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

We report ⁷⁵As nuclear quadrupole resonance (NQR) studies on oxypnictide superconductors LaFeAsO_1−xF_x (x=0.08, 0.15) and LaNiAsO_1−xF_x (x=0, 0.06, 0.10, 0.12). In LaFeAsO_0.92F_0.08 (T_c=23 K), nuclear spin-lattice relaxation rate 1/T₁ shows no coherence peak just below T_c and decreases with decreasing temperature accompanied by a hump structure at T∼0.4T_c, which is a characteristic of the multigap superconductivity. In the normal state, the quantity 1/T₁T increases with decreasing temperature to T_c, indicating that the existence of antiferromagnetic correlation originating from its multiple electronic band structure. On the other hand, LaNiAsO_1−xF_x shows a clear Hebel-Slichter (coherence) peak just below T_c, evidencing that the LaNiAsO_1−xF_x is a BCS superconductor. In the normal state, 1/T₁T is constant in the temperature range for all LaNiAsO_1−xF_x, which indicates electron correlations are weak. We suggest that the contrasting behavior of both superconductivity and electron correlations in LaFeAsO_0.92F_0.08 and LaNiAsO_1−xF_x 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.