Strong-coupling superconductivity in the nickel-based oxypnictide LaNiAsO1-xFx

A series of layered nickel-based LaNiAsO(1-x)F(x) compounds with x=0-0.15 are synthesized by solid-state reactions. The pure LaNiAsO exhibits bulk superconductivity with T(c)similar to 2.75 K. Partial substitution of oxygen with fluorine increases the transition temperature to similar to 3.8 K. The LaNiAsO(0.9)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.