Evolution from itinerant antiferromagnet to unconventional superconductor with fluorine doping in LaFeAs(O_1-xF_x) revealed by ⁷⁵As and ¹³⁹La nuclear magnetic resonance

We report experimental results of ⁷⁵As and ¹³⁹La nuclear magnetic resonance (NMR) in the iron-based layered LaFeAs(O _1-xF_x) (x = 0:0, 0.04, and 0.11). In the undoped LaFeAsO, 1/T₁ of ¹³⁹La exhibits a distinct peak at T_N ∼ 142 K below which the spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the 4% F-doped sample, 1/T₁T exhibits a Curie-Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the 11% F-doped sample, in contrast, pseudogap behavior is observed in 1/T₁T both at the ⁷⁵As and ¹³⁹La site with a gap value of Δ_PG ∼ 172 K. The spin dynamics vary markedly with F doping, which is ascribed to the Fermi-surface structure. As for the superconducting properties for the 4 and 11% F-doped samples, 1/T₁ in both compounds does not exhibit a coherence peak just below T_c and follows a T³ dependence at low temperatures, which suggests unconventional superconductivity with line-nodes. We discuss similarities and differences between LaFeAs(O_1-xF _x) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F doping dependence of T _c and 1/T₁.