Anisotropic s-Wave Gap in the Vicinity of a Quantum Critical Point in Superconducting BaFe₂(As_1-x P_x )₂ Single Crystals: A Study of Point-Contact Spectroscopy

We report on soft c-axis point-contact Andreev reflection (PCAR) spectroscopy combining with resistivity measurements on BaFe2(As0.7P0.3)(2), to elucidate the superconducting gap structure in the vicinity of the quantum critical point. A double peak at the gap edge plus a dip feature at zero-bias has been observed on the PCAR spectra, indicative of the presence of a nodeless gap in BaFe2(As0.7P0.3)(2). Detailed analysis within a sophisticated theoretical model reveals an anisotropic gap with deep gap minima. The PCARs also feature additional structures related to the electron-bosonic coupling mode. Using the extracted superconducting energy gap value, a characteristic bosonic energy Omega(b) and its temperature dependence are obtained, comparable with the spin-resonance energy observed in neutron scattering experiment. These results indicate a magnetism-driven quantum critical point in the BaFe2(As1-xPx)(2) system.