We report the results from a 75As NMR study on the overdoped iron pnictide superconductor, CeFeAsO0.8F0.2. Two peaks are observed in the NMR spectra for temperatures as high as 100 K, which is above the superconducting transition temperature of 39 K and hence they cannot be attributed to the effect of vortices in the superconducting state as previously suggested [A. Ghoshray, et al., Phys. Rev. B 79, 144512 (2009)]. The temperature dependence of the 75As NMR shifts for the two peaks is consistent with hyperfine coupling from magnetic Ce to As and with different Curie-Weiss temperatures. The appearance of two Curie-Weiss temperatures where one is near zero and the other is ∼-18 K suggests that the two peaks arise from ordered and disordered regions where the disorder could be due to regions with significant fluorine site disorder and/or clusters of oxygen vacancies. Similar to some other studies on ReFeAsO1−xFx (Re is a rare earth) we find that there are more than one 75As spin-lattice and spin-spin relaxation rates that can be attributed to ordered and disordered regions in the ReO layer and suggests that inhomogeneity is a common feature in the ReFeAsO1−xFx superconductors.
@article{arxiv.1212.0984,
title = {$^{75}$As NMR study of overdoped CeFeAsO$_{0.8}$F$_{0.2}$},
author = {Damian Rybicki and Thomas Meissner and Grant V. M. Williams and Shen Chong and Marc Lux and Jürgen Haase},
journal= {arXiv preprint arXiv:1212.0984},
year = {2013}
}