  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].
