Superconducting gap and critical behavior in the Iron-Pnictides

In the phase diagram of iron pnictides, superconductivity arises at the border of antiferromagnetism, which raises the question of the role of symmetry of the gap and quantum criticality. Although more than 15-years of extensive research, the microscopic origin of the pairing symmetry inside the superconducting (SC) dome and its link to quantum criticality still remains elusive. Here, we report two new findings on BaFe2_xNixAs2: (1) A sharp peak in the x-dependence of the lower and upper critical fields, the SC critical current density Jc, the size of the jump in the specific heat Delta Cel/T and the Sommerfeld coefficient (gamma) at the optimum composition x = 0.10, where the SC transition temperature Tc reaches a maximum. Our obtained reliable values as a function of doping of the normal-state Sommerfeld coefficient increase with doping, illustrating the strong competition between magnetism and superconductivity and attributed to closing of spin density wave gap with Ni doping. (2) We show that doping induced a sudden change of the gap structure from nodeless to nodal. Our results imply that the superconductivity in BaFe2_xNixAs2 is closely linked to the quantum criticality and is characterized by a complex order parameter.