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

TL;DR

This study uses $^{75}$As NMR to investigate the overdoped CeFeAsO$_{0.8}$F$_{0.2}$ superconductor, revealing inhomogeneity and magnetic regions above the transition temperature, challenging previous vortex-related interpretations.

Contribution

First NMR investigation of overdoped CeFeAsO$_{0.8}$F$_{0.2}$ showing multiple peaks and inhomogeneity related to magnetic and structural disorder.

Findings

Two NMR peaks observed above $T_c$, indicating inhomogeneity.

Different Curie-Weiss temperatures suggest ordered and disordered regions.

Inhomogeneity is a common feature in ReFeAsO$_{1-x}$F$_x$ superconductors.

Abstract

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.