75As-NMR Studies of LaFeAsO1-xFx for Various x Values

TL;DR

This study uses 75As NMR and NQR to analyze how doping levels affect the superconducting properties and inhomogeneity in LaFeAsO1-xFx, revealing an intrinsic T^{5-6} relaxation behavior at optimal doping.

Contribution

It provides detailed NMR/NQR analysis across various doping levels, highlighting the relationship between inhomogeneity and relaxation rates in LaFeAsO1-xFx superconductors.

Findings

Optimal doping yields the smallest inhomogeneity.

The relaxation rate follows a T^{5-6} dependence near Tc.

Inhomogeneity affects the temperature dependence of 1/T1.

Abstract

Results of 75As NMR and NQR measurements are reported for superconducting samples of LaFeAsO1-xFx with various x values. The x dependent widths of the superconducting transition and those of the NQR spectra indicate that the optimally doped LaFeAsO1-xFx sample with the superconducting transition temperature Tc ~28 K has the smallest width or smallest inhomogeneity of the superconducting order parameter \Delta. In the analyses of the temperature (T) dependence of the NMR longitudinal relaxation rate 1/T1, we have tried to use a simple relation 1/T1 ~ T^n to see how fast 1/T1 decreases when the superconducting order parameter grows. The relation holds rather well in the T region of ~0.3Tc < T =< Tc, and n varies from 2.5-6 with varying x, having the maximum value 5-6 at the optimal x. These results indicate that the x dependence of n originates from the spatial distribution of \Delta, and that the T^{5-6} behavior is considered to be intrinsic to this system.