⁷⁵As and ¹³⁹La NMR studies on LaFeAs (O_{1 - x} F_x)

We report experimental results of ⁷⁵As and ¹³⁹La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs (O_{1 - x} F_x) (x = 0.0, 0.04, 0.07 and 0.11) where superconductivity occurs in x greater than 0.04. In the undoped LaFeAsO, 1 / T₁ of ¹³⁹La exhibits a distinct peak at T_N ~ 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 / T₁ T of ⁷⁵As exhibits a Curie-Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the x = 0.11 sample, in contrast, pseudogap behavior is observed in 1 / T₁ T at the ⁷⁵As site with a gap value of Δ_PG ~ 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 / T₁ of ⁷⁵As in all compounds does not exhibit a coherence peak just below T_c and follows a T³ 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 (O_{1 - x} F_x) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of T_c and 1 / T₁. Crown