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

Abstract

We present extensive 75 As NMR and NQR data on the superconducting arsenides PrFeAs 0.89 F 0.11 (T c =45 K), LaFeAsO 0.92 F 0.08 (T c =27 K), LiFeAs (T c = 17 K) and Ba 0.72 K 0.28 Fe 2 As 2 (T c = 31.5 K) single crystal, and compare with the nickel analog LaNiAsO 0.9 F 0.1 (T c =4.0 K) . In contrast to LaNiAsO 0.9 F 0.1 where the superconducting gap is shown to be isotropic, the spin lattice relaxation rate 1/T 1 in the Fe-arsenides decreases below T c with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T c 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 LaNiAsO 0.9 F 0.1 . We will discuss the implications of these results and highlight the importance of the Fermi surface topology.

Crossover from weak to strong pairing in unconventional superconductors

“…Despite its relatively high T c , LiFeAs is characterized by weak coupling barely above the BCS limit, [37][38][39][40][41][42]80,81 possibly related to the absence of notable Fermi surface nesting in its band structure 37 or even a different pairing mechanism. 82,83 In NaFeAs, on the contrary, superconductivity with T c ≈ 10 K coexists with antiferromagnetism.…”

“…However, we cannot fail to mention some deviations from this trend that are best demonstrated by LiFeAs, the bearer of the highest known T c = 18 K among stoichiometric Febased materials, together with its close relative NaFeAs. Despite its relatively high T c , LiFeAs is characterized by weak coupling barely above the BCS limit, [37][38][39][40][41][42]80,81 possibly related to the absence of notable Fermi surface nesting in its band structure 37 or even a different pairing mechanism. 82,83 In NaFeAs, on the contrary, superconductivity with T c ≈ 10 K coexists with antiferromagnetism.…”