Doping Dependence of Normal-State Properties in Iron-Based Oxypnictide Superconductor LaFeAsO1-y Probed by 57Fe-NMR and 75As-NMR/NQR

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We report systematic 57Fe-NMR and 75As-NMR/NQR studies on an underdoped sample ($T_{\text{c}}=20$ K), an optimally doped sample ($T_{\text{c}}=28$ K), and an overdoped sample ($T_{\text{c}}=22$ K) of oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO1-y. A microscopic phase separation between superconducting domains and magnetic domains is shown to take place in the underdoped sample, indicating a local inhomogeneity in association with the density distribution of oxygen deficiencies. As a result, $1/T_{1}T$ in the normal state of the superconducting domain decreases significantly upon cooling at both the Fe and As sites regardless of the electron-doping level in LaFeAsO1-y. On the basis of this result, we claim that $1/T_{1}T$ is not always enhanced by antiferromagnetic fluctuations close to an antiferromagnetic phase in the underdoped superconducting sample. This contrasts with the behavior in hole-doped Ba0.6K0.4Fe2As2 ($T_{\text{c}}=38$ K), which exhibits a significant increase in $1/T_{1}T$ upon cooling. We remark that the crucial difference between the normal-state properties of LaFeAsO1-y and Ba0.6K0.4Fe2As2 originates from the fact that the relevant Fermi surface topologies are differently modified depending on whether electrons or holes are doped into the FeAs layers.
2009-08-15