Ternary iron pnictides $AFe_2As_2$ (A = alkali metal) and LiFeAs: A systematic study of magnetic properties and transport measurements

We have observed superconductivity for the alkali metal $AFe_2As_2$, where A = Cs, Rb, and K, at temperatures below 4 K. In addition to the above mentioned ternary compounds we also were able to synthesize and measure the metastable superconducting $NaFe_2As_2$, which has a considerably higher superconducting transition at 25 K. The ternary iron pnictides $AFe_2As_2$ are all considered strongly hole over-doped superconductors. LiFeAs is superconducting with a $T_c$ of 18 K. Thermoelectric power measurements indicate that a majority of carriers are electron like, which supports our conjecture that LiFeAs is a stoichiometric superconductor. The effects of high pressure on the $T_c$ will also be discussed.