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

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The anisotropy of the nuclear spin-lattice relaxation rate 1/T₁ of ⁷⁵As was investigated in the iron-based superconductor 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/T₁, R ≡ (1/T₁)H ∥ ab/(1/T₁)ₕ ∥ c in the normal state is almost the same (≃ 1.5). The observed anisotropy is ascribable to the presence of the local stripe correlations with Q = (π, 0) or (0, π). In contrast, 1/T₁ 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-Tc superconductivity in iron pnictides.