Doping Induced Gap Anisotropy in Iron-Based Superconductors: a Point-Contact Andreev Reflection Study of BaFe sub(2-x)Ni sub(x)As sub(2) Single Crystals

We report a systematic investigation on c-axis point-contact Andreev reflection (PCAR) in BaFe sub(2-x)Ni sub(x)As sub(2) superconducting single crystals from underdoped to overdoped regions (0.075[< or =] x [< or =]0.15). At low temperatures, an in-gap sharp 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 variety of the conductance spectra with doping can be well described by using a generalized Blonder-Tinkham-Klapwijk formalism with an angle-dependent gap. This gap shows a clear crossover from a nodeless in the underdoped side to a nodal feature in the overdoped region. This result provides 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 sub(+ or -) scenario.