Anisotropic s-wave gap in the vicinity of a quantum critical point in superconducting BaFe₂(As₁₋ₓPₓ)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 BaFe 2 (As 0.7 P 0.3 ) 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 BaFe 2 (As 0.7 P 0.3 ) 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 Ω 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 BaFe 2 (As 1− x P x ) 2 system.