Low work function in the 122-family of iron-based superconductors

We determine the work functions of the iron arsenic compounds $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A=\mathrm{Ca},\mathrm{Ba},\mathrm{Cs}$) using photoemission spectroscopy to be 2.7 eV for ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$, 1.8 eV for ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, and 1.3 eV for ${\mathrm{CsFe}}_{2}{\mathrm{As}}_{2}$. The work functions of these 122 iron-based superconductors track those of the elementary metal $A$ but are substantially smaller. The most likely explanation of this observation is that the cleaving surface exposes only half an $A$-layer. The low work function and good photoemission cross section of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{CsFe}}_{2}{\mathrm{As}}_{2}$ enable photoemission even from a common white LED light.