Strong-coupling superconductivity in layered nickel-based   LaO$_{1-x}$F$_x$NiAs
  A series of layered nickel-based LaO$_{1-x}$F$_x$NiAs compounds with x=0 to 0.15 are synthesized by solid state reactions. The pure LaONiAs exhibits bulk superconductivity with Tc $\sim$ 2.75 K. Partial substitution of oxygen by fluorine increases the transition temperature to $\sim 3.8$ K. The LaO$_{0.9}$F$_{0.1}$NiAs 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. 
Z. Li, G. F. Chen, J. Dong, G. Li, W. Z. Hu, D. Wu, S. K. Su, P.   Zheng, T. Xiang, N. L. Wang, J. L. Luo