Doping dependence of Normal-state Property in the Iron-based Oxypnictide Superconductors LaFeAsO1-y probed by 57Fe-NMR and 75As-NMR/NQR

摘要

We report systematic 57Fe-NMR and 75As-NMR/NQR studies on an underdopedsample (T_c=20 K), an optimally doped sample (T_c=28 K), and an overdopedsample (T_c=22 K) of oxygen-deficient iron (Fe)-based oxypnictidesuperconductor LaFeAsO_{1-y}$. A microscopic phase separation betweensuperconducting domains and magnetic domains is shown to take place in theunderdoped sample, indicating a local inhomogeneity in association with thedensity distribution of oxygen deficiencies. As a result, 1/T_1T in the normalstate of the superconducting domain decreases significantly upon cooling atboth the Fe and As sites regardless of the electron-doping level inLaFeAsO_{1-y}. On the basis of this result, we claim that $1/T_1T$ is notalways enhanced by antiferromagnetic fluctuations close to an antiferromagneticphase in the underdoped superconducting sample. This contrasts with thebehavior in hole-doped Ba_{0.6}K_{0.4}Fe2As2(T_c= 38 K), which exhibits asignificant increase in $1/T_1T$ upon cooling. We remark that the crucialdifference between the normal-state properties of LaFeAsO_{1-y} andBa_{0.6}K_{0.4}Fe2As2 originates from the fact that the relevant Fermi surfacetopologies are differently modified depending on whether electrons or holes aredoped into the FeAs layers.