Infrared spectroscopic study of topological material BaMnSb₂

Abstract

A detailed infrared optical spectrum of the new topological material BaMnSb₂ has been measured at temperatures ranging from 7 K to 295 K. As the temperature decreases, the plasma minimum has a clear blue shift in reflectivity spectrum, indicating that the carrier density changes with temperature. In the real part of the optical conductivity <inline-formula><tex-math id="M1">\begin{document}$\sigma_{1}(\omega)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220011_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220011_M1.png"/></alternatives></inline-formula>, two linearly-increased components can be identified, but neither of their extrapolation pass through the origin, which proves that BaMnSb₂ has a gapped Dirac dispersion near the Fermi level. Comparing with the theoretical calculation by using first-principles methods, the onset of these two linearly-increased components are in good agreement with the band structures. In addition, a range of constant optical conductivity is found in <inline-formula><tex-math id="M2">\begin{document}$\sigma_{1}(\omega)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220011_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220011_M2.png"/></alternatives></inline-formula>, which cannot be described well by the Drude-Lorentz model. Therefore, we introduce a frequency-independent component to fit <inline-formula><tex-math id="M3">\begin{document}$\sigma_{1}(\omega)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220011_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220011_M3.png"/></alternatives></inline-formula> successfully. However, different from the Dirac nodal-line semimetal YbMnSb₂ which shares same fitting results as well as crystal structure, the constant component in BaMnSb₂ has a small proportion of <inline-formula><tex-math id="M4">\begin{document}$\sigma_{1}(\omega)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220011_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20220011_M4.png"/></alternatives></inline-formula>. Through calculation and analysis, we attribute the constant component to the surface state of BaMnSb₂.