An NMR study on the F-doping evolution of the iron oxypnictide LaFeAs(O1-xFx)

We report the experimental results of As-75 and La-139 nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O1-xFx) (x = 0.0, 0.04, 0.07, 0.11 and 0.14) where superconductivity occurs at x greater than or similar to 0.03. In the undoped LaFeAsO, 1/T-1 of La-139 exhibits a distinct peak at T-N similar to 142K 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 As-75 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 As-75 site with a gap value of Delta(PG) similar to 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/T-1 of As-75 in the three samples does not exhibit a coherence peak just below T-c and follows a T-3 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(O1-xFx) 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/T-1.