Effect of magnetism and phonons on localized carriers in the ferrimagnetic kagome metals GdMn6Sn6 and TbMn6Sn6

Abstract

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, $R{\mathrm{Mn}}_{6}{\mathrm{Sn}}_{6}$, 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 $R{\mathrm{Mn}}_{6}{\mathrm{Sn}}_{6}$ family, ${\mathrm{GdMn}}_{6}{\mathrm{Sn}}_{6}$ (in-plane ferrimagnet) and ${\mathrm{TbMn}}_{6}{\mathrm{Sn}}_{6}$ (out-of-plane ferrimagnet), in a broad energy range (50--18 000 ${\mathrm{cm}}^{\ensuremath{-}1}$, equivalent to 6.2 meV--2.23 eV) down to 10 K. At high temperatures, a phonon mode at approximately 160 ${\mathrm{cm}}^{\ensuremath{-}1}$ is observed, which becomes screened out in ${\mathrm{TbMn}}_{6}{\mathrm{Sn}}_{6}$ below $\ensuremath{\sim}$150 K as the localization peak linearly passes through the mode. In ${\mathrm{GdMn}}_{6}{\mathrm{Sn}}_{6}$, the disappearance of the phonon is accompanied by the onset of saturation of the peak position, suggesting an unusual interplay between the two features.