Doping-Induced Evolution of Superconducting Order Parameter in Ba(Fe1-xNix)2As2 Single Crystals*

We report a systematic investigation on the $c$-axis point- contact Andreev reflection (PCAR) in BaFe$_{2-x}$Ni$_x$As$_2$ superconducting single crystals with the Ni concentrations from underdoped to overdoped regions (0.075 $\leq x\leq 0.15$). At low temperatures, an in-gap peak at low-bias voltage is observed in PCAR for overdoped samples, in contrast to the case of underdoped junctions, in which an in-gap plateau is observed. The spectra are fitted using a generalized Blonder- Tinkham-Klapwijk (BTK) formalism with two gaps: one isotropic and another angle dependent. The second gap, resulted from the fitting, shows a clear crossover from a nodeless in the underdoped side to a nodal feature in the overdoped region. This intriguing observation provides strong evidence of the doping induced evolution of the superconducting order parameter when the inter-pocket and intra-pocket scattering are tuned through doping, as expected in the $s_{\pm}$ scenario.