Quasi-two-dimensional behavior of 112-type iron-based superconductors

Fluctuation magnetoconductivity and magnetization above the superconducting transition temperature (Tc) are measured on the recently discovered 112 family of iron-based superconductors (IBS), Ca$_{1-x}$La$_x$Fe$_{1-y}$Ni$_y$As$_2$, which presents an extra As-As chain spacer-layer. The analysis in terms of a generalization of the Lawrence-Doniach (LD) approach to finite applied magnetic fields indicates that these compounds are among the most anisotropic IBS ($\gamma$ up to ~30), and provides a compelling evidence of a quasi-two-dimensional behavior for doping levels near the optimal one.