As-NQR study of superconductivity in LaRAsO1−xFx (R=Fe and Ni)

We report 75As nuclear quadrupole resonance (NQR) studies on oxypnictide superconductors LaFeAsO1−xFx (x=0.08, 0.15) and LaNiAsO1−xFx (x=0, 0.06, 0.10, 0.12). In LaFeAsO0.92F0.08 (Tc=23 K), nuclear spin-lattice relaxation rate 1/T1 shows no coherence peak just below Tc and decreases with decreasing temperature accompanied by a hump structure at T∼0.4TcT∼0.4Tc, which is a characteristic of the multigap superconductivity. In the normal state, the quantity 1/T1T increases with decreasing temperature to Tc, indicating that the existence of antiferromagnetic correlation originating from its multiple electronic band structure. On the other hand, LaNiAsO1−xFx shows a clear Hebel–Slichter (coherence) peak just below Tc, evidencing that the LaNiAsO1−xFx is a BCS superconductor. In the normal state, 1/T1T   is constant in the temperature range 4≤T≤10K for all LaNiAsO1−xFx, which indicates electron correlations are weak. We suggest that the contrasting behavior of both superconductivity and electron correlations in LaFeAsO0.92F0.08 and LaNiAsO1−xFx between them relate to the difference of electronic band structure configuration. We also provide a possible interpretation for the pseudogap-like behavior in the normal state observed in both compounds.