A series of layered nickel-based ${\text{LaNiAsO}}_{1-x}{\text{F}}_x$ compounds with $x=0–0.15$ are synthesized by solid-state reactions. The pure LaNiAsO exhibits bulk superconductivity with $T_c\sim 2.75\text{K}$. Partial substitution of oxygen with fluorine increases the transition temperature to $\sim 3.8\text{K}$. The ${\text{LaNiAsO}}_{0.9}{\text{F}}_{0.1}$ sample shows a sharp superconducting transition and a sharp specific-heat jump at the critical temperature. The magnitude of the specific-heat jump is much larger than that expected from the weak-coupling BCS theory, indicating that this superconductor is in the strong-coupling regime. Furthermore, the temperature dependence of the specific heat deviates strongly from the theoretical result for the single-band $s$- or $d$-wave superconductor but shows a characteristic feature of multigap.

• Received 18 June 2008

DOI:https://doi.org/10.1103/PhysRevB.78.060504