Anisotropic $s$-wave gap in the vicinity of a quantum critical point in superconducting BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_2$ single crystals: a study of point-contact spectroscopy

Here 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 QCP. 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 reveal 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 BaFe$_2$(As$_{1-x}$P$_x$)$_2$ system.