75As and 139La NMR studies on LaFeAs(O1-xFx)

We report 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 and 0.11) where superconductivity occurs in x greater than 0.04. 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 30K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the x = 0.11 sample, in contrast, pseudogap behavior is observed in 1/T-1 T at the 75 As site with a gap value of Delta(PG)similar to 175 K. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron-doping. As for the superconducting properties for x = 0.04, 0.07 and 0.11, 1/T-1 of 75 As in all compounds does not exhibit a coherence peak just below T-c and follows a T-3 dependence at low temperatures. These results seemingly suggests that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between LaFeAs(O1-xFx) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of T, and 1/T-1. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.