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})

TL;DR

This systematic ^75As NMR study investigates how low-lying excitations and magnetic fluctuations in LaFeAs(O_{1-x}F_{x}) evolve with F doping, revealing that AFM fluctuations diminish with doping while superconductivity persists.

Contribution

The paper provides detailed NMR evidence showing the suppression of AFM fluctuations with F doping and their limited role in superconductivity in LaFeAs(O_{1-x}F_{x}).

Findings

Weak magnetic order at x=0.04 near phase boundary.

Suppression of AFM fluctuations with increased F doping.

Persistence of superconductivity despite reduced AFM fluctuations.

Abstract

We report systematic $^{75}$As NMR studies on LaFeAs(O$_{1-x}$F$_{x}$) ($0\le x\le0.14$). 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 $1/T_1T$ 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.