Optical investigation of the thermoelectric topological crystalline insulator Pb0.77Sn0.23Se

Pb0.77Sn0.23Se is a promising thermoelectric alloy that exhibits a temperature dependent band inversion below 300 K. Recent work has shown that this band inversion also coincides with a trivial to nontrivial topological phase transition. To understand how the properties critical to thermoelectric efficiency are affected by the band inversion, we measured the broadband optical response of Pb0.77Sn0.23Se as a function of temperature. We find clear optical signatures suggesting the band inversion occurs at 160 +/- 15 K, and use the extended Drude model to accurately determine a T-3/2 dependence of the bulk carrier lifetime, associated with electron-acoustic phonon scattering. Due to the high bulk carrier doping level, no discriminating signatures of the topological surface states are found, although their presence cannot be excluded from our data.