From Kondo behavior to high temperature superconductivity in Sr(Ni$_{1-x}$Fe$_x$)$_2$As$_2$

SrFe$_2$As$_2$ has an antiferromagnetic groundstate at ambient pressure that can be suppressed by chemical doping or pressure to produce unconventional superconductivity. SrNi$_2$As$_2$ is a non-magnetic conventional superconductor with T$_c$ of 0.6 K. It has been shown that in Sr(Ni$_{1-x}$Fe$_x$)$_2$As$_2$ there is a dome of superconductivity between $x=0.95$ and $x=0.9$. However, little is known about this doping series for small $x$ values. We will present the study of the thermodynamic and transport properties of the doping series of Sr(Ni$_{(1-x)}$Fe$_x$)$_2$As$_2$ for $x\leq 0.6$. In the dilute Fe limit ($x \leq 0.01$) we find strong evidence for single-ion Kondo behaviour. As the concentration of Fe is increased, Fe-Fe interaction effects become significant and the Kondo scale increases. For $0.2\leq x \leq 0.6$ magnetic susceptibility measurements show the presence of a spin glass transition. The presence of Kondo behaviour in Sr(Ni$_{(1-x)}$Fe$_x$)$_2$As$_2$ indicates the formation of local moments interacting with conduction electrons. Therefore, we will address the relevance of this result to the discussion of the itineracy of the magnetism in SrFe$_2$As$_2$, as well as the observed enhancement of the effective mass seen in many pnictide compounds.