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

概要

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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 $T_{\text{c}}$ ${\sim}28$ 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 $1/T_{1}$, we have tried to use a simple relation $1/T_{1}\sim T^{n}$ to see how fast $1/T_{1}$ decreases when the superconducting order parameter grows. The relation holds rather well in the $T$ region of ${\sim}0.3T_{\text{c}}<T\leq T_{\text{c}}$, 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.
2010-09-15