Gap Anisotropy Evolution Across the Superconducting Dome in Single Crystals of BaFe 2 (As 1- x P x ) 2 (0.26 ≤ x ≤ 0.40)

The presence of a quantum critical point (QCP) and the role of the quantum criticality on the pairing states of unconventional superconductors have been subjects of intense scrutiny. Here, we performed c-axis point-contact Andreev reflection (PCAR) spectroscopy combining with in-plane resistivity ρ ab measurement on single crystals of BaFe 2 (As 1- x P x ) 2 (0.26 < x < 0.40) spanning from underdoped to overdoped region. The temperature dependence of resistivity ρ ( T ) exhibits a perfect T -linear behavior associated with a classic picture of quantum criticality with a magnetic QCP at the optimal composition x ≃ 0.30 at which superconductivity is most robust, as previously reported. The evolution of PCAR spectrum with doping level x clearly exhibit a strongly anisotropic or even a nodal gap in underdoped region. On the contrary, a weakly anisotropic gap appears for the optimally and overdoped region. Quantitative analysis based on a sophisticated two-dimensional Blonder-Tinkham-Klapwijk model reveals a crossover of weakly to strongly anisotropic gap at x = 0.30, coinciding with the putative magnetic QCP, and a possible nodal-nodeless gap transition between x = 0.27 and 0.26. Our experimental results shed new light on the role of quantum fluctuations on the superconductivity in BaFe 2 (As 1- x P x ) 2 system.