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

TL;DR

This study uses NMR and NQR techniques to analyze the superconducting properties and spin fluctuations in iron arsenides, revealing multiple gaps and anisotropic spin behavior, and compares these with nickel analogs to understand their electronic structure.

Contribution

It provides detailed NMR/NQR data on iron arsenide superconductors, demonstrating multiple superconducting gaps and anisotropic spin fluctuations, and compares these features with nickel-based analogs.

Findings

Superconducting gaps are multiple and anisotropic in iron arsenides.

Spin fluctuations are weaker and anisotropic compared to cuprates.

Nickel analog shows isotropic gap and no significant electron correlations.

Abstract

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.