Strong-coupling superconductivity in layered nickel-based LaO1-xFxNiAs

Abstract

A series of layered nickel-based LaO1-xFxNiAs compounds with x=0 to 0.15 are synthesized by solid state reactions. The pure LaONiAs exhibits bulk superconductivity with Tc 2.75 K. Partial substitution of oxygen by fluorine increases the transition temperature to 3.8 K. The LaO0.9F0.1NiAs 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 some character of a multi-gap system.