Superconducting gap and nematic resonance at the quantum critical point observed by Raman scattering in BaFe2(As1−xPx)2

We report comprehensive temperature and doping dependences of the Raman scattering spectra for BaFe2(As1−xPx)2 (x=0, 0.07, 0.24, 0.32, and 0.38), focusing on the nematic fluctuation and the superconducting responses. With increasing x, the bare nematic transition temperature estimated from the Raman spectra reaches T=0 K at the optimal doping, which indicates a quantum critical point (QCP) at this composition. In the superconducting compositions, in addition to the pair-breaking peaks observed in the A1g and B1g spectra, another strong B1g peak appears below the superconducting transition temperature, which is ascribed to the nematic resonance peak. The observation of this peak indicates significant nematic correlations in the superconducting state near the QCP in this compound.