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

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Pb₀.₇₇Sn₀.₂₃Se is a novel alloy of two promising thermoelectric materials PbSe and SnSe 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 evidence of the band inversion at 160±15 K, and use the extended Drude model to accurately determine a T³/² 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.