Evolution from Itinerant Antiferromagnet to Unconventional Superconductor with Fluorine Doping in LaFeAs(O_<1-x>F_x) Revealed by ^<75>As and ^<139>La Nuclear Magnetic Resonance(Condensed matter: electronic structure and electrical, magnetic, and optical p

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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〜142K 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_1T exhibits a Curie-Weiss temperature dependence down to 30K, 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_1T both at the ^<75>As and ^<139>La site with a gap value of Δ_<PG>〜172K. 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.
社団法人日本物理学会の論文
2008-07-15