Evolution from Itinerant Antiferromagnet to Unconventional Superconductor with Fluorine Doping in LaFeAs(O1-xFx) Revealed by 75As and 139La Nuclear Magnetic Resonance

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Abstract
We report experimental results of 75 As and 139 La nuclear magnetic resonance (NMR) in the iron-based layered LaFeAs(O 1- x F x ) ( x = 0.0, 0.04, and 0.11). In the undoped LaFeAsO, 1/ T 1 of 139 La exhibits a distinct peak at T N ∼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/ T 1 T 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/ T 1 T both at the 75 As and 139 La site with a gap value of Δ PG ∼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/ T 1 in both compounds does not exhibit a coherence peak just below T c and follows a T 3 dependence at low temperatures, which suggests unconventional superconductivity with line-nodes. We discuss similarities and differences 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/ T 1 .