Evidence for a full energy gap in the nickel pnictide superconductor LaNiAsO1−xFx from A75s nuclear quadrupole resonance

Abstract

We report systematic A75s-NQR and L139a-NMR studies on nickel pnictide superconductors LaNiAsO1−xFx (x=0, 0.06, 0.10, and 0.12). The spin-lattice relaxation rate 1/T1 decreases below Tc 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/T1T is constant in the temperature range Tc∼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.

Source-reported materials — not catalogue approval

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