Doping Dependence of Normal-State Properties in Iron-Based Oxypnictide Superconductor LaFeAsO_1-y Probed by ⁵⁷Fe-NMR and ⁷⁵As-NMR/NQR

Abstract

We report systematic 57 Fe-NMR and 75 As-NMR/NQR studies on an underdoped sample (T c ¼ 20 K), an optimally doped sample (T c ¼ 28 K), and an overdoped sample (T c ¼ 22 K) of oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO 1Ày . 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 1Ày . 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 ¼ 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 1Ày 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.

Antiferromagnetic Order and Superconductivity in Sr4(Mg0.5-xTi0.5+x)2O6Fe2As2 with Electron Doping: 75As-NMR Study

“…However, since its broad SC transition prevents us from deducing SC characteristics precisely, we focus only on normal-state properties of this SC compound. We remark that the T dependences of 1/T 1 T for x=0 and 0.2 resemble those in nondoped (ND) and optimally electron-doped LaFeAsObased compounds, 11,12,25,26 respectively, as compared in Figs. 3(a) and 3(b).…”

Coherence Effect of Sign-Reversing s±-Wave Cooper Pair State in Heavily Overdoped LaFeAsO-based Superconductor: 75As-Nuclear Quadrupole Resonance

“…The 1/T 1 T of H-OVD remarkably decreases upon cooling, the rate of which is steeper than the previous results on other La1111 systems. 13,21 This is primarily attributed to a decrease in the density of states (DOS) at the Fermi level(E F ), namely, the band structure effect derived from the existence of a high DOS just below the E F . 25 Moreover, the value of 1/T 1 T at 240 K decreases with increasing electron-doping levels, which is reasonably explained by an energy shift in the framework of a rigid-band model.…”

Nonmagnetic pair-breaking effect in La(Fe1−xZnx)AsOKitagawa1, Nakai2, Iye3 et al. 2011Phys. Rev. B

“…In LaFeAs(O 1−x F x ) and LaFeAsO 1−δ , low-energy magnetic fluctuations probed with 1/T 1 measurements are dramatically suppressed by F (electron) doping due to variations of the nesting condition. [28][29][30] Therefore, the variations of the electronic state with Zn substitution should be detected with 1/T 1 measurements. As seen in Fig.…”