Doping dependence of normal-state properties in iron-based oxypnictide superconductor LaFeAsO1-y probed by 57Fe-NMR and 75As-NMR/NQR

We report systematic 57Fe-NMR and 75As-NMR/NQR studies on an underdoped sample (Tc=20 K), an optimally doped sample (Tc=28 K), and an overdoped sample (Tc=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/T1T 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/T1T 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 (Tc=38 K), which exhibits a significant increase in 1/T1T 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. (author)