Magnetic fluctuations and superconductivity in LaFeAsO_1-xF _x under pressure as seen via ⁷⁵As NMR

The relationship between antiferromagnetic (AF) fluctuation and superconductivity was investigated in the La1111 series, LaFeAsO_1-x F_x (x=0, 0.05, 0.08, 0.10, and 0.14) by examining nuclear relaxation rates (1/ T₁) at both ambient pressure and 3.0 GPa. The results show that the critical doping level at which low-frequency AF fluctuation vanishes is around the optimally doped regime (x∼0.10). Although the AF fluctuation is enhanced by applying pressure in the underdoped regime (0.05≤x<0.10), the increase in critical transition temperature (T_c) is small, whereas T_c remarkably increases in the overdoped regime (x=0.14), implying that the AF fluctuation is less important to the high- T_c mechanism than the density of states at the electron pocket. The x dependence of T _c at 3.0 GPa is similar to that of R1111 (R=Ce, Pr, Nd, etc.) with T_c ≥40 K at ambient pressure. The relationship between T _c and the pnictogen height or lattice constant indicates that pressure application is equivalent to full rare-earth substitution. This equivalence suggests that high T_c above 40 K is realized when the AF fluctuation is absent.