An infrared view of superconductivity in the iron-based materials

Appropriate chemical doping in the 122 iron arsenides ($A$Fe$_{\mathrm{2}}$As$_{\mathrm{2}})$ usually leads to suppression of the antiferromagnetic order. However, bulk superconductivity does not always occur upon suppression of the antiferromagnetism. In this work we study why some iron-based materials are superconducting while others are not. We have performed cryogenic optical spectroscopy measurements on single crystals of superconducting BaFe$_{\mathrm{1.9}}$Pt$_{\mathrm{0.1}}$As$_{\mathrm{2}}$ and non-superconducting, rare-earth-doped CaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. The \textit{ab}-plane dielectric functions have been obtained and compared to one another to gain insight into the conditions necessary for superconductivity in these materials. We also compare the dielectric functions of a number of superconducting and non-superconducting iron-based materials from the published literature. We shall discuss our findings in light of the electrodynamics formulation of the BCS theory of superconductivity.