab) and out-of-plane (_c) penetration depths in single crystals of electron-doped Ba(Fe_1−xNi_x)₂As₂. As is the case for other pnictides, (T)~Tⁿ over the whole doping range and this behavior extends down to at least T=T_c/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 _c(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. ©2010 The American Physical Society