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

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 Δ. 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 Δ, and that the T^{5-6} behavior is considered to be intrinsic to this system.

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