Effect of magnetism and phonons on localized carriers in the ferrimagnetic kagome metals GdMn${}_6$Sn${}_6$ and TbMn${}_6$Sn${}_6$

Kagome metals possess peculiar optical spectra consisting of contributions from free charge carriers in a Drude-type response, localized carriers seen as a strongly temperature-dependent localization peak, and, in some cases, phonons displaying strong anomalies. The rare-earth kagome metal series, $R$Mn${}_6$Sn${}_6$, provides a marvelous playground to study the electronic properties of kagome metals in the presence of variable magnetic order. Here, we report temperature-dependent reflectivity studies on two members of the $R$Mn${}_6$Sn${}_6$ family, GdMn${}_6$Sn${}_6$ (in-plane ferrimagnet) and TbMn${}_6$Sn${}_6$ (out-of-plane ferrimagnet), in a broad energy range (50 - 18000 cm${}^{-1}$, equivalent to 6.2 meV - 2.23 eV) down to 10 K. At high temperatures, a phonon mode at approximately 160 cm${}^{-1}$ is observed, which becomes screened out in TbMn${}_6$Sn${}_6$ below $\sim$ 150 K as the localization peak linearly passes through the mode. In GdMn${}_6$Sn${}_6$, the disappearance of the phonon is accompanied by the onset of saturation of the peak position, suggesting an unusual interplay between the two features.