Evidence for a full energy gap in the nickel pnictide superconductor LaNiAsO_1-x F_x from ⁷⁵As nuclear quadrupole resonance

We report systematic ⁷⁵As-NQR and L 139 a-NMR studies on nickel pnictide superconductors LaNiAsO_1-x F_x (x=0, 0.06, 0.10, and 0.12). The spin-lattice relaxation rate 1/T₁ decreases below T_c with a well-defined coherence peak and follows an exponential decay at low temperatures. This result indicates that the superconducting gap is fully opened and is strikingly different from that observed in iron-pnictide analogs. In the normal state, 1/T₁ T is constant in the temperature range T_c ∼4 K≤T≤10 K for all compounds and up to T=250 K for x=0 and 0.06, which indicates weak electron correlations and is also different from the iron analog. We argue that the differences between the iron and nickel pnictides arise from the different electronic band structure. Our results highlight the importance of the peculiar Fermi-surface topology in iron pnictides. We also provide a possible interpretation for the "pseudogap-like" behavior in the normal state which is also seen in iron pnictides.