Unusual electronic and vibrational behavior in the high thermopower antimonides FeSb$_2$ and CrSb$_2$*

The antimonides FeSb$_2$ and CrSb$_2$ both exhibit an extraordinarily high thermoelectric power factor at low temperature; however, the origin of this behavior is still a subject of some debate, having been variously attributed to either electronic correlations or the phonon drag effect. % The optical properties of a material provide information about both the electronic and vibrational properties of a material; accordingly, the temperature dependence of the complex optical properties of single crystals of FeSb$_2$ and CrSb$_2$ have been measured along the principle optical axes over a wide frequency range. % At room temperature, the low-frequency optical conductivity of FeSb$_2$ is anisotropic, with the conductivity along the $b$ axis significantly larger than along either the $a$ or $c$ axes. Below $\simeq 100$ K the low-frequency conductivity decreases dramatically, but a step-like feature emerges along the $b$ axis at $\simeq 75$ meV in agreement with first principle calculations, suggesting one-dimensional behavior and signalling the presence of electronic correlations. The infrared-active modes are extremely narrow; curiously, the character of the lattice modes changes between $\simeq 100 - 200$ K, possibly indicating a weak structural distortion.\footnote{C. C. Homes, Q. Du, C. Petrovic, W. H. Brito, S. Choi, and G. Kotliar, Sci. Rep. \textbf{8}, 11292 (2018).} % The high thermopower observed in CrSb$_2$ is attributed to the phonon drag effect. The behavior of the optical conductivity is similar to that of FeSb$_2$, but there is no anisotropy or evidence of low-dimensional behavior. No phonon anomalies are observed in this material; however, just below $T_{\rm N}\simeq 273$ K a peculiar electronic mode appears at $\simeq 50$ meV, which rapidly softens and decreases in intensity at low temperature.