Systematic ^{75}As NMR study of the dependence of low-lying excitations on F doping in the iron oxypnictide LaFeAs(O_{1-x}F_{x})

Abstract

We report systematic ${}^{75}$As NMR studies on LaFeAs(O${}_{1-x}$F${}_x$) ($0\le x\le 0.14$). At $x=0.04$ near the phase boundary, from resistivity, spin-lattice relaxation rate $1/T_1$, and NMR spectrum measurements, we found weak magnetic order at $T_N\simeq 30$ K. Antiferromagnetic (AFM) fluctuations proved through $1/T_1$ are suppressed significantly with F-doping, and pseudogap behavior without pronounced AFM fluctuations is observed at $x=0.11$ where $T_c$ is maximum. This significant suppression of $1/T_{1}T$ upon F-doping while $T_c$ remains nearly unchanged suggests that low-energy AFM fluctuations probed with ${}^{75}$As NMR do not play a crucial role in the superconductivity.

arXiv comment5 pages, 5 figures, published in Phys. Rev. B

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