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We report experimental results of ^{75}As and ^{139}La nuclear magneticresonance (NMR) in the iron-based layered La(O_{1-x}F_{x})FeAs (x= 0.0, 0.04and 0.11) system. In the undoped LaOFeAs, 1/T_1 of ^{139}La exhibits a distinctpeak at T_N ~ 142 K below which the spectra become broadened due to theinternal magnetic field attributed to an antiferromagnetic (AFM) ordering. Inthe 4% F-doped sample, 1/T_1T exhibits a Curie-Weiss temperature dependencedown to 30 K, suggesting the development of AFM spin fluctuations withdecreasing temperature. In the 11% F-doped sample, in contrast, pseudogapbehavior is observed in 1/T_1T both at the ^{75}As and ^{139}La site with a gapvalue of ~172 K. The spin dynamics vary markedly with F doping, which isascribed to the Fermi-surface structure. As for the superconducting propertiesfor the 4% and 11% F-doped samples, 1/T_1 in both compounds does not exhibit acoherence peak just below T_c and follows a T^3 dependence at low temperatures,which suggests unconventional superconductivity with line-nodes. We discusssimilarities and differences between La(O_{1-x}F_{x})FeAs and cuprates, andalso discuss the relationship between spin dynamics and superconductivity onthe basis of F doping dependence of T_c and 1/T_1.

journal of the physical society of japan

Evolution from Itinerant Antiferromagnet to Unconventional  Superconductor with Fluorine Doping in La(O_{1-x}F_{x})FeAs Revealed by  ^{75}As and ^{139}La Nuclear Magnetic Resonance