Y. Ooe | Searcharxiv

arXiv2011

Multiple superconducting gap and anisotropic spin fluctuations in iron arsenides: Comparison with nickel analog

We present extensive 75As NMR and NQR data on the superconducting arsenides PrFeAs0.89F0.11 (Tc=45 K), LaFeAsO0.92F0.08 (Tc=27 K), LiFeAs (Tc = 17 K) and Ba0.72K0.28Fe2As2 (Tc = 31.5 K) single crystal, and compare with the nickel analog LaNiAsO0.9F0.1 (Tc=4.0 K) . In contrast to LaNiAsO0.9F0.1 where the superconducting gap is shown to be isotropic, the spin lattice relaxation rate 1/T1 in the Fe-arsenides decreases below Tc with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below Tc and shows a step-wise T variation as well. These results indicate spinsinglet superconductivity with multiple gaps in the Fe-arsenides. The Fe antiferromagnetic spin fluctuations are anisotropic and weaker compared to underdoped copper-oxides or cobalt-oxide superconductors, while there is no significant electron correlations in LaNiAsO0.9F0.1. We will discuss the implications of these results and highlight the importance of the Fermi surface topology.

cond-mat.supr-con↗

arXiv2010

$^{75}$As NQR and NMR studies of superconductivity and electron correlations in iron arsenide Li$_{x}$FeAs

We report the $^{75}$As-NQR and NMR studies on the iron arsenide superconductor Li$_{x}$FeAs with $T_{\rm c} \sim 17$ K. The spin lattice relaxation rate, $1/T_{1}$, decreases below $T_{\rm c}$ without a coherence peak, and can be fitted by gaps with s$^{\pm}$-wave symmetry in the presence of impurity scattering. In the normal state, both $1/T_{1}T$ and the Knight shift decrease with decreasing temperature but become constant below $T \leq 50 K$. Estimate of the Korringa ratio shows that the spin correlations are weaker than that in other families of iron arsenides, which may account for the lower $T_{\rm c}$ in this material.

cond-mat.supr-con↗