Superconductivity in SrFe$_{2-x}$Co$_x$As$_2$: Internal Doping of the Iron Arsenide Layers

In the strontium iron-cobalt arsenides SrFe$_{2-x}$Co$_x$As$_2$ ($0.2\leq x \leq 0.4$) superconductivity with $T_c$ up to 20\,K is observed in magnetic susceptibility, electrical resistivity, and specific heat data. This first observation of bulk superconductivity induced by electron doping in this family of compounds -- despite strong disorder in the Fe-As layer -- favors an itinerant electronic theory in contrast to the strongly correlated cuprates and renders a $p$- or $d$-wave pairing unlikely. The magnetic ordering present in SrFe$_2$As$_2$ is rapidly suppressed by substitution of Fe by Co. DFT calculations show that this is due to a rigid down-shift of the Fe-3$d_{x^2-y^2}$-related band edge in the density of states.