$^{75}$As NMR study of overdoped CeFeAsO$_{0.8}$F$_{0.2}$

We report the results from a $^{75}$As NMR study on the overdoped iron pnictide superconductor, CeFeAsO$_{0.8}$F$_{0.2}$. 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 $^{75}$As 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 $\sim$-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 ReFeAsO$_{1-x}$F$_x$ (Re is a rare earth) we find that there are more than one $^{75}$As 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 ReFeAsO$_{1-x}$F$_x$ superconductors.

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