Optical properties and carrier localization in the layered phosphide EuCd2P2

The layered phosphide EuCd${}_{2}$P${}_{2}$ is a poor metal that is distinguished by an anomalous increase in the resistivity at $\ensuremath{\simeq}18$ K, well above the Napos{e}el transition at ${T}_{\mathrm{N}}\ensuremath{\simeq}11.5$ K, by nearly two orders of magnitude, before reverting back to a metallic state below ${T}_{\text{N}}$. The addition and removal of free carriers has a small effect on the optical conductivity. However, it has a dramatic effect on the electronic screening of the infrared-active lattice modes, yielding dramatically different line shapes in the reflectivity (see image), providing clues to the mechanism driving this unusual behavior.