Evolution from itinerant antiferromagnet to unconventional superconductor with fluorine doping in LaFeAs(O1-xFx) 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(O1-xFx) (x=0.0, 0.04, and 0.11). In the undoped LaFeAsO, 1/T1 of 139La exhibits a distinct peak at TN - 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 ΔPG - 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 Tc and follows a T3 dependence at low temperatures, which suggests unconventional superconductivity with line-nodes. We discuss similarities and differences between LaFeAs(O1-xFx) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F doping dependence of Tc and 1/T1. (author)