NMR study under pressure on the La1111 pnictides, LaFeAsO _1-x F _x

The relationship between antiferromagnetic (AF) fluctuation and superconductivity was investigated in the La1111 series, LaFeAsO _1-x F _x (x=0.05, 0.08, and 0.14) by examining nuclear relaxation rates (1/T ₁) at both ambient pressure and 3.0 GPa. Although the AF fluctuation is enhanced by applying pressure in the underdoped regime (x=0.05, and 0.08), the increase in critical transition temperature (T _c) is small, whereas T _c increases remarkably in the overdoped regime (x=0.14) where the AF fluctuation is absent, suggesting that high T _c above 40 K originates not from the AF fluctuation but from the density of states at the electron pocket.