Optical properties and carrier localization in the layered phosphide EuCd$_\mathbf{2}$P$_\mathbf{2}$

The temperature dependence of the complex optical properties of the layered phosphide material EuCd$_2$P$_2$ have been measured over a wide frequency range above and below $T_{\rm N} \simeq 11.5$ K for light polarized in the $a-b$ planes. At room temperature, the optical conductivity is well described by a weak free-carrier component with a Drude plasma frequency of $\simeq 1100$ cm$^{-1}$ and a scattering rate of $1/\tau_D\simeq 700$ cm$^{-1}$, with the onset of interband absorptions above $\simeq 2000$ cm$^{-1}$. Two infrared-active $E_u$ modes are observed at $\simeq\,$89 and 239 cm$^{-1}$. As the temperature is reduced the scattering rate decreases and the low-frequency conductivity increases slightly; however, below $\simeq 50$ K the conductivity decreases until at the resistivity maximum at $\simeq 18$ K (just below $2T_{\rm N}$) the spectral weight associated with free carriers is transferred to a localized excitation at $\simeq 500$ cm$^{-1}$. Below $T_{\rm N}$, metallic behavior is recovered. Interestingly, the $E_u$ modes are largely unaffected by these changes, with only the position of the high-frequency mode showing any signs of anomalous behavior. While several scenarios are considered, the prevailing view is that the resistivity maximum and subsequent carrier localization is due to the formation of ferromagnetic domains below $\simeq 2T_{\rm N}$ that result in spin-polarized clusters due to spin-carrier coupling [1].