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-Tc Fe-based superconducting compounds, AFe$_{2}$As$_{2}$ with A = K, Cs, K/Sr, and Cs/Sr, were synthesized. The Tc of KFe$_{2}$As$_{2}$ and CsFe$_{2}$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 (Fe$_{2}$As$_{2})$ layers in the superconductivity of the Fe-based layered systems, and the special feature of having elemental A layers provides new avenues to superconductivity at higher Tc.