### Abstract

We report the experimental results of ^{75}As and ^{139}La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O _{1−x}F_{x}) (x = 0.0, 0.04, 0.07, 0.11 and 0.14) where superconductivity occurs at x > 0.03. In the undoped LaFeAsO, 1/T _{1} of ^{139}La exhibits a distinct peak at Ti_{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/T_{1}T of ^{75}As exhibits a Curie-Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with a decrease in temperature. At x = 0.11 and 0.14, in contrast, pseudogap behavior is observed in 1/ T_{1}T at the ^{75}As site with a gap value of δ_{PG} ∼ 175 and 165 K, respectively. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure because of the electron doping. As for the superconducting properties for x = 0.04, 0.07 and 0.11, the 1 / T_{1} of^{75} As in the three samples does not exhibit a coherence peak just below T_{c} and follows a T^{3} dependence at low temperatures. These results may suggest unconventional superconductivity with a zero gap along the lines, but neither the ﬁeld-induced extra relaxation rate nor the residual density of states at the low temperatures is incompatible with the presence of the line nodes. We discuss the 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 the F-doping dependence of T_{c} and 1/T_{1}.

Original language | English |
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Article number | 045004 |

Journal | New Journal of Physics |

Volume | 11 |

DOIs | |

Publication status | Published - 2009 Apr 27 |

Externally published | Yes |

### ASJC Scopus subject areas

- Physics and Astronomy(all)

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_{1-x}F

_{x}).

*New Journal of Physics*,

*11*, [045004]. https://doi.org/10.1088/1367-2630/11/4/045004