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We report a systematic investigation on c-axis point-contact Andreevreflection (PCAR) in BaFe$_{2-x}$Ni$_x$As$_2$ superconducting single crystalsfrom underdoped to overdoped regions (0.075 $\leq x\leq 0.15$). At optimaldoping ($x=0.1$) the PCAR spectrum feature the structures of twosuperconducting gap and electron-boson coupling mode. In the $s\pm$ scenario,quantitative analysis using a generalized Blonder-Tinkham-Klapwijk (BTK)formalism with two gaps: one isotropic and another angle dependent, suggest anodeless state in strong-coupling limit with gap minima on the Fermi surfaces.Upon crossing above the optimal doping ($x > 0.1$), the PCAR spectrum show anin-gap sharp narrow peak at low bias, in contrast to the case of underdopedsamples ($x < 0.1$), signaling the onset of deepened gap minima or nodes in thesuperconducting gap. This result provides evidence of the modulation of the gapamplitude with doping concentration, consistent with the calculations for theorbital dependent pair interaction mediated by the antiferromagnetic spinfluctuations.

Gap Anisotropy in Iron-Based Superconductors: A Point-Contact Andreev Reflection Study of BaFe$_{2-x}$Ni$_{x}$As$_2$ Single Crystals