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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Abstract
We report systematic 57 Fe-NMR and 75 As-NMR/NQR studies on an underdoped sample ( T c =20 K), an optimally doped sample ( T c =28 K), and an overdoped sample ( T c =22 K) of oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO 1- 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 LaFeAsO 1- 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 Ba 0.6 K 0.4 Fe 2 As 2 ( T 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 LaFeAsO 1- y and Ba 0.6 K 0.4 Fe 2 As 2 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.

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