Universal relationship between low-energy antiferromagnetic fluctuations and superconductivity in BaFe2(As1-xPx) 2
To identify the key parameter for optimal superconductivity in iron pnictides, we measured the 31P-NMR relaxation rate on BaFe2(As1-xPx) 2 (x =0.22 and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For x =0.22 , structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4 GPa, whereas the superconducting transition temperature Tc increases to twice its former value. In contrast, for x =0.28 near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and Tc reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the Tc dome as a function of Weiss temperature θ , which measures the distance to the QCP. Moreover, the Tc-θ curve under pressure precisely coincides with that with a chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on BaFe2(As1-xPx) 2 .