Superconducting Fe-Based Compounds ( A 1 − x Sr x ) Fe 2 As 2 with A = K and Cs with Transition Temperatures up to 37 K

New high-${T}_{c}$ Fe-based superconducting compounds, $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ with $A=\mathrm{K}$, Cs, $\mathrm{K}/\mathrm{Sr}$, and $\mathrm{Cs}/\mathrm{Sr}$, were synthesized. The ${T}_{c}$ of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{CsFe}}_{2}{\mathrm{As}}_{2}$ is 3.8 and 2.6 K, respectively, which rises with partial substitution of Sr for K and Cs and peaks at 37 K for 50%--60% Sr substitution, and the compounds enter a spin-density-wave state with increasing electron number (Sr content). The compounds represent $p$-type analogs of the $n$-doped rare-earth oxypnictide superconductors. Their electronic and structural behavior demonstrate the crucial role of the…