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

Abstract

We report the experimental results of 75As and 139La 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≳0.03. In the undoped LaFeAsO, 1/T1 of 139La exhibits a distinct peak at TN∼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 ΔPG∼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 Tc 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(O1- xFx) and cuprates and also discuss the relationship between spin dynamics and superconductivity on the basis of the F-doping dependence of Tc and 1/T1.