Stripe antiferromagnetic correlations in LaFeAsO1-xFx probed by 75As NMR

The anisotropy of the nuclear spin-lattice relaxation rate 1/T-1 of As-75 was investigated in the iron pnictide LaFeAs(O1-xFx) (x=0.07, 0.11, and 0.14) as well as LaFeAsO. While the temperature dependence of the normal-state 1/T1T in the superconducting (SC) x=0.07 is different from that in the SC x=0.11, their anisotropy of 1/T-1, R equivalent to (1/T-1)(H parallel to ab)/(1/T-1)(H parallel to c) in the normal state is almost the same (similar or equal to 1.5). The observed anisotropy is ascribable to the presence of the local stripe correlations with Q=(pi,0) or (0,pi). In contrast, 1/T-1 is isotropic and R is approximately 1 in the overdoped x=0.14 sample, where superconductivity is almost suppressed. These results suggest that the presence of the local stripe correlations originating from the nesting between hole and electron Fermi surfaces is linked to high-T-c superconductivity in iron pnictides.