An NMR study on the F-doping evolution of the iron oxypnictide LaFeAs(O_1-xF_x)

We report the experimental results of ⁷⁵As and ¹³⁹La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O_{1- x}F_x) (x=0.0, 0.04, 0.07, 0.11 and 0.14) where superconductivity occurs at x≳0.03. In the undoped LaFeAsO, 1/T₁ of ¹³⁹La exhibits a distinct peak at T_N∼142 K below which the La-NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the x=0.04 sample, 1/T₁T of ⁷⁵As exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with a decrease in temperature. At x=0.11 and 0.14, in contrast, pseudogap behavior is observed in 1/T₁T at the ⁷⁵As site with a gap value of Δ_PG∼175 and 165 K, respectively. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure because of the electron doping. As for the superconducting properties for x=0.04, 0.07 and 0.11, the 1/T₁ of ⁷⁵As in the three samples does not exhibit a coherence peak just below T_c and follows a T³ dependence at low temperatures. These results may suggest unconventional superconductivity with a zero gap along the lines, but neither the field-induced extra relaxation rate nor the residual density of states at the low temperatures is incompatible with the presence of the line nodes. We discuss the similarities and differences between LaFeAs(O_{1- x}F_x) and cuprates and also discuss the relationship between spin dynamics and superconductivity on the basis of the F-doping dependence of T_c and 1/T₁.