Anomalous optical properties of a multiband metal due to thermal redistribution: The case of SrMnSb$_{\mathrm{2}}$

We report an optical spectroscopic study of SnMnSb$_{\mathrm{2}}$, a low carrier density metal. As temperature is decreased, our measurements reveal a large increase in the free carrier plasma frequency, which is unusual for a metal. This seemingly anomalous behavior can be accounted for using a `three band' model of the multiband electronic structure of SrMnSb$_{\mathrm{2}}$ that includes two conduction bands and one valence band close to the Fermi level. The temperature dependence of the low-lying interband optical transitions and the Hall number can also be understood using our model. Our results provide a possible explanation for the puzzling optical properties that have been reported in a number of topical low carrier density metals and semimetals.