Superconducting Fe-Based Compounds $(A_{1-x}{\mathrm{Sr}}_x){\mathrm{Fe}}_2{\mathrm{As}}_2$ with $A=\mathrm{K}$ 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 (${\mathrm{Fe}}_2{\mathrm{As}}_2$) 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 $T_c$.

Figure 1

(a) Crystal structure of LaOFeAs; (b) crystal structure of $(\mathrm{K}/\mathrm{Sr}){\mathrm{Fe}}_2{\mathrm{As}}_2$ and $(\mathrm{Cs}/\mathrm{Sr}){\mathrm{Fe}}_2{\mathrm{As}}_2$.

Figure 2

(a) Powder diffraction data/Rietveld refinement: (a) ${\mathrm{KFe}}_2{\mathrm{As}}_2$, (b) ${\mathrm{SrFe}}_2{\mathrm{As}}_2$, (c) ${\mathrm{CsFe}}_2{\mathrm{As}}_2$.

Figure 3

(a) Resistivity of the two end members, ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ and ${\mathrm{KFe}}_2{\mathrm{As}}_2$. The inset shows the superconducting transition of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ on an enlarged scale; (b) resistivity at different fields of ${\mathrm{K}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{Fe}}_2{\mathrm{As}}_2$; (c) resistivity at different fields of ${\mathrm{Cs}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{Fe}}_2{\mathrm{As}}_2$. The insets in (b) and (c) show the $T$ dependence of the critical fields as determined from the midpoint of the resistivity drop.

Figure 4

(a) Field cooled (FC) and zero field cooled (ZFC) magnetic susceptibilities of ${\mathrm{K}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{Fe}}_2{\mathrm{As}}_2$ and ${\mathrm{Cs}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{Fe}}_2{\mathrm{As}}_2$ measured at 10 Oe. The inset shows the magnetic susceptibilities of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ near $T_c$; (b) Seebeck coefficients of ${\mathrm{K}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{Fe}}_2{\mathrm{As}}_2$ (top curve), ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ (middle curve), and ${\mathrm{CeO}}_{0.84}{\mathrm{F}}_{0.16}\mathrm{FeAs}$ (bottom curve).

Figure 5

Superconducting phase diagram of ${\mathrm{K}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$.